466 ELEMENTS OF MATERIA MEDICA.

of a texture somewhat elastic or horny: in this state it acquires, bythe necessary rubbing into powder, a decidedly yellow or huff colour,more or less deep, according to the degree of trituration which it hasundergone. If, on the contrary, the calomel be sublimed into a verycapacious and cold receiver, it falls in a most impalpable and perfectlywhite powder, which only requires one elutriation to fit it for use ; itthen remains perfectly colourless. By a modification of the process, itmay be suffered, as it sublimes, to fall into water, according to Mr.Jewell’s patent.

“ The above circumstances, too, account for the various appearancesunder which calomel occasionally presents itself in commerce : it may beadded, that the buff aspect of this substance indicates the absence ofcorrosive sublimate; though it by no means follows as a consequencethat when snow-white it contains it. When the surface of massive sub-limed calomel is scratched, it always exhibits a buff colour: it alsobecomes yellow when heated, but loses its tint as it again cools.” (Manualof Chemistry, 4th ed. p. 788).

Mr. Jewell’s process ( Repert. of Arts, xiii. 79, 2d Series) for preparingcalomel consists in keeping the receiving vessel filled with steam, so thatthe vaporous calomel is condensed in it, and takes the form of a finepowder, which is much finer than can be obtained by levigation andelutriation. This process has been improved by M. O. Henry (fig. 66).

( h .) By precipitation .—Inthe Dublin Pharmacopoeia,a process is given forprocuring precipitated ca-lomel. It consists in mix-ing seven parts of chlorideof sodium, dissolved in 400parts of boiling water, with17 parts of purified mercury,dissolved in 15 parts of di-luted nitric acid. Doubledecomposition takes place:one equivalent or 60 partsof chloride of sodium de-compose one equivalentor 264 parts of proto-nitrate of mercury, anclproduce one equivalent or238 parts of chloride ofmercury, and one equiva-lent or 86 parts of nitrateof soda.

Properties. —The primary form of the crystals of calomel is the rightsquare prism. The appearance of the crystalline cake of sublimedcalomel has been already noticed. As met with in the shops, it is in theform of a fine odourless or tasteless powder, whose sp. gr. is 7'176 (7’2,Brande). When prepared by Jewell’s process it is perfectly white, butwhen obtained in the ordinary way has a light buff or ivory tint. Itvolatilizes by heat, and, under pressure, fuses. It is insoluble in coldwater and alcohol. According to Donovan {Ann. Phil. xiv. 323)

Henry's modification of Jewell's apparatus for pre-paring calomel by steam (Hydrosublimate ofmercury).

Fio. 66.

(a) Furnace, containing an earthen retort (having a

wide and short neck), in which the ingredi-ents for making calomel are placed.

( b ) An earthen receiver, having three tubulures:

one connecting with the retort; a seconddipping - into water in an earthen jar, and athird connected to a steam-pipe.

(cj Steam-boiler.