FOTASS. 
Ago 
sel and cork it up. The solution gradually 
separates itself into two strata: the lowest 
consists of the impurities, partly dissolved in 
water and partly in a solid state; the upper- 
most consists of the pure potass dissolved in 
alcohol, and is of a reddish-brown colour. 
Decant this alcohol solution into a silver ba- 
son, and evaporate it rapidly till a black 
charry crust forms on the surface, and the 
liquid, below acquires such consistence as to 
become solid on cooling. Then remove the 
black crust, and pour the solution into a por- 
celain vessel. When cold, it concretes into 
a fine white substance, which is pure potass. 
It must be broken to pieces, and put info an 
air-tight phial. 
For this process we are indebted to Ber- 
tho'let. The following, which was proposed 
by Lowitz of Petersburgh, is less expensive. 
The potash of commerce and quicklime are 
to be boiled together as above described. 
The filtered liquor is then to be evaporated 
till a thick pellicle appears on its surface, and 
afterwards allowed to cool ; and all the crys- 
tals which have formed are to be separated, 
for they consist of foreign salts. The evapo- 
ration is then to be continued in an iron pot ; 
and, during the process, the pellicle which 
forms on the surface is to be carefully taken 
off with an iron skimmer. When no more 
pellicle appears, and when the matter ceases 
to boil, it is to be taken off the fire, and must 
be constantly agitated with an iron spatula 
while cooling. It is then to be dissolved in 
double its own weight of cold water. This 
solution is to be filtered and evaporated in a 
retort (not of glass, because potass in this 
state dissolves glass), till it begins to deposit 
regular crystals. If the mass consolidates 
ever so little by cooling, a small quantity of 
water is to be added, and it must be heated 
again. When a sufficient number of crystals 
have been formed, the liquor which swims 
over them, and which has assumed a very 
brown colour, must be decanted off, and kept 
in a well-closed bottle till the brown matter 
lias subsided, and then it may be evaporated 
as before, and more crystals obtained. 
The theory of these processes is obvious: 
the lime separatesthe carbonic acid, for which 
it has a stronger affinity ; and the alcohol or 
the evaporation separate all the other foreign 
ingredients. 
As potass is never obtained at first in a state 
of purity, but always combined with carbonic 
acid, it was long before chemists understood 
to what the changes produced upon it by 
lime were owing. At last, in 1756, Dr. Black 
proved, by the most ingenious and satisfac- 
tory analysis, that the potass which the world 
bad considered as a simple substance, was 
really a compound, consisting of potass and 
carbonic acid ; that lime deprived it of this 
acid ; and that it became more active by be- 
coming more simple. 
That potass was known to the antient 
Gauls and Germans cannot be doubted, as 
they were the inventors of soap, which, Pliny 
informs us, they composed of ashes and 
tallow. These ashes (for he mentions the 
ashes of the beech tree particularly), were 
nothing else but potass ; not, however, in a 
state of purity. The xonm, too, mentioned by 
Aristophanes and Plato, appears to have 
been a ley made of the same kind of ashes. 
The ah hymists were well acquainted with it; 
and it has been in every period very much 
employed in chemical researches. It was 
long distinguished among us by the name of 
vegetable alkali, because it is obtained from 
vegetables, and because it was long thought 
to be peculiar to the vegetable kingdom ; but 
this is now known to be a mistake. It was 
called also salt of tartar, because it may be 
obtained by burning the salt called tartar. 
Mr. Kirwan has given it the name of tartarin. 
Dr. Pearson has called it vegalkali, Klaproth 
kali, and Dr. Black lixiva. By most British 
chemists it is called potash : but this term, in 
common language, signifies the carbonat of 
potass, or the potass of commerce. For in 
fact, till Berthollet published his process in 
the year 1786, chemists had never examined 
potass in a state of complete purity. 
2. Potass is a brittle substance of a white 
colour, and a smell resembling that which is 
perceived during the slacking of quicklime. 
Its taste is remarkably acrid; and it is so ex- 
ceedingly corrosive, that when applied to any 
part of the body, it destroys it almost instant- 
aneously. On account of this property, it 
Iras been called caustic, and is often used by 
surgeons under the name of the potential 
cautery, to open abcesses, and to destroy 
useless or hurtful excrescences. Its specific 
gravity is 1.70. ' 
When heated it melts; at a red heat it 
swells, and evaporates slowly in a white acrid 
smoke. A strong heat gives it a greenish 
tinge, but produces no other alteration in it. 
Potass is not altered by exposure to light. 
When exposed to the air, it soon attracts 
moisture, and is converted into a liquid; at 
the same timeit combines with carbonic acid, 
for which it has a strong affinity. 
