C H E M 
gfetatm in any liquor ; thus the infufu/n of tin, poured 
into milk, whey, ferum of blood, beef-broth, &c. will 
form a precipitate fromthefe liquids, which will be more 
or lei's plentiful, according to the quantity of gelatin they 
contain. Thus, every fubltance whole infufum can pre¬ 
cipitate animal glue’, poffeffes the tanning principle; 
every fubltance which has the tanning principle precipi¬ 
tates the fulphat of iron black; any fubltance which pre¬ 
cipitates fulphat of iron, but does not precipitate the 
folutum of glue, has not the tanning principle. 
The effential point in the art of tanning, is to know, 
and to regulate invariably, the circumftances which de¬ 
termine the paflage of the fibre to the Hate of gelatin ; 
and to feizethe pi oper time,'as well as’the proper method, 
of combining the tanliin ; for, according to Seguin, 
the fibre is oxygenated glue, which, in the fibrous lfate, 
cannot combine with tannin ; but it acquires the pro¬ 
perty of forming that combination in palling to the lfate 
of gelatin, by which it lofes part of its oxygen. 
MALIC ACID. 
This acid exilfs not only in the apple, (whence its 
name,) but alfo in ftrawberry, ralpberry, and moll of 
the fummer fruits. Commonly, however, to obtain 
this acid, the juice of lour apples is exprefled, and fatu- 
rated with vegetable alkali : to this liquor a folution of 
acetit of lead, or fugar of Saturn, is added ; a double 
decompofition takes place, the acetous acid combines 
with the potalh, and the malic acid with the oxyd of 
lead ; the metallic fait, or malat of lead, falls down ; 
this precipitate is walhed and treated with the l'ulphuric 
acid, diluted with water; fulphat of lead is formed, 
and the malic acid remains in the liquor. It is necel- 
fary to add a fufficient quantity of fulphuric acid to de- 
compofe the whole of the malat of lead, which is .known 
by thefrelh acid tafteof the liquor. 
This acid poffeffes the following properties : It cannot 
be obtained in the concrete form. With the three alka¬ 
lis, it forms deiiquefeent neutral falts. With lime it 
forms a lalt which affords fmall irregular cryllals, folu- 
ble in boiling water, in vinegar, and in the malic acid it- 
felf. With clay it forms a fait of very difficult lolubility, 
With magnefia, a deiiquefeent fait. It diffolves iron ; 
and this folution is brown, and does not afford cryftals. 
With zink, which it diffolves well, it affords a lalt in 
very fine cryftals. The nitric acid changes it into the 
oxalic acid. It precipitates the nitrats of mercury, of 
lead, of filver,and of gold, in the metallic ftate. The calca¬ 
reous malat decompofes the ammoniacal citrat, and cal¬ 
careous citrat is formed, which is infoluble in boiling 
water and in the vegetable acids. The folution of cal¬ 
careous malat in water is precipitated by alcohol. Laft- 
ly, the malic acid is readily deltroyed by fire, which 
changes it into the carbonic acid : this lalt partly fatu- 
rates the bales of the malats, which are decompoled by 
heat. Thefe are the properties which eftablilh the pe¬ 
culiar characters of this acid. 
Scheele found it almoft pure, or mixed with a fmall 
quantity of citric acid, in the juice of apples, barberries, 
elderberries, does, the fruit of the lervice-tree, and 
damfons. He found it in combination with half its 
weight of citric acid in gool'eberries, cherries, Itraw- 
berries, rafpberries, and blackberries. He alfo obtained 
5 t from fugar by the nitric acid; and Morveau remarks, 
that the malic acid appears before the oxalic acid. To 
obtain this acid from iugar, and to feparate, at the fame 
time, the oxalic from the malic acid, and to obtain the lat¬ 
ter in a ftate of purity, weak nitric acid is to be poured 
over fugar, and di{filled, till the mixture begins to af* 
fiime a brown colour ; precipitate the oxalic acid by 
means of lime-water; and another acid remains which 
lime does not affect. To obtain this acid pure, the li¬ 
quor is to be faturated with chalk ; filtre, and add al¬ 
cohol, which occafions a mucilage ; this mucilage, well 
Vol. IV. No. zoo. 
