CHEMISTRY. 
2H 
water; attraft the moifture'of the air; give a green call to 
blue vegetable colours; and unite Ilrongly with acids. 
See Alkali, in our firft volume, p. 340. The known 
alkalis are three in number , potajb, foda, and ammoniac. 
Potash. —This fait has never been decompofed ; yet 
there are many fafits which tend to (hew, that it is not a 
Ample fubftance. It is of a'white colour, and extremely 
sauftic, i.e. afting with great power upon animal fub- 
fubliinces, diffolving them, decompoling them, and form¬ 
ing with them a kind of foap, feparating carbon, hydro¬ 
gen, and-azot, in the ammoniacal ftate. It is by burn¬ 
ing plants, which all contain a greater or lefs quantity 
of potaih, that this fubftance is prepared, as we lhall ex¬ 
plain when we come to (peak of carbonat of potaih; at 
prefent we have only to Ipeak of it in a ftate of purity. 
The following is the rnoft Iimple procefs for obtaining 
it pure. 
The apparatus conftfts of feveral Ihallow troughs of 
white wood ; put at the bottom fome river fand, which 
muft be well wafhed; over that lay another bed of fand 
finer fti 11, and cover that with a cloth ftrewed with allies. 
Make a hole at the bottom of the trough, and fix a glafs 
tube to draw off the liquor as it filters. Things being thus 
prepared, take equal parts of quicklime and potaih, if the 
lime be very cauftic; if otherwife, put twenty parts of 
lime to fifteen of potaih. Put water into an iron-pot; 
when it is ready to boil, add the lime, which, as it (lakes, 
caufes ebullition; when the lime is flaked, or killed, put 
in the potaih, and make the mafs into the confiftency of 
thick foup, which leave to cool a little, then pour the 
mixture into troughs, and cover it with water immediate¬ 
ly ; to prevent the water from making holes in the mix¬ 
ture as it is poured in, lay a thin board which will rife 
with the water. Veftels muft be placed, of courfe, to re¬ 
ceive the liquor which runs through the tube; and, in 
order that the ley may not ablorb the carbonic acid con¬ 
tained in the atmofphere, thefe veflels Ihould be (lightly 
clofed, fo as to hinder the circulation of the external air. 
It is neceflary to keep water always upon the mixture; 
and ceafe to collect it when it comes taftelefs through 
the tube : but obferve that the fluid wiil become infipid 
all at’once, fo that the liquor obtained will be nearly dfi 
the fame ftrengthat the end as at the beginning. Melt¬ 
ing-pots may be uled for evaporating the waters ; begin 
with the weakeft, or laft runs, to avoid keeping the ftronger 
ones too long in contadl with the air; make a ftrong ebul¬ 
lition. When concentrated to a certain degree, the ful- 
phat of potaih cryftallizes and is precipitated. 
To obtain the'potaih cauftic, dry, in a folid mafs, pour 
the concentrated liquor into a fmaller pot; then finilh 
the evaporation to fuch a degree, that, as it drops on a 
plate of iron or marble, it coagulates. 
To purify this concrete potaih, put it into a bottle, or 
jar, and pour over it very pure alcohol: the potaih only 
will diflblve; the fulphat and muriat of potafh, the por¬ 
tions of earth, and even of carbonic acid, which it holds 
fo Ilrongly, or which it has recovered from the air during 
evaporation, remain at the bottom of the folution. Draw 
off the clear liquor, put it into a retort, and diftil it. To 
have it very pure, the folution muft be evaporated in a 
filver-pan : it cryftallizes in cooling into white thin plates 
fometimes four lines long. Inftead of leaving it to cryl- 
tallize, it may be concreted to drynefs. 
Potaih^converts.the blue vegetable colours to a green. 
It melts in a moderate heat ; in a very ftrong heat, fuch 
as that of a glafs-houfe, it flies off in vapours. In con¬ 
tact with atmolpherical air, it foon liquefies, and attrafts 
from it the,carbonic acid. In a ftate of purity, it makes 
. no effervefcence with acids. It has, a great affinity for 
water; it draws it from almoft all other bodies from which 
It feparates a good deal of caloric. 