3. It has a very strong affinity for water. 
At the common temperature of the air, one 
part of water dissolves two parts of potass. 
The solution is transparent, very dense, and 
almost of the consistence of oil. It is in this 
state that potass is usually employed by 
chemists. When four parts of potass in pow- 
der, and one of snow are mixed together, the 
mixture becomes liquid, and at the same 
time absorbs a quantity of caloric. This 
mixture was employed by Lowitz to produce 
artificial cold. When the aqueous solution 
of potass is evaporated to a proper consist- 
ency, the potass crystallizes. I he shape of 
its crystals is very different, according to the 
way in which they have been produced. When 
allowed to form spontaneously, they are 
octahedrons in groups, and contain 0.43 of 
water. When formed by evaporation on the 
fire, they assume the figure of very thin 
transparent blades of extraordinary magni- 
tude, which, by an assemblage of lines cross- 
ing each other in prodigious numbers, present 
an aggregate of cells or cavities, commonly so 
very close, that the vessel may be inverted 
without losing one drop of the liquid which it 
contains. 
4. Potass shews no disposition fo unite with 
oxygen, neither is it altered by the action of 
any of the compounds into which oxygen 
enters, though it has a strong tendency to 
vinite with several of the^e compounds. 
5. It unites with none of the simple com- 
bustibles except sulphur. Carbon and hydro- 
gen do not act upon it at all ; neither does it 
produce any alteration in them, but it acts 
upon phosphorus with considerable energy. 
When three parts of sulphur and one of 
potass are triturated together in a glass 
mortar, the sulphur acquires a green colour, 
the mixture becomes hot, and exhales an 
aliaceous odour. It gradually attracts mois- 
ture from the air, and is totally soluble in 
water. When two parts of potass and one of 
sulphur are heated in a crucible, they melt 
and combine, and form sulphuret ot potass. 
The potash of commerce may be also em- 
ployed; for the carbonic acid separates in the 
form of a gas during the combination of tha 
potass and sulphur. When the fusion is com- 
plete, the sulphuret is to be poured upon a 
marble slab ; and as soon as it congeals, it 
must be broken to pieces, and set by into a 
well-corked phial. 
Sulphuret of potass, thus prepared, is of a 
brown colour, not unlike the liver of animals. 
Hence it was formerly called hepar sulphuris, 
“ liver of sulphur but when exposed to the 
air, it soon becomes green, and even white. 
It is hard, brittle, and has a glassy fracture. 
Its taste is acrid, caustic, and bitter, and it 
leaves a brown stain upon the skin. It has 
no other smell than that of sublimed sulphur. 
When exposed to a violent heat the sulphur 
sublimes, and the potass remains in a state of 
purity. This sulphuret converts vegetable 
blues to green, and soon destroys them. 
When heated with charcoal, it dissolves, and 
combines with it. 
When sulphuret of potass is exposed to the 
air, or when it is moistened with water, its pro- 
perties very soon change. It acquires a green 
colour, and exhales the odour of sulphureted 
hydrogen gas. This change is owing to the 
formation of a quantity of sulphureted hydro- 
gen, in consequence of the decomposition of 
the water. This new- formed substance com- 
bines with the sulphuret, and converts it into 
hydrogenated sulphuret of potass, which is 
soluble in water, and has a brownish green 
colour. It may be formed also by boiling in 
water two parts of potass and one part of sul- 
phur. Sulphuret of potass produces no 
change upon air, but hydrogenated sulphuret 
gradually absorbs oxygen. When inclosed 
in a vessel with a quantity of air, it soon ab- 
sorbs all the oxygen of that portion, and 
leaves nothing but azotic gas. This fact, 
which was first observed by Scheele, induced 
him to use hydrogenated sulphuret to mea- 
sure the quantity of oxygen contained in any 
given portion ot atmospheric air. Hydrogen- 
ated ’sulphuret is capable of oxydizing and 
dissolving almost all the metals. We are in- 
debted to M. Berthollet for the first accurate 
account of the difference between these two 
substances. 
Potass cannot be combined with phos- 
phorus by any method at present known. 
But when potass, dissolved in water, is heated 
over phosphorus in a retort, the water is gra- 
dually decomposed, part of the phosphorus is 
converted into phosphoric acid, and a great 
quantity of phosphureted hydrogen gas is 
emitted, which takes fire as usual as soon as 
it comes into contact with the air of the atmo- 
sphere. It was by this process that Gemgem- 
bre first obtained phosphureted hydrogen 
gas. 
6. It does not appear that potass is capable 
of uniting with azote, or even of acting on it 
at all ; but with muriatic acid it unites very 
readily, and forms the compound known by 
the name of muriat of soda. 
7. Potass does not combine with any of 
the metals; but some of tire meta’s which 
