14 
V R U 
PRU 
PRU 
in four limes their weight of cold water, to 
exclude the sulphat of potass. 
Assay a few drops of this solution with ba- 
rytes water, to see whether it contains any 
sulphuric acid, and add some barytes water 
to the remainder if necessary: iiltre off the 
solution from the sulphat of barytes, which 
will have precipitated, and set it by to cry- 
stallize for a few days; that the barytes, if 
any should remain, may be precipitated. If 
the crystals now obtained are of a pale yellow 
colour, and discover no bluei-h streaks when 
sprinkled over with muriatic acid, they are 
lit for use ; but if they still discover blueish 
or green streaks, the solutions and crystalli- 
zations must be repeated. 
These crystals must be kept in a well- 
stopped bottle, which, to preserve them from 
the air, should be filled with alcohol, as they 
are insoluble in it. 
Before they are used, the quantity of iron 
they contain should be ascertained, by heating 
100 grains to redness for half an hour in an 
open crucible : the prussic acid will be con- 
sumed, and the iron will remain in the state 
of a reddish-brown magnetic oxide, which 
should be weighed and noted. This oxide is 
half the weight of the Prussian blue afforded 
by the Prussian alkali : its weight must there- 
fore be subtracted from that of metallic pre- 
cipitates formed by this test. Hence the 
weight of the crystals, in a given quantity of 
the solution, should be noted, that the quan- 
tity employed in precipitation may be known. 
Care must be taken to continue the calcina- 
tion till the oxide of iron becomes brown ; 
for while it is black, it weighs considerably 
more than- it should. 
Another good method of preparing this salt 
has been lately given by Mr. Henry ; but it 
is rather too expensive for general use. It 
consists in first forming a triple prussiat of 
barytes, and adding it in crystals to a solution 
of carbonat of potass till the solution no long- 
er restores the colour of reddened litmus 
paper. After digesting the mixture for half 
an hour, fiitre the liquid, and evaporate it 
gently. The triple prussiat of potass crystal- 
lizes. 
PRUSSIC ACID, is one of the most im- 
portant instruments which the chemist pos- 
sesses. It was discovered about a century 
ago by Diesbach at Berlin ; and a method of 
preparing it was published by Woodward in 
the Philosophical Transactions for 1724, which 
he said he had got from one of his friends' in 
Germany. This method was as follows : 
Detonate together four ounces of nitre and 
as much tartar, in order to procure an extem- 
poraneous alkali; then add four ounces of 
dried bullock’s blood ; mix the ingredients 
well together, and put them into a crucible 
covered with a lid, in which there is a small 
hole; calcine with a moderate tire till the 
blood emits no more smoke or dame capable 
of blackening any white body exposed to it : 
increase the lire towards the end, so that the 
whole matter contained in the crucible shall 
be moderately but sensibly red. In this state 
throw it into two pints of water, and boil it 
for half an hour. Decapt off this water, and 
continue to pour on more till it comes off in- 
sipid. Add all these liquids together, and 
boil them down to two pints. Dissolve two 
ounces of sulphat of iron and eight ounces of 
alum in two pints of boiling water ; mix this 
wifli the former liquor while both are hot. 
An effervescence takes place, and a powder | 
is precipitated, of a green colour mixed with ! 
blue. Separate this precipitate by iiitration, 
and pour muriatic acid upon it till it becomes 
of a beautiful blue ; then wash it with water 
and dry it. 
Different explanations were given of the 
nature of this precipitate by different che- 
mists. All of them acknowledged that it 
contained iron ; but to account for the colour 
was the difficult point. Brown, and Geol- 
froy, and Neuman, discovered in succession, 
that a great many other animal substances 
besides blood communicated to alkalies the 
property of forming Prussian blue; but the 
theories by which they attempted to account 
for its formation were altogether nugatory. 
At last a very important step was made in 
the investigation oi this compound by Mac- 
quer, who published a dissertation on it in 
the year 1752. 
I bis celebrated chemist ascertained the 
following facts: 1. When an alkali is added 
to a solution of iron in any acid, the iron is 
precipitated of a yellow colour, and soluble 
in acids ; but if iron is precipitated from an 
acid by an alkali prepared by calcination with 
blood (which has been called a Prussian al- 
kali), it is of a green colour. 2. Acids dis- 
solve only a part of this precipitate, and leave 
behind an insoluble powder, which is of an 
intense blue .colour. The gieen precipi- 
tate, therefore, is composed of two different 
substances, one of which is Prussian blue. 
3. The other is the brown or yellow oxide of 
iron ; and the green colour is owing to the 
mixture of the blue and yellow substances. 
