POT 
evaporated, till a thick pellicle is formed on 
the surface. It is then set by till crystals 
are formed on it, which are crystals of ex- 
traneous salts, that arc to be removed. The 
evaporation is to be continued, and the se- 
veral pellicles removed as fast as they are 
formed. When the fluid cease's to boil, and 
no more pellicles arise, it is removed from 
the file, and kept stirring till it is cold. It 
is then dissolved in double its weight of 
water; the solution is filtered and evapo- 
rated in a glass retort, till regular crystals 
begin to be deposited. When a sufficient 
quantity has been formed, the liquid is de- 
canted, and the salt is re-dissolved after it 
is suffered to drain, in the same quantity of 
water. The decanted liquor is preserved 
in a well-closed bottle for several days, till 
it subside and become clear. It is then de- 
canted, evaporated, and crystallized again, 
and the process repeated as long as the 
crystals afford with the least quantity of 
water solutions that are perfectly limpid. 
Potash thus obtained is a white solid 
substance, which is susceptible of crystalli- 
zation in long compressed, quadrangular 
prisms, terminating in sharp-pointed pyra- 
mids. These crystals, which are only ob- 
tained from very concentrated solutions are 
soft and deliquescent. The taste is ex- 
tremely acrid ; and it is so corrosive, that 
it destroys the texture of the skin the mo- 
ment it touches it: hence it has derived the 
name of caustic, and is employed in surgery 
for the purpose of opening abscesses, or for 
destroying excrescences. Its specific gra- 
vity is about 1 .7. By a similar mode to 
that above described, pure soda may be 
prepared, substituting the carbonate of 
soda for the pearl-ash. They both possess 
the following properties : — 1. They convert 
vegetable blues into a green eolour. 2. 
They powerfully attract moisture. 3. They 
readily dissolve in water, and produce heat 
during the solution. They are not volati- 
lized by a moderate heat, hence they are 
caUed fixed alkalies. Fixed alkalies have 
till very lately been numbered among the 
simple substances, not, however, without 
exciting in the minds of chymists a suspicion 
that they were compounds. Professor Davy 
has, in the course of the present and pre- 
ceding years, put the matter beyond all 
doubt, and has proved to the satisfaction of 
every chemist, that they are compound of 
o.xygen and certain metallic bases, to which 
he has given the names of 
POTASIUM, and SoDiuM,or S odaicjm. 
Of these, and of the experiments which led 
POT 
to the discoveries, we shall proceed to give 
some account, having attended the repeti- 
tion of his experiments at the lectures deli- 
vered last spring at the ^^oyal Institution. 
Mr. Davy, in his first attempts to decom- 
pose the alkalies, made use of the aqueous 
solutions, and failed. He next made use of 
the potash in a state of igneous fusion, which 
he brought within the sphere of the galvanic 
battery : with this also he was unsuccessful 
in the main point; but some brilliant phe- 
nomena were produced. The potash ap- 
peared a conductor in a high degree: a 
most intense light was exhibited at the ne- 
gative wire, and a column of flame, which 
seemed to be owing to the developement of 
combustible matter, arose from the point of 
contact. Mr. Davy next tried several ex- 
periments on the electrization of potash 
rendered fluid by heaf, with the hope of 
being able to collect the combustile matter, 
but he was still unsuccessful, “ and I only,” 
says he, “ attained my object by employing 
electricity as the common agent for fusion 
and decomposition.” Potash perfectly dried 
by ignition is a non-conductor ; by a very 
slight addition of moisture, which does not 
perceptibly destroy its aggregation, it is 
rendered a conductor, and in this state it 
readily fuses and decomposes by strong 
electrical powers. A small piece of pure 
potash was placed upon an insulated disc of 
platina, connected with the negative side of 
the battery, in a stale of intense activity ; 
and a platina wire, communicating with the 
positive side, was brought in contact with 
the upper surface of the alkali, a vivid ac- 
tion took place, and the potash began to 
fuse at both points of electrization. There 
was a violent effervescence at the upper 
surface ; at the lower, or negative surface, 
there was no liberation of elastic fluid ; but 
small globules, having a high metallic lustre, 
appeared; these were similar in visible 
character to quicksilver: some of them 
burnt with explosion and bright flame as 
soon as they were formed, and others re- 
mained, and were merely tarnished, and 
finally covered with a white film, which 
formed on them. “ These globules,” said 
the- professor, “ numerous experiments soon 
shewed to be the substance I was in search 
of, and a peculiar inflammable principle the 
basis of potash. I found that the platina 
was in no way connected with the result, 
except as the medium for exhibiting the 
electrical powers of decomposition ; and a 
substance of the same kind was produced 
when pieces of copper, silver, gold, plum-i 
