CHEMISTRY. 
thefe folutions. Fluoric acid decompofes it alfo, as does 
boracic acid ; but this lalt will not decompofe the calca¬ 
reous fait without heat. 
Carbonat of lime promotes the vitrification of fome 
earthy and ftony fubftances. Mixed with filiceous earth, 
it caufes fufion, when the latter is in the proportion of 
one-third, or one-fourth. This fait, when naturally 
mixed with argillaceous earth, forms the fubftance cal¬ 
led marl. This fubftance prefents a great number of va¬ 
rieties, with refpedl to colour, denlity, &c. and melts 
into a greenilh yellow glafs, when urged by a ftrong 
heat. It is ufed with great fuccefs as a manure to ferti¬ 
lize lands. 
Barytes decompofes carbonat of lime, but not without 
heat. Muriat of ammoniac is decompofed by it; the re- 
fults are, muriat of lime, and carbonat of ammoniac, 
formerly called fal volatile of England, ox concrete volatile 
Jalt. This operation is made, by diltilling in a ftone- 
\vare retort, a mixture of one pound of fal ammoniac, 
and two pounds of chalk or cretaceous fpar, in powder. 
Thefe two fubftances mull be very dry. A receiver or 
cucurbit of glafs is adapted to the retort, and the fire is 
gradually railed to a low red heat, the receiver being 
kept cool with wet cloths, or by a fmall ftream of cold 
water, which runs on it during the whole operation. 
White vapours pafs over, which condenle in very pure 
and white cryftals on the fides of the receiver. This is 
the carbonat of ammoniac. This experiment offers a 
frefb proof that the variation of temperature changes the 
affinities: for, carbonat of ammoniac decompoles mu¬ 
riat of lime when cold, which fhews an anomaly; the 
contrary happens when heat is applied; the heat tends 
to detach the carbonic acid from the lime, and the am¬ 
moniac from the muriatic acid : hence it is not wonder¬ 
ful that the equilibrium of the divellentanc| quiefeentaf- 
finities fhould be totally deftroyed by the difference be¬ 
tween cold and heat. 
Carbonat of Potash. —To combine potalh, and 
indeed any of the alkalis, with carbonic acid, we have 
above delcribed an apparatus which Ihould always be 
contrived fo that the furfaces of the fluids may be re¬ 
newed; and this will be better performed by means of 
the apparatus with double fyphons, which we have de- 
feribed and figured, for obtaining carbonic acid gas ; in 
which apparatus thefyphons of themfelves agitate andlhift 
the liquors, and thus continually renew their furfaces. 
The carbonic acid, though weak, neutralifes alkaline 
fubftances, and forms falts ; yet the alkalis preferve with 
it the property of reltoring vegetable tints, giving them 
a green colour, and they continue fomething of an alka¬ 
line tafte. Thus carbonic acid has npt fo much of a latu- 
rating property, with relpecil to alkalis and earths, as 
other acids. 
It is not long fince carbonat of potalh has been known 
in a ftate of purity; it was formerly fuppofed to be deli- 
quefeent; it was called tartarian alkali, becaufe it was 
obtained by burning tartar of wine to alhes. When 
moiftened by the air, it was called oil of tartar per deli- 
quinm : this property arofe only from the fixed fait of the 
tartar not being laturated with the carbonic acid. Boh- 
nius obtained this fait in the form of regular cryftals; 
but he could not explain the procefs. Black and Berg¬ 
man have lince thrown great light upon the fubjedl. 
As we now prepare this fait from its principles, by a 
diredt combination of carbonic acid gas with potalh, we 
havefeen the'polfibility of producing a fait, with proper¬ 
ties entirely different from that of which we have been 
fpeaking: it has been called neutral carbonat of potalh, 
to diftinguilh it from the other non-faturated alkalis, 
which are met with in commerce. This neutral carbo¬ 
nat of potalh is no longer cauftic ; it is faitilh only, with 
a Highly urinous talte. It is greatly altered by fire, lofing 
therein 0'5’. of its weight, and what remains is cauftic 
potalh. Diftilled in a retort, it lofes its water of cryftal- 
lization, and its acid affumes the aeriform Hate; the pot- 
259 
alh remains at the bottom of the retort, retaining always 
a little of the carbonic acid, which it is very difficult to 
feparate. It undergoes no alteration by being expofed 
to the air. 
