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T Thermochemistry Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Endothermic reactions absorb heat. Exothermic reactions release heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories. The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. These experiments mark the foundation of thermochemistry.

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TThermochemistry

Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the

course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.

Endothermic reactions absorb heat. Exothermic reactions release heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories.The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. These experiments mark the foundation of thermochemistry.

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Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:

1. Lavoisier and Laplace’s law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.[2]

2. Hess' law (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.

These statements preceded the first law of thermodynamics (1845) and helped in its formulation.

Thermometer

Thermometers measure temperature, by using materials that change in some way when they are heated or cooled. In a mercury or alcohol thermometer the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit (used in the United States) or Celsius (used in Canada) and Kelvin (used mostly by scientists).

Trojan horse

Still seeking to gain entrance into Troy, clever Odysseus (some say with the aid of Athena) ordered a large wooden horse to be built. Its insides were to be hollow so that soldiers could hide within it.

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Once the statue had been built by the artist Epeius, a number of the Greek warriors, along with Odysseus, climbed inside. The rest of the Greek fleet sailed away, so as to deceive the Trojans.

One man, Sinon, was left behind. When the Trojans came to marvel at the huge creation, Sinon pretended to be angry with the Greeks, stating that they had deserted him. He assured the Trojans that the wooden horse was safe and would

bring luck to the Trojans.

Only two people, Laocoon and Cassandra, spoke out against the horse, but they were ignored. The Trojans celebrated what they thought was their victory, and dragged the wooden horse into Troy.

That night, after most of Troy was asleep or in a drunken stupor, Sinon let the Greek warriors out from the horse, and they slaughtered the Trojans. Priam was killed as he huddled by Zeus' altar and Cassandra was pulled from the statue of Athena and raped.

Trojan war

The apple of discord

The Trojan War has its roots in the marriage between Peleus and Thetis, a sea-goddess. Peleus and Thetis had not invited Eris,

the goddess of discord, to their marriage and the outraged goddess stormed into the wedding banquet and threw a golden apple onto the table. The apple belonged to, Eris said, whomever was the fairest.

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Hera, Athena, and Aphrodite each reached for the apple. Zeus proclaimed that Paris, prince of Troy and thought to be the most beautiful man alive, would act as the judge.

Hermes went to Paris, and Paris agreed to act as the judge. Hera promised him power, Athena promised him wealth, and Aphrodite promised the most beautiful woman in the world.

Paris chose Aphrodite, and she promised him that Helen, wife of Menelaus, would be his wife. Paris then prepared to set off for Sparta to capture Helen. Twin prophets Cassandra and Helenus tried to persuade him against such action, as did his mother, Hecuba. But Paris would not listen and he set off for Sparta.

In Sparta, Menelaus, husband of Helen, treated Paris as a royal guest. However, when Menelaus left Sparta to go to a funeral, Paris abducted Helen (who perhaps went willingly) and also carried off much of Menelaus' wealth.

Trepanation

Trepanning, also known as trephination, trephining or making a burr hole, is a surgical intervention in which a hole is drilled or scraped into the human skull, exposing the dura mater to treat health problems related to intracranial diseases. It may also refer to any "burr" hole created through other body surfaces, including nail beds. It is often used to relieve pressure beneath a surface. A trephine is an instrument used for cutting out a round piece of skull bone.

Evidence of trepanation has been found in prehistoric human remains from Neolithic times onward. Cave paintings indicate that people believed the practice would cure epileptic seizures, migraines, and mental disorders. The bone that was trepanned was kept by the prehistoric people and may have been worn as a charm to keep evil spirits away.

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Evidence also suggests that trepanation was primitive emergency surgery after head wounds to remove shattered bits of bone from a fractured skull and clean out the blood that often pools under the skull after a blow to the head. Such injuries were typical for primitive weaponry such as slings and war clubs.

There is some contemporary use of the term. In modern eye surgery, a trephine instrument is used in corneal transplant surgery. The procedure of drilling a hole through a fingernail or toenail is also known as trephination. It is performed by a physician or surgeon to relieve the pain associated with a subungual hematoma (blood under the nail); a small amount of blood is expressed through the hole and the pain associated with the pressure is partially alleviated.

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UUltraviolet Radiation

Ultraviolet (UV) radiation is similar to visible light in all physical aspects, except that it does not enable us to see things. The light that enables us to see things is referred to as visible light and is composed of the colors we see in a rainbow. The ultraviolet region starts right after the violet end of the rainbow.

In scientific terms, UV radiation is electromagnetic radiation just like visible light, radar signals and radio broadcast signals (see Figure 1). Electromagnetic radiation is transmitted in the form of waves. The waves can be described by their wavelength or frequency and their amplitude (the strength or intensity of the wave). Wavelength is the length of one complete wave cycle. For radiation in the UV region of the spectrum, wavelengths are measured in nanometers (nm), where 1 nm = one millionth of a millimeter.

