As it is obviously a desideratum to procure preparations of protoxide of mercury of uniform strength, Mr. Evans has been led to seek a process, by which to obtain this oxide in a pure state. After repeated experiments, he has pitched upon the following formula: Dissolve four ounces of caustic hydrate of potassa in a pound of water, and to the clear solution, decanted from any impurities, add four ounces of calomel, and shake the mixture frequently. Pour off the liquid, and wash the precipitate formed with water, and then dry it at a gentle heat.
In regard to the medical efficacy of the protoxide obtained in this way, Mr. Evans reports the following to be the results obtained by Dr. Coates, at whose suggestion the article was prepared. As a substitute for calomel, it is more apt to vomit and purge, two grain doses operating several times. As an alterative, it was found incomparably more efficacious than the blue pill, being more certain and regular in its operation. Dr. C. thinks, that one-fourth of a grain of the precipitated protoxide, as prepared by Mr. Evans, is equal to three or four grains of the blue mass.—Journ. of the Philad. Col. of Pharm. May, 1826.
The method here proposed for obtaining the black oxide of mercury by Mr. Evans, was first suggested and put in practice by Mr. Phillips. See his "Experimental Examination of the last edition of the Pharmacopœia Londinensis, London, 1811," page 114. His words are, "When solution of potash is employed, the several inconveniences attendant upon the use of lime-water are avoided, and a blackish coloured protoxide is obtained without heating the solution. As potash is much more soluble than lime, it is scarcely necessary to employ one-tenth part of the quantity of water; this not only renders the process more convenient, but the quantity of air contained in the water being less, very little of the oxide, perhaps none of it, is converted into peroxide." See also the experiments, and observations of Mr. Donovan, on Mercurial Ointment, &c. published in the Medical Journals, several years ago.
71. Goulard's Extract of Lead. Mr. Daniel B. Smith proposes the following formula for obtaining Goulard's extract of uniform strength:
| Acetate of lead, crystallized, | 15 ounces, troy. |
| Protoxide of lead, | 9 ounces, troy. |
| Distilled water, | 4 pints. |
"Boil them together for fifteen minutes and filter. The filtered liquid will weigh about five and a quarter pounds, is transparent, colourless, and of the specific gravity of 1.267. (30° Baumé.)"
We conceive that Mr. Smith has erroneously denominated the sugar of lead, a binacetate. The best usage is to deem that the primary saline compound, which contains a single proportional of acid and base. Accordingly we call the saturated carbonate of potassa, a bicarbonate; and Dr. Thomson calls borax, a biborate of soda, on account of its containing two proportionals of acid to one of base, notwithstanding the alkaline qualities of this salt. Goulard's extract is, therefore, a sub-binacetate of lead, or according to Dr. Thomson's recently suggested nomenclature, a diacetate.—Ibid.
FOOTNOTES:
[29] Series of Engravings to the morbid anatomy; fasc. 1. pl. 6. fig. 1, 2.
[30] Lond. Med. Review; vol. 4.