Potafh combines very well with fulphur, forming a 
combination which long went by the name of liver of 
fulphur, but is how called fulphure. Thefe combinations 
may be made either in the humid or the dry way. £0 
the former method, boil the alkali and the fulphur toge¬ 
ther in water, but not in meta^ pots, for thele combina¬ 
tions a£l upon moll metals, and diffolve them more or 
lefs. I11 the dry way, melt equal parts of the fulphur 
and the alkali in an earthen crucibles; the refult is a fo¬ 
lid mafs of a reddilh brown colour, refembiing the liver 
of certain animals, whence thefe fulphures had formerly 
the name of livers. 
The fulphure of potaih is fufible; it is foluble in 
water, forming fulphurated hydrogen. This fulpimrated 
hydrogen unites in great quantity with the alkaline bale, 
making together a combination which Berthollet calls 
hydro-J'ulphure. To obtain hydro-fulphure of potalli, 
take potafh prepared with alcohol ; enfure a perfefit fatu- 
ration, by letting the liquor take up an excels of lulphu- 
rated hydrogen ; then drive off this excefs by heat. It 
may be known what proportions of fulphurated hydrogen 
are prefent in a fulphure, by precipitating, with the fo¬ 
lution, a folution of copper. 
Alkaline fulphures, or combinations of fulphur with 
an alkaline bale, can only exift in the dry ftate ; when 
diffolved in water, fulphurated hydrogen is formed; Ber- 
tholiet calls this combination of fulphur and fulphurated 
hydrogen with a bale, by the name hydrogenated, fulphure. 
Thus we have fulphures, hydro-fulphures, and" liy- ' 
drogenated fulphures: thefe cannot be confidered as in 
a ftate of faturation of each of their rel’pective principles, 
till the fuperabundant fulphur has been precipitated by 
fulphurated hydrogen. See Berthollet’s Memcire, in the 
Annales de Chimie, vol. xxv. p. 253. It is the more ne- 
ceflary to point out the prefence of the fulphurated hy¬ 
drogen in the hydrogenated fulphure, as it is by means 
of this that the fulphur remains united to the alkali and 
to the water. 
Sulphure of potalli, like all other fulphures, is de¬ 
compofed by the action of fire, which drives off the ful¬ 
phur, and fets the bale at liberty. They attraft the oxy¬ 
gen of the air. Acids decompofe them alfo, by attack¬ 
ing their bafes and precipitating the fulphur ; in this cafe, 
fome fulphurated hydrogen gas is conlfantly produced, 
whofe rapid difengagement produces an effervefcence 
which is more or lefs brilk, according to the ftate of the 
fulphures and the acids. If oxygenated muriatic acid 
gas be palled through a folution of fulphure of potaih, 
the fulphure is changed into a fulphat. 
Potaih combines with filex in the dry way and abforbs 
it in its fufion, forming a tranfparent body called giafs. 
The glafs has different properties, according to the rela¬ 
tive quantities of fand and fixed alkali it contains. If 
three or four parts of potaih be uled to one of filex, a 
loft glafs is produced, which attracts the humidity of the 
air, becomes opake, and at laft fluid. This glafs is folu¬ 
ble in water, by virtue of the fuperabundant alkali it 
contains ; and the folution is called liquor of flints. To 
prepare this liquid filiceous potaih, take one part of fand 
reduced to powder, and four parts of potalli. Put thele 
into a crucible, which is to be but half filled, and place the 
crucible in a forge-furnace. When the matter begins to 
melt, it fwells conliderably; it continues turbid till the 
alkali has diffolved all the filex ; keep the crucible open 
as long as the effervefcence lafts; then cover the crucible, 
increaie the fire fo as to make a complete fufion; then 
the contents of the crucible are to be poured into a very 
dry iron mortar, or on an iron plate ; the matter con- 
denfes as it cools into the form of glafs : this matter is 
to be pulveriled and difioivedin water ; then it becomes 
the liquor of flints. Acids decompofe it: they feize the 
alkali, and precipitate the earth, called earth of flints. In 
order that this precipitation may be well performed, the 
liquor of flints muft not be too much diluted; for in this 
cafe the particles of earth are in a ftate of fuch extreme 
divifion, that they remain fufpended in the liquid, which 
muft be evaporated -before any fenfible lubiidence can 
take place. Several chemifts think that the earth of flints 
is not fimilar to filiceous earth, and that it has been 
changed 
