C fl E M 
upd, muriatic acid being poured on it, it immediately 
becomes of a more beautiful and deeper blue, and is to 
be then dried by a mild heat, or by expofure to the air. 
Many chemiils have, iince the time of Woodward, at¬ 
tended to the theory and- preparation of Prufiian blue. 
With regaid to its preparation, it is now known that a 
great number of lubftances are capable of commuitieat¬ 
ing to the alkali the property of precipitating iron of a 
blue colour. In the manufactories, they take another 
mode: Equal parts of the blood, horns, nails, and fkin, 
of oxen, are reduced to a coal, of which they mix ten 
parts with thirty parts of potufh ; calcine this mixture in 
an iron pot; in twelve hours the mixture will be in a 
foft parte; then they are poured into tubs of water ; fil- 
tre, and mix this iolution with another, compoled of 
three paits of alum, and one of fulphat of iron. 
Prufiian blue takes flame fooner than fulphur. It de¬ 
tonates (trongly with the i'uper-oxygenated muriat of pot- 
afli. Diflilled in the pneumatic apparatus, ammoniacal 
gas, carbonat of ammoniac, and hydrogen gas, are ob¬ 
tained ; oxyd of iron and fotne alumine remain in the re¬ 
tort. Ammoniac heated over Prufiian blue, decompofes 
it by feparating the colouring principle. Fourcroy difi- 
covered, that lime-water put in digeftion over Prufiian 
blue had the property of decompofirig it with the help of 
a little heat. Thus may be prepared the pruffiat of lime ; 
for the lime-water faturates itfeif entirely with the co¬ 
louring principle,’which in this cafe, performs the office 
of an acid ; hence it has been called Pruffic acid. Pruf- 
liat of lime is the moft certain mode of difeovering the 
prefence of iron in mineral water. Pure fixed alkalis de¬ 
prive Prufiian blue of its colour immediately, and with¬ 
out heat; they are to be preferred to the alkaline carbo- 
nafs. Magnefia alio attacks the colouring matter of 
Prufiian blue, but much more weakly than. lime. Ba¬ 
rytes has the fame property. 
Scheele has. finewn, that the red oxyd of mercury fe- 
parates the colouring matter from Prufiian blue; and this 
is the mode of obtaining the Pruffic acid. The procefs 
confifls in uniting in a glafs cucurbit, or a matrals, two 
parts of Prufiian blue in powder, one part of red oxyd of 
mercury, and fix parts of water; boil this mixture for 
half an hour, ftirring it continually ; it will aflumea yel¬ 
low colour, inclining to green ; filtre, and pour over the 
refidue two parts more of boiling water; leave the whole 
together to cryltaliize by evaporation. In this - experi¬ 
ment, the Pruffic acid quits the ox; d of iron to unite 
with the oxyd of mercury, with which it has a ftronger 
itttraftion, and forms a lolublefalt, which cryftallizes in 
tetrahedral prilms, terminated by quadrangular pyramids, 
whole planes anlv.erto the angles of the prifin. This is 
Pruffiat of mercury'-, it is decontpofed neither by acids nor 
alkalis. To obtain Pruffic acid from this, dlflolve the 
pruiliat of mercury in water, or take the liquor as above 
delcribed, previous to evaporation. Pour the quantity 
obtained in the preceding experiment, into a bottle over 
450 grains of iron-filings; add 180 grains of concentrat¬ 
ed iulphuric acid (Berthoilet prefers the muriatic acid,) 
and agitate the mixture Itrongly for fome minutes. The 
whole becomes of a black colour by the redu< 5 lion of the 
mercury; the liquor loles its mercurial tafte, and afiumes 
that of the colouring lixiviation; the iron unites with 
the oxygen of the mercury, and combines with the ful- 
phuric acid. Let it fettle; then decant the liquor, and 
put it into a retort on afand-bath ; adapt a receiver with 
a little dillilled water to abforb the acid ; then lute care¬ 
fully. By the'help of a gentle heat, the colouring prin¬ 
ciple paffes firft, as being more volatile than water: 
ftop the operation when about one-fourth of the liquor 
has palled over. This liquor will contain a little ful- 
phuric acid ; to feparate which, re-diftil with a very flight 
heat over pulverized chalk; and then the Pruffic acid 
will be obtained in its greateft purity. 
This acid has an odour peculiar to itfeif, fomewhat like 
bitter almonds; it is of al'weet tafte 5 it does not redden 
Vol. IV. No. 198. 
