VOL. 3. No. 1 USE on CHARTS IN TEACHINO GENERAL GIEMISTRY 69

The name and symbol of each element, its group in the periodic classi- fication, and its valences are included in the first line of each chart. Under occurrence, on the next line, only the more important, naturally occnr- ring compounds are placed. Their mineralogical names can he omitted, for the most part, except where they are of particular significance. Water of hydration can be treated in the same way.

The more important compounds of the element are then arranged from left to right in the order of increasing valence of the element; that is, the hydride, if it exists, is placed a t the extreme left, since in i t the element has a negative valence, followed by the free element with a valence of zero, and then the oxides, in which the valence is positive.

Vertical lines are then drawn down from these formulae and are con- sidered to represent their respective formulae. An arrow pointing to a line shows a method by which the compound can he prepared, whereas an arrow pointing away from the line signifies a chemical reaction into which the compound will enter. At the lower end of each line is placed the valence of the element in the compound for which the line stands.

Whenever lines, which have no connection, cross, the convention employed by electrical draughtsmen is used. If it is desired to show, for example, that both sodium and sodium oxide can be converted into sodium chloride upon treatment with hydrochloric acid, this would he indicated as follows:

N a

I HCI HCI

I

This also shows that sodium will not yield sodium oxide when treated with hydrochloric acid.

Compounds which are unstable or which have not been isolated are enclosed in brackets.

On each arrow may he placed a number which refers to an equation for the reaction represented on the chart.

After the student has constructed a series of charts they offer him a rapid method of reviewing the chemistry of any particular element.

Mining Society Medal Presented Inventor of Smoke Precipitator. Dr. Frederick Gardner Cottrell has been awarded the gold medal for 1924 of the Mining and Metal- lurgical Society of America. The award is in recognition of "distinguished service in the development of a method of electrical precipitation of solid and liquid particles from smelter smoke and in recognition of his public spirit in making a gift of the proceeds of his invention for the support of scientific research." Dr. Cottrell is at present director of the Fixed Nitrogen Research Laboratory of the U. S. Department of Agriculture in Washington.-Science Service