CHEMISTRY. 
the Roman'alunt, and one of its furfaces is covered with 
pyramidal cryftals. 4 Alum from Smyrna. The moll 
ancient manufactures of alum appear to have been efta- 
blifhed near Smyrna and Conftantinople. This alum is 
only found in cabinets of natural hiftory. 5. French 
alum. It is prepared in many manufactories in France; 
efpecially in thole of Chaptal, and at Javille near Paris: 
they mix the acid water which proceeds from the rectifi¬ 
cation of fulphuric acid, with the clay of Gentilly, in 
wooden troughs, and expofe it to the air for lix months 
then walh the mats with ley. 6. Alum extracted from 
efflorefcent Ichilti and volcanic products. It is alfo ex¬ 
tracted from earths and Hones in many parts of Germany, 
where there have been manufactories of it fince the year 
1544; and alio in England, Spain, Sweden, and molt 
parts of Europe. 
The addition of a certain portion of alkali is now 
fhown to be necelfary to the formation of alum. The 
neceffity of this addition had been known for a long time 
in the manufacturing of this iubftance; but it was con¬ 
ceived, that the only ufe of the alkali was to faturate the 
excefs of acid. Bergman, however, had remarked, that 
the addition of foda or lime did not promote the cryftal- 
lization of alum when ufed as fubllitutes for potalh and 
ammoniac. He did not, however, feem to know that 
the potalh entered into the formation of the alum. This 
curious faCt was fully afcertained by the experiments of 
Vauquelin. He found, that the addition of the fulphat 
of potalh was equally efficacious in promoting the cryltal- 
lization of alum with the potalh iti'elf, and that the fame 
effeCt was produced by the fulphat of ammoniac. Berg¬ 
man had alfo obferved, that not only the common alum, 
but alio the Roman, when decompofed by ammoniac, 
afforded a fmall quantity of fulphat of potalh. Alum 
dill'olves in live times its own weight of cold water ; but 
boiling water diffolves more than half its weight, eight 
ounces of water in this Hate diffolving five ounces of the 
fait. It cryllallizes very well by cooling. Its cryftals are 
triangular pyramids, with truncated angles. When they 
are depofited on threads in the middle of the folution, 
very regular oCtahedrons are formed, whofe pyramids 
are obliquely truncated at the middle, between the ver¬ 
tex and the bafe. This fait melts with a mild heat, emit¬ 
ting abundance of aqueous vapours, at the fame time 
that it fwells up, and becomes converted into a very large 
light mafs, of an opaque white, with a great number of 
cavities. This phenomenon is produced, as in borax, 
by the difengagement of water, whofe vapour blows up 
and extends the laline mafs. The alum in this ftate is 
called calcined, alum , and weighs' little more than half 
its former weight. It is fomewhat altered; reddens the 
fyrup of violets ; has a ftronger tafte ; lofes its water of 
cryftallization, and a very linall part of its acid ; for, if 
the experiment be made in clofe veffels, the water ob¬ 
tained is acid. Yet calcined alum has always an excefs 
of acid; which happens becaufe the molecules of the 
alum, having an extreme affinity for each other, let the 
acid run, which there remains interpofed. 
Sulphat of alumine {lowly efflorefees in the air. Several 
combuftible bodies decompofe it: we will take charcoal for 
an example : Mix eight parts of charcoal in powder with 
one of alum; put the whole into a retort, and adapt it 
to the pneumatic apparatus. By the help of heat, car- 
-bonic acid gas is brought under the jar; and what re¬ 
mains in the retort is a fulphit of alumine. 
Alum is decompofed alio by animal and vegetable fiib- 
ftances: this decompofition produces a fubltance. which 
takes fire on expofure to the air, and is called pyrophorus , 
or the pkofphorus of Homberg . Take three parts of alum 
to one of honey, meal, or lugar ; either of thefe will have 
the lame effeft. Dry the mixture in an into ladle, ftir- 
ring it with a fpatula. The mixture firft liquefies, then 
puffs up, and at length fubfides into lumps or clods: then 
pulverile it grofsly, and finilh the drying of it, which 
leaves a black carbonaceous powder. It is then put into 
Voir. IV. No. 192. 
