MER

( 82 ) MER

deficcation. Fourcroy found that ioo parts ofthe muriat fuperfaturated with ammonia gave86 parts of white precipitate, whereas caufticfoda gave only 75 of brick-red oxyd. Thewhite precipitate therefore contains fomethingbelides oxyd of mercury. It was infoluble inwater, and when diftilled per fe in a retort itgave out ammonia, azotic gas, and 86 per cent,of calomel was left. By further analyfis Four-croy found this white precipitate to confift of81 of oxyd of mercury, 16 of muriatic acid, and3 of ammonia. It is therefore a triple com-pound, but rendered infoluble by the greatexcefs of mercury. Sulphuric acid converts itto corrofive muriat'and ammoniaco-fulphat ofmercury. Muriatic acid rediffolves this preci-pitate, and produces the fame fait as is formedby the addition of muriat of ammonia to corro-five muriat, and which is of a very peculiarnature.

When mercury, muriatic acid, and ammoniaare in a ftate of mutual faturation without de-compofition, (as happens when muriat ofammonia is added to corrofive muriat) a moltintimate combination takes place between thethree fubftances, and the mercurial muriat byhelp of the fal ammoniac is rendered full twentytimes more foluble in water than before, forone part of fal ammoniac diffolved in 3 ofwater, will enable the latter to diffolve no lefsthan 5 parts of the mercurial fait when thetemperature is raifed. This triple combinationis not broken by fublimation, for the fait thatrifes in this procefs ftill contains ammonia andis equally refoluble in a fmall proportion ofwater. This triple fait, long known to theolder chemifts was called by them Sal alemhroth.In modern nomenclature it is termed the ammo-niaco-mcrcurial muriat.

If to the folution of this fait (formed by equalweights of muriated ammonia and corrofivemuriat) be added a carbonated fixed alkali, aneffervefcence takes place, and a white precipi-tate is formed, fimilar to that produced byfuperfaturating corrofive muriat with ammonia,which, as already mentioned, ftill confifts of• muriatic acid, ammonia, and oxyd of mercury,but the latter fo much in excefs as to be render-ed infoluble. This is the common •whitepreci-pitate of pharmacy, which mull be carefullydiftinguifhed from the fimple fub-muriat orcalomel. If however too much fixed alkali beadded, the precipitate is yellow. Eight partsof corrofive muriat will yield about yf of whiteprecipitate.

The exact compofition of corrofive muriat

will be mentioned after defcribing the mildmuriat or calomel.

Corrofive muriat is one of the melt activemercurial medicines, and is one of the moltviolent poifons in an over-dofe, of all the metallicfalts, inflaming and diforganizing the wholealimentary canal. But it preferves dead animalmatter very perfectly, which when wafhed witha faturated folution of the fait and expofed tothe air is foon covered with a faline cruft orvarnifh which no infedt can touch.

The next combination of mercury with mu-riatic acid to be mentioned, is the Sub-muriat,mild or f%veet Muriat , or Calomel . It differs fromcorrofive fublimate not in the number andnature of its conftituent parts but only in theirproportion, and is found by numerous experi-ments to contain in equal weights more mer-cury, or, (what is the fame thing) lefs acid,and the mercury lefs oxydated. Thefe twocircumftances will explain the many cafes of themutual converfion of the one fait into the other,and produ&ion of one from the other by variouschemical agents.

The fub-muriat is made in two ways, eitherby precipitation from folution, or by fublima-tion. The latter is the moft antient, and is thusperformed. Take one pound of corrofive fubli-mate and nine ounces of running mercury, rubthem in a mortar with juft water enough to pre-vent the fublimate from flying off in duft till thewhole becomes a grey powder,and no globules ofmercury can any longer be perceived. Put thispowder into one or more glafs fubliming veffels(common phials will anfwer in the fmall way,which fhould be no more than half full) andfet them on a fand bath covered up to thebeginning of the neck with the hot fand, andfublime with a heat gradually increafed. Thefirft fubliming of the calomel is perceived by awhite vapour which iffues from the mouth oftheveffel. The calomel is not pure after thefirft operation, for a fmall portion of undecom-pofed mercury and of corrofive fublimate firft rifeand occupy the top of the veffel. Below this isa white cake, which is the true fub-muriat, andat the bottom of the veffel remains a fmallquantity of reddifh oxyd which cannot befublimed. ■ The whole contents of the fublimingveffel are then to be rubbed together and againfubmitted to the fame procefs, and whenfinifhed the cake of calomel which occupies themiddle of the vefiel is to be carefully feparatedand wafhed repeatedly with hot water to free itfrom any adhering particles of corrofive fub-limate.