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8 C H E M 
a bellows. If thefe be expofed to the light for a few 
minutes, and afterwards carried into an obfcure place, 
they fhine like a burning coal. They even fhine when 
immerfed in water. They lole this property by degrees, 
but recover it on being again heated. Maby other fub¬ 
ftances likewife exhibit this appearance. Magnefia, 
chalk, calcareous 1'ulph'at, and fluat, &c. become lumi¬ 
nous after having been heated. Macquer obferVed the 
lame property in earth of alum, fulphat of potaffi, Bri- 
ancon chalk, black flint calcined ; which proves, that the 
prelence of an acid is not abfolutely neceflary tor the pro- 
dufiticn of this phenomenon, though it feerns in fome 
refpefit to contribute to its intenfity. 
This fait isperfeftly infoluble in water, and is not afit- 
ed on by eatths or falino-terreftrial fubftances. Pure 
fixed alkalis do not decompofe it. This l.aft is one of its 
moft fingtflar properties. In faff, the other earthy and 
lalino-terreltrial fubftances have lefs affinity than fixed 
alkalis to the fulphuric acid ; but barytes on the contra¬ 
ry, has more. Whence we have obi’erved, after Berg¬ 
man, that this earth decompofes the fulphats of potafli 
and foda, as it does likewife fait containing ammoniac. 
Mineral acids have no afition on fulphat of barytes, be- 
caufe the fulphuric acid adheres more ftrongly than any 
other to the earth, which forms the bafe ot this falf. 
Neither do neutral falts produce any. change in it, if we 
except the carbonats of potafh and ioda, which produce 
a decompofition by double affinity. The barytes is fepa- 
rated from the fulphuric acid, becaufe it is attacked by 
the carbonic acid, at the fame time that one of the alka¬ 
lis feizes the former acid. To produce this decompofi¬ 
tion, a mixture of two parts of carbonat of potafh, and 
one part of fulphat of barytes reduced to powder, are 
ftrongly heated in a crucible. The matter which is femi- 
vitrified is to be waflied in diddled water; and the li¬ 
quid, after filtration and evaporation, affords fulphat of 
potafli: the fubftance which remains on the filtre is car¬ 
bonat of barytes; which, when well wafhed, is in the 
form of a very white and fine pulverulent matter, but ufu- 
aily impure, becaufe it always contains a portion of ful¬ 
phat of barytes, which has efcaped the decompofition. 
In the preparation of pure barytic earth, Vauquelin 
prefers the nitric to the muriatic acid. The nitrat of 
barytes is afterwards decompofed by expofure to the fire. 
If the earth obtained in this manner be ufed for the pre¬ 
paration of the muriat of barytes, it is better not to add 
to it direfit ly the muriatic acid,‘but rather to decompofe 
by it the muriat‘of foda. The muriat of barytes' will 
crydadize, and leave the alkali in a date of caudicity. 
The alkali may be cryftallized, by adding to it carbonic 
acid. See "Jcwn dePbyf. 1794- 
Sulphat of barytes is not applied to any confiderable 
ufe. Phofphoric cakes are prepared of this fubltance, 
and-the ponderous earth is extracted for chemical experi¬ 
ments. Dr. Withering, in the Philofophical Tranfafifions 
for 1784., points out a valuable chemical purpofe, to which 
..the barytic muriat may be applied ; namely, the purifica¬ 
tion of the marine acid from the admixture of fulphuric 
acid, by which it is often adulterated. The lolution of 
this fait caufes the fulphuric acid to fad to the bottom, 
together with the earth, in the form of ponderous fpar. 
Sulphat of Potash. —This -fubftance exifts in cer¬ 
tain vegetables, whence it is extrafited by burning them 
to allies. The afhes of fome vegetables contain it in 
great abundance, efnecially thofe which grow at a dif- 
tance from the fea; for thofe hear the lalt water rather 
contain fulphat of foda. Some of the falts met with in 
commerce contain half their weight of fulphat of potaffi. 
This fait has formerly borne different names, as <vitri- 
olated tartar, fe.l de duobus, arcanum duplication, and poly- 
chref} fait. 
To prepare this fait, put into an earthen or done pan, 
four parts of potaffi which didblve in twelve parts of 
hot water s pour on by degrees fulphuric acid acidulated ; 
a. brilk efterve cence is produced if the potafli of com- 
I S T R Y. 
merce be ufed, for with pure potaffi there will be wone. 