Different wavelengths of electromagnetic radiation cause different types of effects on people. For example, gamma rays are used in cancer therapy to kill cancerous cells and infrared light can be used to keep you warm.

UV radiation has shorter wavelengths (higher frequencies) compared to visible light but have longer wavelengths (lower

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frequencies) compared to X-rays. UV radiation is divided into three wavelength ranges:

What are some health effects of exposure to UV radiation?

Some UV exposure is essential for good health. It stimulates vitamin D production in the body. In medical practice, UV lamps are used for treating psoriasis (a condition causing itchy, scaly red patches on the skin) and for treating jaundice in new born babies.

Excessive exposure to ultraviolet radiation is associated with different types of skin cancer, sunburn, accelerated skin aging, as well as cataracts and other eye diseases. The severity of the effect depends on the wavelength (see Figure 2), intensity, and duration of exposure.

Effect on the skin

The shortwave UV radiation (UV-C) poses the maximum risk. The sun emits UV-C but it is absorbed in the ozone layer of the atmosphere before reaching the earth. Therefore, UV-C from the sun does not affect people. Some man-made UV sources also emit UV-C. However, the regulations concerning such sources restrict the UV-C intensity to a minimal level and may have requirements to install special guards or shields and interlocks to prevent exposure to the UV.

The medium wave UV (UV-B) causes skin burns, erythema (reddening of the skin) and darkening of the skin. Prolonged exposures increase the risk of skin cancer.

Unsaturated FatAn unsaturated fat is a fat or fatty acid in which there is one or more double bond in the fatty acid chain.A fat molecule is monounsaturated if it contains one double bond and polyunsaturated if it contains more than one double bond.Where double bonds are formed, hydrogen atoms are eliminated.

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Thus, a saturated fat is "saturated" with hydrogen atoms.The greater the degree of unsaturation in a fatty acid (ie, the more double bonds in the fatty acid), the more vulnerable it is to lipid peroxidation (rancidity).Antioxidants can protect unsaturated fat from lipid peroxidation.Foods containing unsaturated fats include avocado, nuts, and soybean, canola, and olive oils.Meat products contain both saturated and unsaturated fats.Unsaturated fats are liquid at room temperature

Uranus: The Blue Planet

Uranus spins on an axis that often points directly at the sun. This odd alignment is thought to be result of a collision with some other body, possibly a planet size object, early in its history. Its bright blue- green color comes from methane gas in its atmosphere.

Unusual Uranus

Once considered one of the blander-looking planets, Uranus has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet from the sun is so distant that it takes 84 years to complete one orbit.

Uranus, with no solid surface, is one of the gas giant planets. (The others are Jupiter, Saturn, and Neptune.) Its atmosphere is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus's cloud tops, which lie beneath a layer of methane gas.

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Stats

Distance from sun1,783,939,400 miles(2,870,972,170 km)

Length of Year30,687 days

Length of Day17 hours,15 minutes (retrograde) planet rotates in the opposite direction of other planets in its system

Average temp.

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As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through and resulting in the blue-green color that we see.

The planet's atmospheric details are very difficult to see in visible light. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of "icy" materials (water, methane, and ammonia), with higher-density material at depth.

Off-Kilter Planet

Uranus's rotation axis is nearly horizontal, as though the planet has been knocked on its side. This unusual orientation may be the result of a collision with a planet-size body early in Uranus's history, which apparently radically changed the planet's rotation. Additionally, while magnetic fields are typically in alignment with a planet's rotation, Uranus's magnetic field is tipped over.

Even though Uranus is tipped on its side and experiences seasons that last over 20 years, the temperature differences on the summer and winter sides do not differ greatly because the planet is so far from the sun. Near the cloud tops, the temperature of Uranus is -357 degrees Fahrenheit (-216 degrees Celsius).

Because of the planet's unusual orientation, Uranus's rings are perpendicular to its orbital path about the sun. The ten outer rings are dark, thin, and narrow, while the 11th ring is inside the others and is broad and diffuse.

Uranus has 27 known moons, named mostly for characters from the works of William Shakespeare and Alexander Pope. Miranda is the strangest-looking Uranian moon, appearing as though it were made of spare parts. The high cliffs and winding valleys of the moon may indicate partial melting of the interior, with icy material occasionally drifting to the surface.

Urbain, Georges

(12 April 1872 – 5 November 1938in Paris) French Chemist, Professor of Sorbonne. He studied at the elite École supérieure de physique et de chimie industrielles de la ville de Paris (ESPCI ParisTech). He discovered the element lutetium (number 71) in 1907.

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Lutetium is very difficult to separate from other elements and the principal commercially viable ore of lutetium is the rare earth phosphate mineral monazite.

Lutetium is one of the rare and expensive elements and is used in preparing specialised glasses and lenses. They are also used in oil refining and in certain medical diagnostic techniques. This metal

is always found with almost all the other rare-earth metals but never by itself.