I S T R Y. 301 
blue paper; it thickens the folutions of foap and cf al¬ 
kaline fulphures. It precipitates alumine from its nitric 
Iolution; it decompoles oxygenated muriatic acid, ab- 
fyrbs its oxygen, and becomes fragrant. In thisftate, it 
feems to ha\e no great tendency to alkaline fubffances.. 
It no longer precipitates iron of a blue, but of a green 
colour; and this green precipitate is foluble in acids. It 
becomes blue again by contadl with the rays of the fun, 
or by the addition of fulpbureous acid and iron. 
When, by means of oxygenated muriatic acid, the 
Pruffic acid is brought to the ftate of making a green pre¬ 
cipitate with iron, ammoniac is formed as fooh as it is 
mixed with alkaii or lime. An acid, poured into titis 
mixture, no longer reftores to Pruffic acid its own pecu¬ 
liar odour ; Berthoilet fuppofes this acid to be entirely 
deftroyed. Though perfeftly-pure potalh be ufed, an 
acid poured in after it has adfted, will produce an eft’ei vef- 
cence, and difengages carbonic acid which is entirely 
formed. Berthoilet, who has publilhed fome very inte- 
refting papers on the Pruffic acid and its combinations, 
concludes, from the experiments here detailed, that 
azot, hydrogen, and carbon, united in proportion, ami 
with a degree of condenfiition not at prelent known, form 
what is called Pruffic acid. 
Vauquelin has publilhed fome remarks on the formation 
of this acid. To afeertain whether it contains oxygen, 
he made the following experiments : 1. Put into a retort, 
one hundred parts of muriat of ammoniac, fifty of lime, 
and twenty-five of charcoal finely powdered: adapt a re - 
ceiver containing a flight foluticn of fulphat of iron, in 
which plunge the beak of the retort. Give a brilk heat, 
and continue the ahtion of the fire till nothing comes 
over. 2. Into the fame apparatus put one hundred parts 
of muriat of ammoniac, fifty parts of the femi-vitreous 
oxyd of lead, and twenty-five parts of charcoal. Heat 
as before ; agitate brifkly the liquors contained in the re¬ 
ceivers, and leave thehi expofed to the air for feveraL 
days, that the combination may be complete between the 
oxyd of iron and the Pruffic acid, and that the Pruiliat 
of iron may abforb as much oxygen, as is neceffiuy to 
pafs to the ftate of blue Pruffiat, not to be adled upon by 
acids. Pour into thefe liquors equal quantities of ful- 
phuric acid much weakened with w r ater, and a Pruffiau 
blue is produced whofe qualities are as one to fix, com¬ 
pared with the preceding experiment, that is to fay, the 
product was fix times as plentiful w ith oxyd of lead, than 
in that experiment where he ufed lime to difengage the 
ammoniac. 
> What can caufe the great difference in the refults of 
thefe two experiments ? fays Vauquelin. Can it be the 
prel’enceof oxygen in the oxyd of lead; or becaufe that 
oxyd, by diiengaging the ammoniac more flowly, gives 
time fordiflblving more carbon? Thefe queftions we rea¬ 
dily fet down as he has left them, becaule they may 
awaken the attention of future chemiils, and lead to new 
experiments and ufeful contlulions. Though Vauquelin, 
has not refolved thefe queftions, yet it appears certain, 
that, whenever a fubftance containing oxygen is intro¬ 
duced into a mixture proper for forming Pr uffic acid, a 
greater quantity of that acid will be produced. The lame 
chemilt remarked, that an alkaline Pruffiat, how dry foever, 
by decompofition in well-clofed veffels, always afforded 
carbonat of ammoniac. Curaudeau has fhewn, that, by 
heating cauftic potalh with common charcoal dult in a 
crucible of platina, a liquor is obtained by lixiviation, 
which precipitates iron of a true Prufiian blue. 
The Pruffic acid combines with different bales. With 
potalh and the pure acid, a flight heat produces a fait 
which cryftallizes in fquare plates, the edges bevelled,- 
formed in octahedrons with the two oppolite pyramids 
truncated : to obtain thefe cryltals, evaporate the liquor 
to drynefs, re-diffolve, filtrate, and then concentrate, 
with a gentle heat. In this ftate, the Pruffiat no longer 
affords a blue colour with acids. This fait may alfo be 
prepared by laturating cauftic potalh with the colouring 
4H matter, 
