CHLORIDE OF MERCURY.

467

and others (Graelin, Handb. d. Chemie , i. 1299; Geiger’s Handb. d.Pharm. by Liebig, i. 561), calomel suffers partial decomposition by longboiling in water, and a solution is obtained which contains mercury andchlorine (bichloride of mercury?).

By exposure to light, calomel becomes dark coloured, in consequence,according to Dumas ( Traite. de Chimie, iii. 605), of the transformation ofa small portion into mercury and bichloride. Others have ascribed thischange to the evolution of chlorine and combination of the metal withoxygen. Both hypotheses are inconsistent with the statement of Vogel(Landgrebe, TJeber das Licht, 97), that this blackened calomel is insolublein nitric acid. Is it not probable that the change depends on the forma-tion of a subchloride, as Wctzlar has shown to be the case with chlorideof silver ? By digestion in hot and concentrated hydrochloric acid, weobtain bichloride of mercury and reguline mercury. Boiling sullipuricacid forms bipersulpliate and bichloride of mercury, with the evolution ofsulphurous acid.

Characteristics. —Iodide of potassium produces at first a greyish,afterwards a greenish-yellow precipitate (iodide of mercury). Whenheated in nitric acid, calomel is converted into bichloride and bipernitrateof mercury; and on the application of the tests already mentioned ibrmercurial preparations generally, we readily obtain evidence of the pre-sence of mercury. Having thus shown it to be a mercurial compound,we may easily prove it to be calomel by observing that it is insoluble inwater, and that on the addition of lime-water a blackish grey precipi-tate (protoxide of mercury) is obtained, while the supernatant liquor

is found, on the addition of the nitrate of silver, to give evidence of the

presence of chlorine in solution, l’rotochloride of tin, added to calomel,abstracts the chlorine, and becomes bichloride of tin, while globules ofmetallic mercury are obtained.

Composition. —The following is the composition of calomel:—

Turner,

Davy,

Eq. Eq.Wt. Per Cent. Zaboada*

Mercury . 1 . . . . 202 ... 84'87 ... . 85

Chlorine.1 . . . . 3G . . . 15*12 .... 15

Chloride of Mercury . 1 . . . . 238 . . . 90*99 . . . .100

Purity. — When pure, calomel is completely vaporized by heat.Water or alcohol which has been digested on it, should occasion no pre-cipitate or change of colour on the addition of lime-water, caustic potash,ammonia, nitrate of silver, or hydrosulphuric acid, by which the absenceof bichloride of mercury may be inferred. I have met with calomelwhich, in consequence of being imperfectly washed, contained bichloride.It had been given to several patients before its purity was suspected, andhad operated on them most violently. When mixed with potash it be-came black, like pure calomel: the quantity of bichloride being in-sufficient to produce any perceptible alteration in the colour of the preci-pitate. But water which had been digested on it, gave, with the above-mentioned tests, the characteristic indications of bichloride of mercury.

Physiological Effects, (a.) On animals.— Wepfer {Hist. CicutceAquat.), Viborg, Flormann (Wibmer, Wirk. d. Arzn.), Gaspard(Magendie, Journ. de Physiol), and Annesley ( Diseases of India ), have