The word Lutetium, derives from the Latin word Lutetia which means Paris. It was independently discovered in the year 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl

Auer von Welsbach, and American chemist Charles James. The three scientists found this metal as an impurity in ytterbia. Swiss chemist Jean Charles Galissard de Marignac initially thought that ytterbia consisted only of ytterbium.

Lutetium was the last natural rare earth element to be discovered. The synthetic rare earth promethium was produced later in the laboratory from uranium fission products.

The scientists, Georges Urbain, Baron Carl Auer von Welsbach, Charles James proposed different names for the element. However, George Urbain chose neoytterbium and lutecium, while Baron Carl Auer von Welsbach chose aldebaranium and cassiopeium. Both of these articles accused each other of publishing results based on the other party.

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The Commission on Atomic Mass, which was then responsible for the attribution of new element names, settled the dispute in the year 1909 by granting priority to George Urbain and adopted his names as the official ones. The names ere based on the fact that the separation of lutetium from Marignac's ytterbium was first described by George Urbain. After Urbain's names were recognized, neoytterbium was reverted to ytterbium.

Carl Auer von Welsbach isolated lutetium from ytterbia and he called the element cassipoium after the constellation Cassiopeia. Until the 1950s, some German chemists called lutetium by Baron Carl Auer von Welsbach's name cassiopeium. But in the year 1949, the spelling of element 71 was changed to lutetium. Georges Urbain successfully separated lutetium from ytterbia in 1907.

He separated ytterbia into two constituents by a series of fractional crystallizations of ytterbium nitrate from nitric acid solution and obtained two rare earth oxides. One retained the name of ytterbium while the other he called lutecium which was later changed to lutetium. However, Welsbach's 1907 samples of lutetium had been pure, while Urbain's 1907 samples only contained traces of lutetium. This later misled George Urbain into thinking that he had discovered element 72, which he named celtium, which was actually very pure lutetium.

Charles James, who stayed out of the priority argument, worked on it on a much larger scale and possessed the largest supply of lutetium at the time. Charles James also succeeded in isolating lutetium in 1906-1907 and he patented a bromate fractional crystallization process for isolating the rare earth metals. His fractional crystallization process was considered be the best technique of separating rare earths until the discovery of ion exchange techniques in the 1940s. Pure lutetium metal was first produced in the year 1953.

Uric acid

Uric acid is a chemical created when the body breaks down substances called purines. Purines are found in some foods and drinks. These includeliver, anchovies,

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mackerel, dried beans and peas, and beer.Most uric acid dissolves in blood and travels to the kidneys. From there, it passes out in urine. If your body produces too much uric acid or doesn't remove enough if it, you can get sick. A high level of uric acid in the blood is called hyperuricemia.

This test checks to see how much uric acid you have in your blood. Another test can be used to check the level of uric acid in your urine.

How the Test is Performed

A blood sample is needed. Most of the time blood is drawn from a vein located on the inside of the elbow or the back of the hand.

How to Prepare for the Test

You should not eat or drink anything for 4 hours before the test unless told otherwise.

Many medicines can interfere with blood test results.

Your health care provider will tell you if you need to stop taking any medicines before you have this test.

Do not stop or change your medications without talking to your doctor first.

Your doctor may also tell you to stop taking any drugs that may affect the test results. Never stop taking any medicine without talking to your doctor.

Normal Results

Normal values range between 3.5 and 7.2 mg/dL.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

The example above shows the common measurement range for results for these tests. Some laboratories use different measurements or may test different specimens.

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VVacoule

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist,animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size, its structure varies according to the needs of the cell.

The function and importance of vacuoles varies greatly according to the type of cell in which they are present, having much greater prominence in the cells of plants, fungi and certain protists than those of animals and bacteria. In general, the functions of the vacuole include:

Isolating materials that might be harmful

or a threat to the cell Containing waste products containing water in plant cells

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Maintaining internal hydrostatic pressure orturgor within the cell

Maintaining an acidic internal pH Containing small molecules Exporting unwanted substances from the cell Allows plants to support structures such as leaves and

flowers due to the pressure of the central vacuole In seeds, stored proteins needed for germination are kept in

'protein bodies', which are modified vacuoles.

Vacuoles also play a major role in autophagy, maintaining a balance between biogenesis(production) and degradation (or turnover), of many substances and cell structures in certain organisms. They also aid in the lysis and recycling of misfolded proteins that have begun to build up within the cell. Thomas Boller and others proposed that the vacuole participates in the destruction of invadingbacteria and Robert B Mellor proposed organ-specific forms have a role in 'housing' symbiotic bacteria. In protists, vacuoles have the additional function of storing food which has been absorbed by the organism and assisting in the digestive and waste management process for the cell.

VelocityVelocity is the rate of change of the position of an object, equivalent to a specification of its speed and direction of motion, e.g. 60 km/h to the north. Velocity is an important concept in kinematics, the branch of classical mechanics which describes the motion of bodies.

Velocity is a vector physical quantity; both magnitude and direction are required to define it. The scalar absolute value(magnitude) of velocity is called "speed", a quantity that

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is measured in metres per second (m/s or m·s−1) in the SI (metric) system.

