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Some chemifts have thought proper to repeatthe fublimation of calomel for five, fix, ormore times (feparating after each time thewhite powder that riles firft and fixes to thetop of the bottles) with a view of more com-pleatly combining the fublimate with the mer-cury and rendering it a more uniform and fafermedicine. But it is clearly (hewn that no goodwhatever can arife from fucli frequent fublima-tion, but on the contrary, the calomel everytime becomes in fome degree decompofed andits properties injured.

Calomel nine times fublimed forms whathas been called by fome the Mercurial Panacea.

The fub-muriat of mercury is a white folidmafs fometimes crydallized in tetrahedral prifmswhen llowly fublimed. It differs molt effentiallyfrom corrofive fublimate in many fenfible andchemical properties. It is nearly, if not en-tirely infoluble in water, it has but little tafte,but what there is is metallic and naufeous, itbecomes yellowilh by long expofure to lightand is luminous when rubbed in the dark. Itrequires a greater heat to be fublimed than thecorrofive rnuriat does, whence after fublimationof a mixture of the two, the upper portionis chiefly the corrofive rnuriat and the lowerpart is the calomel. When lime-water or thealkalies are added to calomel it immediatelyblackens, which circumftance is a very ufefulteft for this fubftance.

The fub-muriat may alfo be prepared in thernoift way and without fublimation by addingnitrat of mercury to rnuriat of foda. A doubledecompofition takes place here, the muriaticacid uniting with the mercury into the fub-muriat (which being infoluble in water fallsto the bottom) and the nitric acid and foda ofthe fait remain united in the liquor. For thisprocefs we are indebted to the illuftrious Scheele,whence the preparation is fometimes calledScheele’s Calomel . It is thus made. Takea folution of mercury in nitrous acid fo ftrongas to be perfeftly faturated even while boiling.For this about fix ounces of acid, and as muchmercury may be ufqd. Then pour the folutionrather gradually into another veflel containingfour ounces of common fait diflblved in half apint of boiling water.

When the white fub-muriat has entirelyfubfided, pour off the fupernatant liquor andwafh the precipitate thoroughly with warmwater, then dry it in a gentle heat.

'1 he fub-muriat prepared in this way fcarcelyif at all differs from the calomel formed byfublimation, being equally infoluble in water,

blackened by lime-water, and (hewing all theother properties of calomel. This precautionhowever is to be obferved, which is, that asthe nitrous folution when loaded with mercury(as in the prefent cafe) is in part decompofableby mere water, it (hould be dropped graduallyinto the folution of fait with frequent dirring,(and not the fait added to the nitrat as directedby fome) and alfo, as Mr. Chenevix has ob-ferved, it will more entirely fecure the decom-pofition of the nitrat if a little muriatic acid beadded to the folution of common fait before itis ufed. The fub-muriat thus prepared mayalfo if neceflary be afterwards fublimed, and itwill be found to rife in the fubliming veflelalmoft without any refidue.

Mercury is not known to exift in any otherform of combination with muriatic acid than ascorrofive rnuriat and fub-muriat, no interme-diate (late of faturation with the acid havingbeen ever obferved, and as before mentioned,the difference between the two falts is, that thecorrofive rnuriat contains more acid than thefub-muriat, lefs mercury, and the mercury is ina lower date of oxygenation. A number offacts have been obferved by chemifts relating tothe mutual change of one of thefe falts into theother by different reagents fome of which maybe mentioned.

Beaume endeavoured to form fome inter-mediate rnuriat of mercury both by mixing andfubliming proportions of corrofive fublimateand running mercury, different from thofe thatconftitute calomel, and by mixing corrofivernuriat and calomel and fubliming, but in noinftance was there produced any thing butcorrofive rnuriat and calomel.

The proportion of fub-muriat produced inScheele’s method depends on the degree inwhich the nitric acid is faturated with themercury. If an excefs of acid remains, butlittle calomel is produced on adding the commonfait, and the folution contains corrofive rnuriat.

The mercury in calomel is lefs oxygenatedthan in corrofive rnuriat, and many curiousexperiments have been made illuftrating thisfa£t. Thus, as Proud obferves, if muriaticacid be boiled on calomel, corrofive rnuriat isformed, and fome running mercury appears, fothat the calomel is divided into two portions,one of which takes the oxygen belonging to.both and unites with the acid into corrofivernuriat, and the other therefore is'reduced tothe metallic date.

Luffac found that on mixing corrofive fub-limate and red oxyd of mercury, and fubliming,

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