5 1 
which the sulphat of iron is not, it was easy 
to obtain it apart by distillation. Accord- 
ingly he distilled the mixture in a gentle 
beat: the colouring matter came over by the 
time that one-fourth of the liquor had passed 
into the receiver. It was mixed, however, 
with a small quantity of sulphuric acid; 
from which he separated it by distilling a 
second time over a quantity of carbonat of 
; lime. The sulphuric acid combines with the 
lime, and remains behind, which the colour- 
ing matter cannot do, because carbonic acid 
lias a stronger affinity for lime than it lias. 
Thus lie' obtained the colouring matter in a 
state of purity. 
it remained now to discover its component 
parts. He formed a very pure Prussian blue, 
I which he distilled, and increased the lire till 
the vessel became red. The small quantity 
| of water which he had put into the receiver 
: contained a portion, of the blue colouring 
I matter and of. ammonia; and the air of the 
r receiver consisted of azote, carbonic acid gas, 
•and the colouring matter. He concluded, 
from this and other experiments, that the 
: colouring matter is a compound of ammonia 
and oil. But when he attempted to verify 
this theory by combining together ammonia 
and oil, he could not succeed in forming co- 
louring matter. This obliged him to change 
his opinion; and at last he concluded that 
the colouring matter is a compound of am- 
monia and charcoal. He mixed together 
equal quantities of pounded charcoal and 
potass, put the mixture into a crucible, and 
I kept it red-hot for a quarter of an hour : he 
then added a quantity of sal ammoniac in 
i small pieces, which lie pushed to the bottom 
of the melted mixture, kept it in the fire for 
I two minutes till it had ceased to give out va- 
| pours of ammonia, and then threw it into a 
I quantity of water. The solution possessed 
all the properties of the- Prussian alkali. Thus 
Mr. Scheele succeeded in forming the colour- 
ing matter. 
This colouring matter was called prussic 
acid by Morveau, in the first volume of the 
chemical part of the Encyclopedic Metho- 
dique ; an appellation which is now generally 
received. 
These admirable experiments of Scheele 
were repeated and carried still farther, by 
Berthollet in 1787, who applied to the expla- 
I nation of the composition of the colouring 
matter the light which had resulted from his 
previous experiments on the component parts 
of ammonia. This illustrious chemist, scarce- 
ly inferior to Scheele in ingenuity and ad- 
dress, ascertained, in the first place, that the 
prussic alkali is a triple salt, composed of 
prussic acid,* the alkali, and oxide of iron ; 
that it may be obtained in octahedral cry- 
stals ; and that when mixed with sulphuric 
acid, and exposed to the light, it lets tall a 
precipitate of Prussian blue. His next object 
was, to ascertain the component parts of 
prussic acid. When oxymuriatic acid is 
poured into prussic acid obtained by Scheele’s 
process, it loses its oxygen, and is converted 
into common muriatic acid. At the same 
time the prussic acid becomes more odorous 
and more volatile, less capable of combining 
With alkalies, and precipitates iron from its 
solutions, not blue, but green. Thus prussic 
: acid, by combining with oxygen, acquires 
new properties, and is converted into a new 
substance, which may be called oxvprussic 
PRUSSIC ACID. 
acid. If more oxymuriatic acid gas is made 
to pass into prussic acid, and it is exposed 
to the light, the prussic acid separates from 
the water with which it was combined, and 
precipitates to the bottom in the form of an 
aromatic oil; which heat converts into a va- 
pour insoluble in water, and incapable of 
combining with iron. When the green pie- 
cipitate, composed of oxyprussic acid and 
iron, is mixed with a pure fixed alkali, the 
oxyprussic acid is decomposed, and con- 
verted into carbonat of ammonia. 
From these experiments, Berthollet con- 
cluded, that prussic acid does not contain 
ammonia ready-formed ; but that it is a triple 
compound of carbon, hydrogen, and azote, 
in proportions which he was not able to as- 
certain. This conclusion has been stiil far- 
ther verified by Mr. Clouet, who found that 
when animoniacal gas is made to pass through 
a red-hot porcelain tube containing charcoal, 
a quantity of prussic acid is formed. This 
experiment does not succeed unless a pretty 
strong heat is applied to the tube. 
Fourcroy and several other chemists be- 
lieve, that the prussic acid contains also a 
portion of oxygen in its composition, resting 
chiefly upon the following experiments of 
Vauquelin : 
Exper. I. Put into a retort 100 parts of 
the muriat of ammonia, 50 parts of lime, and 
25 parts of charcoal in fine powder ; adapt to 
the retort a receiver containing a slight so- 
lution of the sulphat of iron, and immerse 
into it the beak of the retort ; then apply a 
brisk heat, and continue the action of the fire 
until nothing more is disengaged. 