For example, "5 metres per second" is a scalar (not a vector), whereas "5 metres per second east" is a vector.

If there is a change in speed, direction, or both, then the object has a changing velocity and is said to be undergoing anacceleration.

Constant velocity vs. acceleration

To have a constant velocity, an object must have a constant speed in a constant direction. Constant direction constrains the object to motion in a straight path (the object's path does not curve).

Thus, a constant velocity means motion in a straight line at a constant speed.

For example, a car moving at a constant 20 kilometers per hour in a circular path has a constant speed, but does not have a constant velocity because its direction changes. Hence, the car is considered to be undergoing acceleration.

Distinction between speed and velocity

Speed describes only how fast an object is moving; whereas velocity gives both how fast and in what direction the object is moving.[1] If a car is said to travel at 60 km/h, its speed has been specified. However, if the car is said to move at 60 km/h to the north, its velocity has now been specified.

The big difference can be noticed when we consider movement around a circle. When something moves in a circle and returns to its starting point its average velocity is zero but its average speed is found by dividing the circumference of the circle by the time taken to move around the circle. This is because the average velocity is calculated by only considering the displacement between the starting and the end points while the average speed considers only the total distance traveled.

The average velocity   of an object moving a through  

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displacement   during a time interval   is described by the formula:

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The velocity vector v of an object that has positions   at

time   and   at time , can be computed as the derivative of position:

More simply, for motion in one dimension, velocity can be defined as the slope of the position vs. time graph of an object.

Average velocity magnitudes are always smaller than or equal to average speed of a given particle. Instantaneous velocity is always tangential to trajectory. Slope of tangent of position or displacement time graph is instantaneous velocity and its slope of chord is average velocity.

The equation for an object's velocity can be obtained mathematically by evaluating the integral of the equation for its acceleration beginning from some initial period time   to some point in time later .

The final velocity v of an object which starts with velocity u and then accelerates at constant acceleration a for a period of time   is:

Verbiest, FR. Ferdinand S.J.

(1623 - 1688) A Jesuit scientist in China. He invented the first ever car, the Automobile. In addition to his astronomical studies, he experimented with steam power and built a small three-wheeler with a steam boiler that drove through a nozzle, a small

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turbine and a reduction gear to his own rear.

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He also wrote no less than 30 books. Among these may be found a 32 volume handbook on astronomy, a Manchu grammar, a Chinese missal and a entitled Kiao-li-siang-kiai which is the statement of the fundamental teachings of Christianity and which became the basis for later Chinese Christian literature.

His scientific achievements occasioned several princes and many mandarins and scholars to become Catholics, so that by the time Verbiest died there were about 800,000 Catholics living in 1,200 communities. Verbiest is listed as one of 108 Chinese heroes of the popular novel Shui Hu Chuan, and his portrait is shown with Chinese features in a famous Japanese print. After he died he was buried next to the other two giants of the Jesuit mission: Matteo Ricciand Adam Schall. Their tomb is difficult to visit since it is on the campus of a College of Political Science, but it is immaculately preserved.

Verbiest died a century after the Jesuit mission had begun, and a century before the tragic decision of Pope Clement XI regarding the Chinese rites, which practically ended this vibrant, promising mission. The work in China had to be started over again in a later century and with much less success. It is not difficult to imagine the consequences for the Church and its missionary endeavors if such a decision were made in the fifth century when St Patrick was trying to cope with the rites of the Gaels. 

Virtual image

An image from which rays of reflected or refracted light appear to diverge, as from an image seen in a plane mirror.

A virtual image is formed at the position where the paths of the principal rays cross when projected backward from their paths beyond the lenses. Although a virtual image does not form a visible projection on a screen, it is no sense “imaginary”, i.e.,it has a definite position and

133size and can be “seen” or imaged by the eye, camera, or other optical instrument.

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Voltmeter

A voltmeter is an apparatus that is used for evaluating voltage. A voltmeter can be used in determining if there is extra electricity left in a battery. The initial simple voltmeter was invented by Hans Oersted in 1819.

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WWater

Water is a chemical compound with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms that are connected by covalent bonds. Water is a liquid at standard ambient temperature and pressure, but it often co-exists on Earth with its solid state, ice, and gaseous state, steam (water vapor).

Water covers 71% of the Earth's surface, and is vital for all known forms of life. On Earth, 96.5% of the planet's water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products.[3]

Water on Earth moves continually through the water cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. Evaporation and transpiration contribute to the precipitation over land.

Safe drinking water is essential to humans and other lifeforms even though it provides no calories or organic nutrients. Access to safe drinking water has improved over the last decades in

135almost every part of the world, but approximately one billion people still lack access to safe water and over 2.5 billion lack

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access to adequate sanitation. There is a clear correlation between access to safe water and GDP per capita. However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability. A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of the fresh water used by humans goes to agriculture.

