CHEMISTRY. 
genious contrivance, which muft not be paffed over in 
jilcnce. At the end of the operation, when the veflels 
are fuftered to cool, a vacuum is formed within ; and the 
■external air, prelling on the fur face of the water in the 
laft open bottles, forces it to return by the tubes into the 
firft collateral bottles, and from them into the bottle 
neared the receiver. If the firft bottle were not empty, 
and of a fufficient magnitude to contain all the water of 
the following bottles, the acid liquors would pafs into 
the receiver; and, as the ftrongeft nitrous acid is con¬ 
tained in that veffel, it would be diluted by the addition 
of the fluid contained in all the bottles. This inconve¬ 
nience would be llill more prejudicial in other didilla- 
tions we fhall have occafion to fpeak of, where it would, 
not only diminiih the force, but likewife alter the purity 
of the produdh During the operation, the heat mud be 
gradually raifed till nothing more comes over. The dif- 
engagement, and pafiage of gas through the water in the 
bottles ferve to diieCt the operator in managing the pro- 
cefs. If it be too rapid, the heat mud be diminifhed, 
led the whole mafs in the retort fhould fwell up and pais 
into the receiver. If, on the contrary, it be too flow, the 
dre mud be raifed, to prevent abforption. This'valuable 
apparatus has therefore the additional advantage of di¬ 
recting the operator to conduit his procefs to the belt ad¬ 
vantage. 
Nitrat of potafli diddled with fulphuric acid, furnifhes 
about 0-43 of its own weight of nitric acid, of a faint 
orange-colour. It will eafily be accounted for why the 
nitrat of potafli (hould give an orange-coloured acid, al¬ 
though completely faturated with oxygen, if it be con- 
fidered, that the oxygen is eafily ieparated from the ni¬ 
tric acid by the contaCt of light, or of a moderate heat; 
and, by recollecting alfo, that .nitric acid has a ltrong 
affinity for nitrous gas. It is, in faff, by the influence of 
a double affinity, that a portion of the nitric acid is de- 
compoled : the one, between the caloric, the light, and 
the oxygen ; the other, between the niiric acid and the 
nitrous oxyd. This theory is confirmed by the oxygen 
gas which is difengaged towards the end of the diltilla- 
tion of nitric acid, and which may be Collected with the 
pneumatic apparatus; and by the nitrous gas, which is 
feparatedfroni the coloured acid when mixed with water. 
The refidue from this decompoiition is very acid l'ulpliat 
of potafli; it is commonly in a white mafs, opaque, and 
femirvitreous. 
The acid obtained by this procefs, never pofteffes that 
degree of purity neceffary for nice experiments. It is 
proper, therefore, to rectify it by a frelh dillillation with 
nitre, or nitrat of fllver, or nitrat of barytes, or even the 
femi-vitreous oxyd of lead if the latter be ufed, care 
muft be taken to feparate the firft produCt. A very pure 
acid may be obtained in either of tiiefe ways. 
Another procefs is ufed in the arts, for ex trailing the 
nitric acid from faltpetre : With two partsof nitre mix 
fix parts of clay, which has been prevaoully calcined to 
deprive it of the greateft part of its moilture, and to con¬ 
vert the martial pyrites, from which it is fcarcely ever 
free, into fulphuiic acid,.which remains interpofed be-, 
tween the parts of the earth. This mixture is introduced 
into earthen retorts of a peculiar form, termed mines, 
which are placed in a row, on long furnaces, called gal¬ 
leries; their necks are inferted in bottles of the lame 
form, and material, which lerve as receivers, and which 
are to be luted with clay. When the retorts are covered, 
and the receivers properly adjufted, light the fire; in- 
creafe the heat by degrees; as loon as red vapours begin 
to appear, unlute the receivers, and take out the iiquor 
they contain, which the workmen call phlegm .- refix the 
receivers, and continue the dillillation rill no more va¬ 
pours come over. 