When common potalh is put in water for diflolution, 
it atfirft abforbs a certain quantity of it, which it folidi- 
fies; then the potalh is diflolved in the excels of water 
which is added. In general, when a fait abforbs and fo- 
lidifies water, there lliould naturally be difengaged fome 
caloric, which conftitutes the liquid form of w 7 ater. The 
contrary happens when cold is produced. The abforp- 
tion of the water by the potalh, is in truth a combina¬ 
tion in confequence of affinity or attraction ; which is 
very different from Ample extenfion by folution in a li¬ 
quid : hence a fenfible heat is difengaged. This effedb 
only takes place with the potalh met wfith in commercb. 
If, on the other hand, a fait well cryftallized be put in¬ 
to water, there is furely a production of cold; for, in- 
that cafe, there is no water to be folidified : it is a folid 
converted into a liquid. But, if a dried fait be ufed, it 
employs the ftrong affinity it has for the water, it feizes 
it, and difengages its caloric ; then when faturated with 
folid water, it dilfolves, and returns to the clafs of cryf¬ 
tallized falts. Four parts of cold water are required to 
diftblve one of this fait: by evaporation, and cooling, 
cryftals in various fliapes are produced; the moll com¬ 
mon is the quadrangular prifm, or laminae with dihedral 
triangular lummits, fo that the faceanfwers to one of the 
folid angles of the prifm. 
Carbonat of potalh is decompofed by all the acids.- 
Indeed the carbonic acid may be feparated by the im¬ 
pure acid refidues tempered with water, and by the refi-'- 
dues of diftillation, fuch as acid fulphat of potalh, &c. 
diflolved in water: the carbonic acid is always dilengh- 
ged with effervefeence. The boracic acid will not de¬ 
compofe the carbonat of potalh without heat; but, by 
raifing the temperature, the decompofition takes place. 
This fait may be ufed, like potalh, as a flux for vitrifia- 
ble earths, becaufe the caloric decompofes it, by diflipa- 
ting the carbonic acid. If one part of filex and three of 
potalh be made into a palte, and melted either in a cru¬ 
cible, or by means of the blow-pipe, a glafs will be ob¬ 
tained. 
Barytes, ftrontian, and lime, decompofe this fait. If 
a folution of thefe terreftrial fubftances be poured into a 
folution of carbonat of potalh, a precipitation immedi¬ 
ately takes place; the carbonic acid unites with the 
earth, and forms an infoluble fait, while the potalh is 
held in folution by the liquor. By this decompofition 
may be prepared the lapis cauficus, or cauftic potalh. 
But particular care mult be taken to have a quick eva¬ 
poration, and in fdver veffels, to have it pure, and that 
it may not abforb the carbonic acid; then dry it, melt 
it, and pour it on marble. 
The neutral falts are not changed by this carbonat. 
The magnefian falts give no precipitate in the cold by 
admixture wfith carbonat of potalh ; but, by boiling the 
mixture, a precipitation is obtained. The fame eftedl 
takes place by expofing the mixture to the air; carbonat 
of magneliais produced, w-hich.may even be obtained in 
the cryftallized form. Hence an excellent mode prefents 
itfelf of feparating lime, barytes, and alumine, from 
magnefia; for, by pouring carbonat of potalh into a fo¬ 
lution of thefe earthy fubftances, the lime is prefently 
depofited, by feparating the fulphat, nitrat, &c. if com¬ 
bined with thefe acids, in a liquor previoufly analyzed j 
then, by heating the liquor, magnefia is obtained. 
The ules of carbonat of potalh in the arts are many; 
it is employed in medicine as a very adlive folvent; in 
obftrudlions of the mefentery and the urinary paffages, it 
is not adminiftered but in fmall dofes, together with fome 
fubftance which may moderate its adlion. 
Carbonat of Soda.- —This fait was formerly called. 
natrum, natron , mineral alkali, and fall offoda. This car¬ 
bonat, when ufed for the arts, is extracted from the alhes. 
of 