Waves

Waves, rhythmic motions by which energy, including light and sound, is transmitted through matter or space. Only electromagnetic waves, such as radio, light, and infrared waves, can pass through a vacuum. Other kinds of waves can travel only through matter, such as air or water.

All vibrations produce waves. Vibrating vocal cords, for example, produce sound waves; and the vibrations, or oscillations, of an electrical circuit produce radio waves. Waves themselves may be regarded as vibrations. Water waves, for example, are vibrations

set up in water by the wind, by earthquakes, and by objects that drop into or pass through water. Waves may be reflected (bent backward) by various substances, and they may be refracted (bent at an angle) when they pass from one substance to another. Diffraction is the bending of a wave about an obstacle. Interference results when two waves meet.

How Waves Move

Waves transmit energy, but not matter. When waves pass through a

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substance, such as air, the molecules that make up the substance move to and fro but do not progress forward with the wave. The molecules nearest the source of the wave move first. Their movements set adjoining molecules into motion, and in this way the wave travels forward.

In a longitudinal wave, the molecules move to and fro parallel to the direction in which the wave advances. Sound waves are longitudinal waves made up of a series of compressions and rarefactions. In a transverse wave, the molecules move back and forth at right angles to the direction in which the wave advances. A wave on the surface of a body of water is a transverse wave. Electromagnetic waves are also transverse waves. In electromagnetic waves, electric and magnetic fields—rather than molecules—move back and forth.

How Waves Are Described

Three important characteristics of waves are frequency, wavelength, and amplitude. The frequency is the number of waves that pass a given point per unit of time. It is usually expressed in hertz, the number of cycles, or complete vibrations, per second. For example, the frequency of sound waves produced by the lowest note on a piano is 27.5 hertz (27.5 cycles per second), meaning that 27.5 complete waves are formed during each second that the note is sounded.

Wavelength is the distance between two corresponding parts of adjacent waves. In a transverse wave, the wavelength is commonly measured between the crests or troughs of two adjacent waves; in a longitudinal wave, between points of maximum compression or rarefaction. The wavelength multiplied by the frequency gives the speed of a wave, the distance the wave travels per unit of time.

The amplitude of a wave is the greatest amount by which a changing property of a wave varies from its normal value. In a water wave, for example, the amplitude is half the distance between its trough and crest. The more energy a wave has, the greater is its amplitude. The amplitude of a sound wave, for example, is large for a loud sound and small for a soft one.

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Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape

repeats. It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is commonly designated by the Greek letter lambda (λ). The concept can

also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids. The SI unit of wavelength is the meter.

Assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to frequency of the wave: waves with higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths.

Examples of wave-like phenomena are sound waves, light, and water waves. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric and the magnetic field vary. Water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary.

Wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in sinusoidal waves over deep water a particle near the water's surface moves in a circle of the same diameter as the wave height, unrelated to wavelength.

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Sinusoidal waves

In linear media, any wave pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by[8]

,

where v is called the phase speed (magnitude of the phase velocity) of the wave and f is the wave's frequency. In a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear.

For sound waves in air, the speed of sound is 343 m/s (at room temperature and atmospheric pressure). The wavelengths of sound frequencies audible to the human ear (20 Hz–20 kHz) are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light.

Sinusoidal standing waves in a box that constrains the end points to be nodes will have an integer number of half wavelengths fitting in the box.

A standing wave (black) depicted as the sum of two propagating waves traveling in opposite directions (red and blue)

Standing waves

A standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, and the wavelength is twice the distance between nodes.

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The upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box (an example of boundary conditions) determining which wavelengths are allowed. For example, for an electromagnetic wave, if the box has ideal metal walls, the condition for nodes at the walls results because the metal walls cannot support a tangential electric field, forcing the wave to have zero amplitude at the wall.

The stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities.[9] Consequently, wavelength, period, and wave velocity are related just as for a traveling wave. For example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum.

Weather and climate

Weather is the condition of the atmosphere at a particular place over a short period of time. For example, on a particular day in Pakistan, the weather is warm in the afternoon. But later in the day, when there are clouds blocking Sun's rays, the weather would become cooler.

Climate refers to the weather pattern of a place over a long period, maybe 25 to 30 years, long enough to yield meaningful averages ([1][2]). For example, although the weather in Pakistan may be cool and dry today, Pakistan's climate is hot today.

Meteorology studies weather, while Climatology studies climate.

Temperature is how hot or cold the atmosphere is, how many degrees it is above or below freezing (0°C). Temperature is a very important factor in determining the weather, because it influences or controls other elements of the weather, such as precipitation, humidity, clouds and atmospheric pressure.

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Humidity is the amount of water vapor in the atmosphere. Precipitation: is the product of any condensation process. It includes snow, hail, sleet, drizzle, fog, mist and rain. Atmospheric pressure (or air pressure) is the weight of air resting on the earth's surface. Pressure is shown on a weather map, often called a synoptic map, with lines called isobars. Wind is the movement of air masses.

Wind

Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in our solar system occur on Neptune and Saturn.