4. When heat is applied to this Prussian blue, 
its blue colour is destroyed, and it becomes 
exactly similar to common oxide of iron. It 
is composed, therefore, of iron and some 
other substance, which heat has the property 
of driving off. 5. If it is boiled with a pure 
alkali, it loses its blue colour also, and at the 
same time the alkali acquires the property of 
precipitating of a blue colour solutions of 
iron in acids, or it has become precisely the 
same with the Prussian alkali. 6. Prussian 
blue, therefore, is composed of iron and 
something which a pure alkali can separate 
from it, something which has a greater affi- 
nity for alkali than for iron. 7. By boiling a 
quantity of alkali with Prussian blue, it may 
be completely saturated with this something, 
which may be called colouring matter. 
8. No acid can separate this colouring mat- 
ter from iron after it is once united with it. 
9. When iron dissolved in an acid is mixed 
with an alkali saturated with the colouring 
matter, a double decomposition takes place, 
the acid unites with the alkali, and the co- 
louring matter with the iron, and form:. Prus- 
sian blue. 10. The reason that, in the com- 
mon method of preparing Prussian blue, a 
quantity of yellow oxide is precipitated, is, 
that there is not a sufficient quantity of co- 
louring matter (for the alkali is never satu- 
rated with it) to saturate all the iron displaced 
by the alkali ; a part of it, therefore, is mixed 
with Prussian blue. Muriatic acid dissolves 
this oxide, carries it off, and leaves the blue 
in a state of purity. 
Such were the conclusions which Macquer 
drew from his experiments; experiments 
which not only discovered the composition 
of Prussian blue, but threw a ray of light on 
the nature of affinities, which has contributed 
much towards the advancement of that im 
portant branch of chemistry. 
The nature of the colouring matter, how 
ever, was still unknown. At length, in 1772- 
Morveau announced his suspicion that the 
colouring matter was probably an acid. 
Such was the knowledge of chemists re- 
specting the nature of this colouring matter, 
when Scheele all at once removed the veil, 
and explained its properties and composition. 
He observed that the Prussian alkali, after 
being exposed for some time to the air, lost 
the property of forming Prussian blue ; the 
colouring matter must therefoie have left it. 
He put a small quantity of it into a large glass 
globe, corked it up, and kept it some time 
but no change was produced either in the 
air or the Prussian alkali. Something must 
therefore displace the colouring matter when 
the alkali is exposed to the open air, which is 
not present in a glass vessel. Was it carbo- 
nic acid gas? To ascertain this, lie put a 
quantity of Prussian alkali into a glass globe 
tilled with that gas, and in 24 hours the al- 
kali was incapable of producing Prussian blue. 
It is therefore carbonic acid gas which dis- 
places the colouring matter. He repeated 
this experiment with this difference, that he 
hung in the giobe a bit of paper which had 
been previously dipped into a solution of sul- 
phat of iron, and on which he had let fall (wo 
drops of an alkaline lixivium in order to pre- 
cipitate the iron. This paper was taken out 
in two hours, and became covered with a tine 
blue on adding a little muriatic acid. Car- 
bonic (acid, then, lias the property of sepa- 
rating the colouring matter from alkali with- 
out decomposing it. 
He found also that other acids produce 
the same effect. Hence he concluded, that 
the colouring matter might be obtained in a 
separate state. Accordingly he made a great 
many attempts to procure it in that state, 
and at last discovered the following method, 
which succeeds perfectly : 
Mix together ten parts of Prussian blue in 
powder, live parts of the red oxide of mer- 
cury, and thirty parts of water, and boil the 
mixture for some minutes in a glass vessel. 
The blue colour disappears, and the mixture- 
becomes yellowish-green. Pour it upon a 
iiltre ; and' after all the liquid part has passed, 
pour ten parts of hot water through the fiitre 
to wash the residuum completely. The ox- 
ide of mercury decomposes Prussian blue, 
separates its colouring matter, and forms 
with it a salt soluble in water. The liquid, 
therefore, which has passed through the fiitre 
contains the colouring matter combined with 
mercury. The other component parts of the 
Prussian blue, being insoluble, do not pass 
through the tiltrp. Pour this mercurial liquid 
upon 2 \ parts of clean iron filings, quite free 
from rust. Add at the same time one part 
of concentrated sulphuric acid, and shake the 
mixture. The iron filings are dissolved, and 
the mercury formerly held in solution is pre- 
cipitated in the metallic state. The cause of 
this sudden change is obvious: the iron de- 
oxidizes the mercury, and is at the same instant 
dissolved by the sulphuric acid, which lias a 
stronger affinity for it than the colouring 
matter lias. '[ here remain in solution, 
therefore, only sulphat of iron and the co- 
ouring m liter. 
Now the colouring matter being volatile, 
3 