233 
a fmall long-necked matrafs, which muft be only three- 
parts full; place this matrafs in a crucible with (and, and 
cover the bulb. Heat is applied till a bluifli flame ap¬ 
pears to iffue out of the neck of the phial. After this 
lias continued feveral minutes, the crucible is taken out 
of the fire ; and when the whole is cool, the pyrophorus 
is poured quickly into a dry bottle, which is immediately 
well clofed. This is truly a fulphure of alumine, mixed 
with a fmall quantity of carbon. If a little of this ful¬ 
phure be poured upon paper in the open air, it takes 
fire. If the pyrophorus is flow in burning, the combuftion 
may be haftened by any humid vapour, fuch as that of 
the breath. The inflammation of pyrophorus only takes 
place in a moift air ; it will not burn in a dry air; the 
humidity is abforbed by the fubltance, and the difengag- 
ed caloric of this humidity raifes the temperature of the 
fubltance; then it burns itfelf; the carbon and fulphur, 
in this combination, regenerate fulphuric acid and ful¬ 
phat of alumine, and a little carbonic acid is difengaged. 
It is fo certain that pyrophorus is a compofition of a ful¬ 
phure and of carbon, that, when dilfolved in water, 
there is a precipitation of carbon, and the liquor which 
remains is a hydro-fulphure. 
All the acids decompofe this fulphure; nitric acid pre¬ 
cipitates a reddilh fulphur, becaufe, being oxygenated 
by the acid, it is in the ltate of oxyd of fulphur, and 
nitrat of alumineds formed. Azotic gas and carbonic 
acid are obtained under the jar, when the experiment is 
made in clofe veffels. When the nitrous gas is all decom¬ 
pofed, it gives out its oxygen to the pyrophorus, and 
burns it, the fulphur is acidified, and a little fulphat of 
alumine is again found. Concentrated fulphuric acid, 
and fulphurous acid, flame alfo with pyrophorus; the 
fame effeft takes place with oxygenated muriatic acid gas. 
It appears, from the experiments of Scheele, that 
alum affords pyrophorus only in proportion to the quan¬ 
tity of fixed alkali which it contains, or in proportion to 
the quantity of alkali obtained from the combuftion of 
the carbonaceous fubltance employed. Aluminous earth 
does not appear to be effential to the formation of py¬ 
rophorus, for it may alfo be formed by burning a mixture 
of equal parts of Glauber falts and meal, four parts of 
fulphat of potalh with five, of meal, equal parts of pot- 
afli and meal with one-fourth of fulphur ; or, according 
to Bergman, one part of mineral alkali with a fourth of 
fulphur, and one-third of charcoal powder; or, accord¬ 
ing to Scheele, fulphat ,of potalh calcined with three parts 
of charcoal powder. Pyrophorus, according to the ex¬ 
periments of Lavoilier; diminilhes the volume of atmof- 
pheric air from 100 to 72^. The diminution of the air 
by the combuftion of this fubltance, had before this been 
obferved by Hales, and confirmed by Prieftley. Oxygen 
gas was reduced to one-feventh of its volume by com¬ 
buftion with pyrophorus, and when the remainder was 
walhed with lime water, and inflamed anew, only a very 
fmall portion of the original volume remained undecom- 
pofed. 
Alum is decompofed by barytes, lime, ftrontian, mag- 
nefia, glucine, and all the alkalis, which, poured into a 
folution of this fait, precipitate the alumine. Alum is 
of very extenfive ufe in medicine and the arts. See vol. i. 
p. 381, of this work, under the article Alum. 
Sulphat of Zircone, or Jargon. —That the com¬ 
binations of zircone with acids may aft with facility, it 
is necelfary that it Ihould be in a ftate of extreme divifion 
and very moift, fuch as when juft precipitated from thefe 
folvents ; if it has been dried by fire, or even by the heat 
of a ftove, it unites but difficultly in theie combinations. 
This earth adheres to the acids, but the aftion of a gen¬ 
tle heat is fufficient to break the combination ; this is de- 
monltrated alfo by alkalis, and all other earths, which 
leparate acids from it. 
Sulphuric acid and zircone unite eafily; and the fait 
refulting from the combination is white, inloluble in 
water, 4nd taftelefs. Heat decompofes it, and leaves the 
3 Q zircone 