In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle

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can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds.

Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind affects the spread of wildfires. Winds disperse seeds from various plants, enabling the survival and dispersal of those plant species, as well as flying insect populations. When combined with cold temperatures, wind has a negative impact on livestock. Wind affects animals' food stores, as well as their hunting and defensive strategies.

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XXenon

Xenon is a chemical element in the periodic table that has the symbol Xe and atomic number 54.

A colorless, very heavy, odorless noble gas, xenon occurs in the earth's atmosphere in trace amounts and was part of the first noble gas compound synthesized.Xenon is a member of the zero-valence elements that are called noble or inert gases, however, "inert" is not a completely accurate description of this chemical series since some noble gas compounds have been synthesized.In a gas filled tube, xenon

emits a blue glow when the gas is excited by electrical discharge.This gas is most widely and most famously used in light-emitting devices called Xenon flash lamps, which are used in photographic flashes, stroboscopic lamps, to excite the active medium in lasers which then generate coherent light, to produce laser power for inertial confinement fusion, in bactericidal lamps (rarely), and in certain dermatological uses.

Xray fish

X-Ray Tetra Classification and EvolutionThe X-Ray Tetra is a small species of schooling Fish that is

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naturally found in the Amazon River's coastal waters in South America. The X-RayTetra is also known as the Golden

Pristella Tetra and the Water Goldfinch due to the faint golden colouration of their translucent skin. They were first described by Ulrey in 1894 and have since become one of the most popular freshwater Fish kept in artificial aquariums today. Although the X-Ray Tetra is the only known species in it's genus, it is closely related to other small and colourful South American Fish, including the nearly 100 other Tetra species.

X-Ray Tetra Anatomy and AppearanceThe most distinctive feature of the X-Ray Tetra is the translucent layer of skin that covers it's small body, allowing theFish's backbone to be clearly seen. The scales of the X-RayTetra are a silvery-yellowish colour that is very faint,

looking almost golden in some lights. The X-Ray Tetra also has a re-tipped tail and strikingly striped dorsal and anal fins that are yellow, black and white in colour. This is a relatively small species of Fish that actually has a bony internal structure known as the Weberian apparatus, which is used in picking up sound waves, and contributes to their acute sense of hearing (this bony

structure is also found in many of their relatives). Females are generally slightly larger and rounder than the more slender males, although the two are very similar in appearance.

X-Ray Tetra Distribution and HabitatThe X-Ray Tetra is found in the Amazonian coastal waters of Brazil, Guiana, Guyana, and Venezuela in South America. They differ greatly from other Tetra species as they are able to tolerate the harder brackish water closer to the coast, as well as their usual freshwater environments. They inhabit clear-water streams and tributaries during the dry season, and with the coming of the rains, the X-Ray Tetra then move into the flooded marshlands where the water is softer and more acidic. It is

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during the wet season that the X-Ray Tetra breed as they have better water conditions and a higher abundance of food.

X-Ray Tetra Behaviour and LifestyleLike many other small, colourful Fish, the X-Ray Tetra is a schooling species of Fish inhabiting the region between the bottom and middle of the water as a group. They are incredibly peaceful and are often tolerant of the other species that they share their habitats with. The X-Ray Tetra is one of the most adaptable species of Tetra as it is able to inhabit both fresh and

brackish water happily, in both acidic and alkaline conditions. It is widely observed that those X-Ray Tetra that are kept in tanks, can change quickly from being peaceful to becoming skittish in the presence of larger, predatory Fish, with the samebehaviour known to be displayed if the school size is not big enough.

X-Ray Scanners

Travelers often worry about radiation exposure from airport scanners, but a new report says that the risk is actually low: passengers absorb less radiation from airport X-ray backscatter scanners than they do while standing in line waiting for the scan itself.

A person would have to receive more than 22,500 full-body scans a year to reach the standard maximum safe yearly dose, according to “Radiation Dose from Airport Scanners,” conducted by a task force and commissioned by the American Association of Physicists in Medicine, a nonprofit organization.

“We think the most important single take-away point for concerned passengers is to keep an appropriate perspective: the effective radiation dose received by a passenger during screening is comparable to what that same passenger will receive

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in 12 seconds during the flight itself or from two minutes of natural radiation exposure” on the ground, Christopher Cagnon, chief of radiology physics at U.C.L.A. Medical Center and a lead author of the study, said in a statement, noting that exposure levels are greater in the air because at cruising altitude there is less atmosphere to shield passengers and crew from cosmic radiation.

The scanners used in the study have since been largely pulled by the Transportation Security Administration because of privacy concerns and replaced with ones that emit radio waves, the group said.