The decompoiition of nitrat of potafli does not take 
place here, as with fulphuric acid, on account of the 
greater affinity of the clay for the potafli, but by a dou¬ 
2 39 
ble attraction : one between nitric acid and caloric; the 
other between 'the alumine and the potafli, with which 
it forms a kind of femi-vitreous frit. Yet there is a con- 
fiderable decompoiition of nitric acid in this operation, 
as is proved by the red vapours which get through the 
lutings. It has been obferved, that the prelence of py¬ 
rites favours greatly the decompoiition of nitrat 6f pot- 
afh ; for, the more of it is contained in the clay, the more 
acid is obtained from them, and at a much lower degree 
of heat. The refidue from this operation is called cement 
by the diftillers of aqua-fortis, and is ufeful in mafonry. 
Muriatic acid alio deconipofes nitrat of potafli, but by 
a different operation from that with fulphuric acid : it is 
not by feparating immediately the potafli that this de- 
compofition is effected ; it is, on the contrary, by attack¬ 
ing the nitric acid, which it leizes upon, and part of the 
oxygen, whence arifes oxygenated muriatic acid, and 
nitrous acid in vapours: thus, as the muriatic acid takes 
up part of the oxygen of the nitric acid, it pafles into 
the ftate of nitrous acid ; and, as this has lefs affinity 
with the potafli than the muriatic acid, it is driven away, 
and munat of potalh is formed. It muft be oblervcd, 
that thefe effeCts take place only by the aid of heat; and 
that a good deal of muriatic acid muft be uled, becaufe 
one part of it unites with the oxygen, while another part 
unites with the potafli: fo that here we obferve a double 
affinity. 
Nitrat of potafli is of great life in the arts. Burned 
with different proportions of tartar, it forms the lub¬ 
ftances called fluxes, which are employed in the art of af- 
faying, to fule and reduce metallic lubftances. It is fre¬ 
quently uled in medicine as a febrifuge diuretic antifep- 
tic fait; it is adminiftered in any convenient liquid, from 
ten or twelve grains, to the quantity of half a drachm, 
or more ; and it d_aily produces the mod happy effects. 
Of the CONSTRUCTION of NITRE BEDS. 
Of late years, nitre has occupied much of the attention 
of the French chemifts. Conilderable light lias been 
thrown on the theory of its formation, and feveral im¬ 
provements fuggefled in the various procelfes of its ma¬ 
nufacture. The rti'ult of their inquiries and labours on 
this iRtereftin® fubjeCt, are worthy of our utmoft regard. 
A hundred parts in weight of nitrat of potafli, confift,. ac¬ 
cording to Mr. Kirwan, of 41-2 of nitric acid, 46-15 of 
potalh, and 12-83 of the water of compofition. The 
acid, ns lias been fully proved, refults from the com¬ 
bination of leven parts of oxygen, with three of azot. 
The alkali is fuppofed to be derived immediately and 
entirely from the vegetable matter employed in the pro¬ 
cefs of nitrification; though there is fome realon to fuf- 
peCt, it may in part be formed by the developement and 
lublequent union of its elementary principles. We can 
be at no lofs to account for the origin of tiie water. 
The principal circumftances to which it is neceliary 
to attend in the conltruction of. artificial nitre beds, are,, 
firft, the choice of materials. Thefe, as Fourcroy remarks, 
are fuch vegetable and animal bodies as are moll fufetp- 
tible of decompoiition, with a proper admixture of cal¬ 
careous earth. Though, in the vegetable kingdom, there 
be a confideiable latitude of choice, yet experience has 
Iliown, that all vegetables are not equally fuited for the 
production of nitre. In general, thole vegetables are 
to be preferred which are molt eafily d'econipoied, and 
which ieem to contain moft of the elementary principles 
of that fubftance. Of this kind are fome of the poifonous 
plants, and plants of a itrong and foetid odour, fuch as 
hemlock, tobacco, lungwort, henbane, cabbage, hoar- 
bound, nettles, &c. Hie extraft of thefe plants is found, 
by keeping, to be covered with cryltals of nitre. The 
cruciform and leguminous plants arifwer extremely well 
for the production of nitre. The dry and fibrous parts 
are'of leall ufe. - Among animal lubftances, the loft are 
to be preferred to the hard, becaufe they are more readily 
decompofed. 