Then continue to pour on the acid till the liquor has no 
longer an alkaline nor acid tade, or till it will not change 
the colour of paper dained with turnfcle : this is what 
is called the point of fiituration; drain the liquor, and 
evaporate it to a thin pellicle., It crydallizes in coding 
into a lalt of a folid eighteen-fided ffiape, terminated at 
each extremity by a fix-dded pyramid.' 
Sulphat of potaffi has a difagreeable bitter tade. It is 
not much altered by the afition of heat; when thrown oh 
fed-het coals, it flies into a number of finall fragments, 
attended with noiie ; this phenomenon, called decrepi¬ 
tation, depends on the ludden rarefaction of the water 
contained in its crydals. It loles none of its effential pro¬ 
perties by this decrepitation. It decrepitates in the lame 
manner, and becomes dry, friable, and even pulverulent, 
by the lofs of its water, when expofed to the afition of 
heat in a crucible. It becomes red-hot before it melts, 
and is not fufed but by the application of a confideralile 
degree of heat. The melted mafs, when differed to 
cool, is opaque, and not at all changed in its principles; 
for folution in water reftores its cryftalline and tranfpa- 
rent appearance. If it be kept in ftrong fulion, in an 
open veil'd, it is volatilized without decompofition. The 
reafon why the melted fulphat becomes opaque in cool¬ 
ing, aspnentioned above, is becaufe, when a (alt is dif- 
folved in water, or by fire, its molecules or elementary 
particles being divided and feparated, the light goes 
through it; but, as the mafs grows cold, they concen¬ 
trate, or draw clofe together, lb that the light can 110 
longer penetrate, all is dark. 
Sulphat of poiaffi is not changed by expofure to the 
air. It may be decompofed by charcoal. Expofe equal 
parts of charcoal and fulphat of potaffi to a llrong heat 
in a crucible, and fulphure of potaffi is the produfit; 
the charcoal abforbs the oxygen of the fulphuric acid, 
aud is difengaged in the date of. carbonic acid gas. 
Thus, in the arts, may fulphat of potalh be changed in¬ 
to a fulphure with the help of charcoal; and afterwards, 
decornpofing this fulphure by means of lime, an infolu¬ 
ble fulphure of lime is produced, and the potalh is let 
free. 
This fait is foluble in fixteen parts of water, fome fay 
eighteen, at the temperature of 15 0 of Reaumur; but 
boiling water will diffolve near one fourth of its weight 5 
it crylbillizes in cooling, provided the water be perfefitly 
faturated with it; otherwise it is neceflary to recur to 
evaporation. It is partly decompofed by nitric acid: 
put into a glafs/etort equal parts of fulphat of potafli in 
powder, and nitric afid, at 34. 0 . Adapt a recipient, and 
place the retort in a fand-lieat, and diftil. Or, put the 
mixture into a matrafs, and heat it till the fait be dif- 
folved ; pour the liquor into a glafs veffel ; in cooling it 
furniffies cryftals of nitre ; ftill all the fulphat is not de¬ 
compofed, acid fulphat of potaffi may yet be obtained. 
Muriatic acid works the fame effefit. 
Of all terreftrial fubftances, only barytes decompofes 
this fait; which happens, according to Bergman, be¬ 
caufe it lias a ftronger affinity than potafli to the fulphuric 
acid. If a finall quantity of this earth be added to a fo¬ 
lution of fulphat of potalh, a precipitate is formed, con¬ 
fiding of fulphat of barytes or ponderous fpar, which is 
perfefitly infoluble ; the potafli, in its caultic and pure 
llate, remains difl'olved in the liquid. 
Sulphat of potafh is not ufed, except in medicine; it 
is a good purgative, and is fometimes given alone, in 
the dole of half an ounce or an ounce. It is moft com¬ 
monly adminiftered in a dofe of one or two drachms, to¬ 
gether with other purgative medicines. It is likewife- 
ufed as a folvent in chronical diforders, and efpecially 
in coagulations of the milk; it is then given in dofes of 
a few'drachms, in proper liquids; but its virtue, in this 
refpefit, is interior to that of many other more foluble 
and lefs naufeous falts. 
Acid Sulphat of Pate/h.— Sulphat of potafli has the pro¬ 
perty 