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YYangchuanosaurus

Yangchuanosaurus is often likened to fulfilling the same ecological niche as Allosaurus, except in Asia instead of North America. Not only does Yangchuanosaurus have a similar morphology to Allosaurus it also had access to similar prey items such as stegosaurs and sauropods. Yangchuanosaurus had a characteristic growth on top of its nose as well as smaller horns and ridges, and also possessed a

tail that made up half of its total length.        The first specimen of Yangchuanosaurus was discovered by a construction worker working upon the construction of a dam in Sichuan Province. This revealed an eight meter individual. In 1983 however the second specimen and species of Yangchuanosaurus was discovered from the same formation, and this time the remains revealed an individual estimated to be just under eleven meters long. This gives Yangchuanosaurus another similarity to Allosaurus as it has an average length of eight meters with some individuals suggesting up to eleven meters as well.       There was once a third species of Yangchuanosaurus, however this has since been found to represent another but closely related Asian theropod called Sinraptor.

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Yggdrasil

In Norse mythology, Yggdrasil ("The Terrible One's Horse"), also called the World Tree, is the giant ash tree that links and

shelters all the worlds. Beneath the three roots the realms of Asgard, Jotunheim, and Niflheim are located. Three wells lie at its base: the Well of Wisdom (Mímisbrunnr), guarded by Mimir; the Well of Fate (Urdarbrunnr), guarded by the Norns; and the Hvergelmir (Roaring Kettle), the source of many rivers.

Four deer run across the branches of the tree and eat the buds; they represent the four winds. There are other inhabitants of the tree,

such as the squirrel Ratatosk ("swift teeth"), a notorious gossip, and Vidofnir ("tree snake"), the golden cock that perches on the topmost bough. The roots are gnawed upon by Nidhogg and other serpents. On the day of Ragnarok, the fire giant Surt will set the tree on fire.

Other names for the tree include: Ask Yggdrasil, Hoddmimir's Wood, Laerad and Odin's Horse.

Yttrium

Yttrium and lanthanum hydride films with switchable optical properties in many substances, changes in chemical composition, pressure or temperature can induce metal-to-insulator transitions1. Although dramatic changes in optical and electrical properties accompany such transitions, their interpretation is often complicated by attendant changes in crystallographic

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structure2. Yttrium, lanthanum and the trivalent rare-earth elements form hydrides that also exhibit metal–insulator transitions3–5, but the extreme reactivity and fragility of these materials hinder experimental studies. To overcome these difficulties, we have coated thin films of yttrium and lanthanum with a layer of palladium through which hydrogen can diffuse. Real-time transitions from metallic (YH2 or LaH2) to semiconducting (YH3 or LaH3) behavior occur in these films during continuous absorption of hydrogen, accompanied by pronounced changes in their optical properties. Although the timescale on which this transition occurs is at present rather slow (a few seconds), there appears to be considerable scope for improvement through the choice of rare-earth element and by adopting electrochemical means for driving the transition. In view of the spectacular changes in optical properties—yttrium hydride, for example, changes from a shiny mirror to a yellow, transparent window—metal hydrides might find important technological applications.

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ZZero gravity

Question: Can a Candle Burn in Zero Gravity?

Answer: Yes, can candle can burn in zero gravity. However, the flame is quite a bit different. Fire behaves differently in space and microgravity than on Earth.

A microgravity flame forms a sphere surrounding the wick. Diffusion feeds the flame with oxygen and allows carbon dioxide to move away from the point of combustion, so the rate of burning is slowed. The flame of a candle burned in

microgravity is an almost invisible blue color (video cameras on Mir could not detect the

This photograph shows a candle flame burning in microgravity. The flame expands outward and produces very little soot. NASA Marshall Space Flight Center (NASA-MSFC) blue color). Experiments on Skylab and Mir indicate the temperature of the flame is too low for the yellow color seen on Earth.

Smoke and soot production is different for candles and other forms of fire in space or zero gravity compared to candles on earth. Unless air flow is available, the slower gas exchange from diffusion can produce a soot-free flame. However, when burning stops at the tip of the flame, soot production begins. Soot and smoke production depends on the fuel flow rate.

It isn't true that candles burn for a shorter length of time in space. Dr. Shannon Lucid (Mir), found that candles that burn for

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10 minutes or less on Earth produced a flame for up to 45 minutes. When the flame is extinguished, a white ball surrounding the candle tip remains, which may be a fog of flammable wax vapor.

Zinc

Atomic Number: 30

Symbol: Zn

Atomic Weight: 65.39

Discovery: known since prehistoric time

Electron Configuration: [Ar] 4s2 3d10

Word Origin: German zinke: of obscure origin, probably German for tine. Zinc metal crystals are sharp and pointed. It could also be attributed to the German word 'zin' meaning tin.

Isotopes: There are 30 known isotopes of zinc ranging from Zn-54 to Zn-83 . Zinc has five stable isotopes: Zn-64 (48.63%), Zn-66 (27.90%), Zn-67 (4.10%), Zn-68 (18.75%) and Zn-70 (0.6%).

Properties: Zinc has a melting point of 419.58°C, boiling point of 907°C, specific gravity of 7.133 (25°C), with a valence of 2. Zinc is a lustous blue-white metal. It is brittle at low temperatures, but becomes malleable at 100-150°C. It is a fair electrical conductor. Zinc burns in air at high red heat, evolving white clouds of zinc oxide.

