56 Mr. Ferguson on the Preparation of Soap Cerate.

the ducts of a gland, without having any direct communi- cation with its arteries, nevertheless contain matters solely derived from these arteries. In this view of the subject, the umbilical vein of the foetus is a duct, bearing the nu- tritive and necessary portions of the mother's blood carried to the placenta by the maternal arteries; and on the other hand, the maternal placental veins receive from the arteries of the foetus, whatever it may be necessary to evacuate or eliminate from its blood. It is obvious, that several well-known, but hitherto unexplained physiological phenomena, as well as the placental nutrition of the foetUS, are explicable on this hypothesis; I advance it, however, with diffidence, and have now submitted it to considera. tion, chiefly with a view of exciting further discussion and investigation, concerning this obscure and difficult subject.

ART. VlII.--Observations on the Preparation of Soap Cerate. By WILLIAm[ FEaGUSON, Esq.

As , considerable difficulties have been encountered in the preparation of soap cerate; according to the formula of the London Pharmacopoei% a few observations on their cause, and the best method of making the preparation, may not be considered useless. The Pharmacopoeia directs a pound of semivitreous oxide of lead to be boiled in a gallon of vine- gar, tili it incorporates with it, and eight ounces of soap h'aving been dissolved in the solution, the water is to be evapol'ated slowly, and the residuum formed into acerate, with oil and wax.

The properties of soap eerate depending entirely on the salt of lead which it contains, it is impracticable to make

useful preparation according to this formula. I have always found, that upon adding the soap to the solution of

Mr. Ferguson on the Preparation of Soat~ Cerate. ,57

subaeetate of lead, an immediate deeo~fiposition of the materials takes place ; the potash of the soap, decomposing the metallic salt, combines with its acid, and forms acetate of potash, while the oil unites :with the oxide of lead, forming common lytharge plaster, which rolls about in a heavy mass in the solution of acetate of potash. Should the evaporation now be continued, the residuum, instead of consisting of subacetate of lead, combined mechani- cally with soap, will contain only lytharge plaster, acetate of potash, and a little protoxide of!cad.

By a slight change in the process, a cerate may be made which will contain the metallic salt unde2omposed, and in consequence be possessed of the properties expected to be found in the preparation. Instead of adding the soap to the un-evaporated solution of subacetate of lead, it should be boiled down slowly, till it is the consistence of treacle, and in that state be added to the other materials, melted together, and suffered to cool till they begin to grow whi te ; the whole must now be stirred constantly while stiffening, like Turner's cerate, or ointment of the nitrate of mercury.

The quantity of vinegar necessary varies with the pro. portidn of acetic acid it contains, 50 parts of acetic acid requiring 108 of lytharge to saturate them, about 7.28 ounces of pure acid will be sufficient to decompose I pound of lytharge ; of purified pyroligneous acid (specific g~avity, 1020,) 24.34 ounces contain the requisite quantity, but if common fi-uit vinegar (specific gravity 1014) is used, a little more than nine pints will be required; vinegar of this strength containing but one-twentieth of its weight of acetic acid. An inconvenience, however, attends the use of pyroligneous acid, as from the quantity of carbon precipitated during the evaporation, the ceratc will be of a dark colour.

~'ol~. I. _No. 1, H