: S t r y. 325 
waflied with alcohol, is to be difTolved again in diftilled 
water : decompofe the malat of lime with acetit of lead ; 
and then difengage the malic acid by means of the ful- 
pliuric acid. 
Gum-arabic, manna, fugar of milk, gum-tragacanth, 
ftarch, the fecula of potatoes, and many animal fub- 
ftances, fuch as ifinglafs, white of egg, yolk of egg, 
and blood, treated as above, will fumiffi malic and 
oxalic- acid, 
BENZOIC ACID. 
This acid exifts in benjamin, balfam of Peru and 
Tolu, liquid ftyrax, ftorax, cinnamon, vanilla, the 
urine of horfes, cows, and children; in general urine 
which does not contain phofphoric acid, turnifties tire 
benzoic, though fometimes mixed with lime. This acid 
was formerly obtained by diftillation and fublimation, 
and was called flowers of benjamin. But, as this method, 
affords it only in fmall quantity, Scheele, after feveral 
lefs-fuccefsful experiments, propofed the following pro- 
cefs : Four ounces of quick-lime are extinguifhed in 
twelve ounces of water, and eight pounds of water are 
added when the ebullition has ceafed ; fix ounces of this 
lime-water are poured over one pound of benzoin in pow¬ 
der, with fufficient agitation to mix thefe two fubltance*,; 
the whole of the lime-water is added by degrees. This 
mixture by parts prevents the benzoin from unitng into 
a mafs. The liquor is heated over a gentle fire for 
half an hour, with continual agitation ; it is then 
taken from the fire, and buffered to fettle for feveral 
hours; the clear liquor is then decanted, and eight 
pounds of water thrown on the refidue, which, after 
being boiled for half an hour, is buffered to fubfide, and, 
when clear, is added to the preceding fluid. This vvafn- 
ing and ebullition is repeated twice more, and the waft¬ 
ing is ended by pouring hot water through the refidue 
upon a filtre : all thefe waters are afterwards reduced to 
two pounds by evaporation: a fmall quantity of refin fepa- 
rates ; the evaporated liquor being cooled, muriatic acid 
is added drop by drop, until no more precipitate falls 
down, and the liquid exhibits a tafte fenfibly acid ; the 
fait of benzoin is the precipitate in the form of a 
powder; it is edulcorated on the filtre. If cryftals be 
defired, it may be diffolved in five or fix times its weight 
of boiling water, which being filtered through a cloth, 
and luffered to cool (lowly, the fait is depofited in flat and 
very long prifms. 
In this procefs, the lime abforbs the benzeic acid, and 
forms with it calcareous benzoat, which is very foluble : 
the refin feparates from this fait, which has but a fmall 
affinity with it; the muriatic acid, whofe attraction for 
lime is ftronger than that of the benzoic acid, feizes the 
earth, and leparates the vegetable acid. The liquor re¬ 
duced to two pounds by evaporation, is not fufficient to 
hold the acid in folution, and almoft the whole is depo¬ 
fited. The calcareous benzoat has not the fmell of ben¬ 
zoin ; but, as Icon as the benzoic acid is feparated by the 
muriatic acid, it refumes the lively fmell which is pecu¬ 
liar to that ball’amic fubltance. By this procefs Scheele 
obtained twelve or fourteen drams of benzoic acid from 
one pound of benzoin; whereas fublimation affords no 
more than nine or ten. He informs us, likewife, that 
the purification of this fait by hot water and by cryftal- 
lization, caufes the lofs of a great quantity, and that this 
purification is not neceffary for pharmaceutic ufes. In 
fa£t, this fait, well cryftallized, is very difficult to reduce 
to powder,and the purification has no other objeft than to 
feparate abouttwograins of refininthe poundofbenzoin. 
Laltly, he remarks that the filtration of this acid dif¬ 
folved in water cannot be made but through a cloth, as 
it feparates quickly, and, in proportion as the liquor 
cools, the fait doles the pores of the paper, and the fil¬ 
tration cannot proceed. 
Cbaptal propoles to diftil the benjamin, and to let all 
4 O the 