Uses: Zinc is used to form numerous alloys, including brass, bronze, nickel silver, soft solder, Geman silver, spring brass, and aluminum solder. Zinc is used to make die castings for use in the electrical, automotive, and hardware industries. The alloy Prestal, consisting of 78% zinc and 22% aluminum, is nearly as strong as steel yet exhibits superplasticity. Zinc is used to galvanize other metals to prevent corrosion. Zinc oxide is used in paints, rubbers, cosmetics, plastics, inks, soap, batteries, pharmaceuticals, and many other products. Other zinc compounds are also widely used, such as zinc sulfide (luminous

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dials and fluorescent lights) and ZrZn2 (ferromagnetic materials). Zinc is an essential element for humans and other animal nutrition. Zinc-deficient animals require 50% more food to gain the same weight as animals with sufficient zinc. Zinc metal is not considered toxic, but if fresh zinc oxide is inhaled it can cause a disorder referred to as zinc chills or oxide shakes.

Sources: The primary ores of zinc are sphalerite or blende (zinc sulfide), smithsonite (zinc carbonate), calamine (zinc silicate), and franklinite (zinc, iron, and manganese oxides). An old method of producing zinc was by reducing calamine with charcoal. More recently, it has been obtained by roasting the ores to form zinc oxide and then reducing the oxide with carbon or coal, followed by distillation of the metal.

Zinc Trivia: Zinc is the 24th most abundant element in the Earth's crust. Zinc is the fourth most common metal used today (after iron, aluminum and copper). Zinc exposed to air will form a layer of zinc carbonate by reacting with carbon dioxide. This layer protects the metal from further reactions with air or water. Zinc burns white-green in a flame test. Zinc is the last period four transition metal. Zinc oxide (ZnO) was once called "philosopher's wool" by alchemists because it looked like wool when collected on a condenser after burning zinc metal. Half of the zinc produced today is used to galvanize steel to prevent corrosion. The U.S. penny is 97.6% zinc. The other 2.4% is copper.

ZIRCONIUM

ZirconiumAtomic Number: 40Symbol: ZrAtomic Weight: 91.224Discovery: Martin Klaproth 1789 (Germany); zircon mineral is mentioned in biblical texts.Electron Configuration:[Kr] 4d2 5s2

Word Origin: Named for the mineral zircon. Persianzargun: gold-like, which describes the color of the gemstone known as zircon, jargon, hyacinth, jacinth, or ligure.

152Isotopes: Natural zirconium consists of 5 isotopes; 15 additional isotopes have been characterized.

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Properties: Zirconium is a lustrous grayish-white metal. Finely-divided metal may ignite spontaneously in air, especially at elevated temperatures, but the solid metal is relatively stable. Hafnium is found in zirconium ores and is difficult to separate from zirconium. Commercial-grade zirconium contains from 1% to 3% hafnium. Reactor-grade zirconium is essentially free of hafnium.Uses: Zircaloy(R) is an important alloy for nuclear applications. Zirconium has a low absorption cross section for neutrons, and is therefore used for nuclear energy applications, such as for cladding fuel elements. Zirconium is exceptionally resistant to corrosion by seawater and many common acids and alkalis, so it is used extensively by the chemical industry where corrosive agents are employed. Zirconium is used as an alloying agent in steel, a getter in vacuum tubes, and as a component in surgical appliances, photoflash bulbs, explosive primers, rayon spinnerets, lamp filaments, etc. Zirconium carbonate is used in poison ivy lotions to combine with urushiol. Zirconium alloyed with zinc becomes magnetic at temperatures below 35°K. Zirconium with niobium is used to make low temperature superconductive magnets. Zirconium oxide (zircon) has a high index of refraction and is used as a gemstone. The impure oxide, zirconia, is used for laboratory crucibles that will withstand heat shock, for furnace linings, and by the glass and ceramic industries as a refractory material.

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INDEX TThermochemistry, pp.117-118

Thermometer, p.118

Trojan horse, pp.118-119

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Trojan war, pp.119-120

Trepanation, pp.120-121

UUltraviolet

Radiation, pp.122-123

Unsaturated fat, pp.123-124

Uranus, pp.124-125

Urbain, Georges, pp.125-127

Uric acid, pp.127-128

VVacuole, pp.129-130

Velocity, pp.130-132

Verbiest, Fr. Ferdinand S.J, pp.132-133

Virtual image, p.133

Voltmeter, p.134

WWater, pp.135-136

Waves, pp.136-137

Wavelength, pp.138-140

Weather and climate, pp.140-141

Wind, pp.141-142

X

X-ray scanners, pp.145-146

X-ray fish, pp.143-145

Xenon, p.143

YYangchuanosaurus, p.147

Yggdrasil, p.148

Yttrium, pp.148-149

ZZero gravity, pp.150-151

Zinc, pp.151-152

Zirconium, pp.152-153