Exper. II. Put into a retort 100 parts of 
the muriat of ammonia, 50 parts of semi- 
vitreous oxide of lead, and 25 parts of char- 
coal : adapt a receiver containing a solution 
, of sulphat of iron, and proceed as before. 
Stir well the liquors contained in the re- 
ceivers, and expose them to the air for seve- 
ral days, in order that the combination be- 
tween the oxide of iron and the prussic acid 
maj be perfect, and that the prussiat of iron 
may absorb as much oxygen as is necessary 
for its passing to the state of blue prussiat, 
and for its being proof against acids : then 
pour into these liquors equal quantifies of sul- 
phuric acid well diluted with water, arid you 
will have Prussian blue, the quantities of 
which will be as one to six; that is to say, 
the Prussian blue of the experiment in which 
Yauquelin employed oxide of lead, was six 
times more abundant than that of the expe- 
riment in which he employed only lime to 
disengage the ammonia. 
Having thus traced the gradual progress of 
philosophers in ascertaining the nature of the 
prussic acid, it only remains to give an ac- 
count of its properties, which were first exa- 
mined by the indefatigable Scheele. 
Prussic acid obtained by Seheele’s process 
is a colourless liquid like water. It has a 
strong odour, resembling that of the flowers 
of the peach, or of bitter almonds. Its taste 
is sweetish, acrid, and hot, and apt to excite 
cough. It does not alter the colour of vege- 
table blues. 
It is exceedingly volatile, and evidently 
capable of assuming the gaseous form ; though 
it has never been obtained apart, nor exa- 
mined in that state. At a high temperature 
(when united to a base), it is decomposed 
3 T 2 
and converted into ammonia, carbonic acid, 
and carbureted hydrogen gas. 
It unites difficultly with alkalies and earths, 
and is separated from them much more easily 
than from metallic oxides. More exposure 
to the light of the sun, or to a heat of 110°, is 
sufficient for that purpose. I hese combina- 
tions are decomposed also by all the acids. 
It has no action on metals; but it unites 
with their oxides, and forms with them salts, 
which are almost all insoluble, if we except 
prussiats of mercury and manganese. These 
compounds are not decomposed by acids. 
Yet the prussic acid is not capable of taking* 
the metallic oxides from the other acids. 
Prussic acid has a great tendency to cuter 
into triple compounds, combining at once 
with an alkali and a metallic oxide; and 
these compounds are much more permanent 
and difficult to decompose than its singte 
combinations. 
1 he affinities of this acid, as far as they 
have been ascertained, are as follows: 
Barytes, 
- Strontian, 
Potass, 
Soda, 
Lime, 
Magnesia, 
Ammonia. 
It does not seem capable of combining with 
alumina. 
This acid is of great importance to the che- 
mist, in consequence of the property which 
it has of forming insoluble compounds with 
metallic oxides, and almost with metallic 
oxides alone. This puts it in our power to 
ascertain the presence of a metallic body 
held in solution. When the prussic acid is 
dropt in, a precipitate appears if a metal is 
present. The colour of this precipitate indi- 
cates the metal, and the quantity of it enables 
us to ascertain the quantity of metal con- 
tained in the solution. It is -used especially 
to indicate the presence of iron, which it does 
by the blue colour that the solution assumes ; 
and to free solutions from iron, which it does 
by precipitating the iron in the form of Prus- 
sian blue. 
It is evident, however, that the pure prus- 
sic acid cannot be employed tor these pur- 
poses, because it is incapable of taking me- 
tallic oxides from other acids. It is always 
employed combined with an alkaline or 
earthy basis. In that state it decomposes all 
metallic salts by a compound affinity. The 
base most commonly employed is potass or 
lime ; and indeed it is most usually employed 
in the state of a triple salt, composed of prus- 
sic acid, potass, and oxide of iron, which is 
preferred because it is not apt to be decom- 
posed by the action of the atmosphere. 
Of all the metallic solutions tried by 
Scheele, pure prussic acid occasioned only a 
precipitate in three, namely, 
1. Nitrat of silver precipitated white. 
2. Nitrat of mercury . . . black. 
3. Carbonat of iron . . . green becom- 
ing blue. 
It has no action on the oxides of 
Platinum, 
Iron, 
Tin, 
Lead, 
Bismutji, 
Antimonv, 
Manganese, 
Arsenic acid, 
Molybdic acid. 
