474

ELEMENTS OF MATERIA MEDICA.

iodide of potassium be employed, the red precipitate disappears, owingto the formation of a soluble double salt, in which the biniodide of mer-cury acts the part of an acid, and the iodide of potassium that of a base.This double salt may be called the hydrargyro-iodide of potassium.Bichloride of mercury and the biniodide of mercury also form a doublesalt: hence, if a great excess of the former salt be employed, the redprecipitate disappears, owing to the formation of a soluble double salt.

6. Protochloride of tin occasions, with bichloride of mercury, a whiteprecipitate of calomel, while perchloride of tin remains in solution. Veryshortly this protochloride of mercury is converted into reguline mercury,which falls down in a finely divided state as a greyish powder.

7. Hydrosulphuric acid in excess, passed through a solution of bichlo-ride of mercury, occasions a black precipitate of the bisulphuret of mer-cury : a solution of hydrochloric acid is formed. If the hydrosulphuricacid be not in excess, a white precipitate of the chloro-sulphuret ofmercury is obtained, and w'hicli consists of two atoms bisulphuret ofmercury and one of the bichloride.

8. Ferrocyanide of potassium causes a white precipitate (ferrocyanideof mercury).

9. Albumen is another test for corrosive sublimate, though not one ofmuch value, since it will produce the same white precipitates with manyother substances.

10. Galvanism. —Drop the suspected solution on a piece of gold, as asovereign, and apply a key, so that it may touch, simultaneously, the gold

and the solution; an electric current is immediatelyproduced, the bichloride is decomposed, the mer-cury attaches itself to the negative electrode (orpole), namely the gold, while the chlorine uniteswith the iron of the positive electrode (or pole) toform chloride of iron. The relative position of thegold, the key, and the solution, will be evident fromthe fig. 67 ; and the arrows point out the directioncurrent. The silver stain left on the gold is readilyIn Dr. Christison’s work will be found other methodsof applying galvanism; but the one just mentioned is perhaps the mostuseful, since it can always be readily made use of; whereas a more com-plicated apparatus is found in the hands of a few persons only. Thus itmight be applied at a moment’s notice to detect corrosive sublimate inthe matters vomited by a patient.

The relative delicacy of some of these tests is thus stated by Devergie{Med. Leg. ii. 676):—

Fig. 67.

of the electricremoved by heat.

Degi'ee of Dilution.

Ferrocyanide of potassium.stops at . . 1,500

Lime water. „ . . 4,000

Potash or its carbonate. ,, .. 7,000

Iodide of potassium. ,, . . 8,000

Ammonia. ,, . . 36,000

Hydrosulphuric acid, or hydrosulphate of ammonia . . ,, . . 60,000

Protochloride of tin, or galvanic pile. ,, . . 80,000

The preceding tests have not determined the nature of corrosive sub-limate, further than that it is a permercurial salt. To prove that it is achloride, the simplest method of proceeding is to add to the suspected