PR US SI ATS. 
513 
-gated by budding on any kind of plum- 
stocks. 
PRUSSIATS, salts formed with prussic 
acid. Of these the most important are, 
1. Prussiat of lime, formed by dissolving 
lime in prussic acid, filtring the solution, and 
separating the uncombined lime. It is de- 
composed by all the other acids, and by al- 
kalies. 
2. Prussiat of magnesia. This salt may 
be formed by putting pure magnesia into 
prussic acid, in a few days the earth is 
dissolved, and the compound formed. The 
magnesia is precipitated by the alkalies and 
lime, and by exposure to the air. 
3. Prussiais of iron. As the prussiats of 
iron enter as ingredients into the triple salts 
formed by the prussic acid, it will be neces- 
sary to give some account of them before 
•entering upon the consideration of these triple 
salts. 
It has been demonstrated by chemists, that 
there are no fewer than four prussiats ot iron ; 
namely, 
1 . White prussiat, 
2. Blue prussiat, 
3. Yellow prussiat, 
4. Green prussiat. 
The white prussiat discovered by Mr. 
Proust is composed of prussic add and pro- 
toxide of iron. It becomes gradually blue 
when exposed to the atmosphere, because the 
oxide absorbs oxygen, and is converted into 
peroxide. 
Blue prussiat, or Prussian blue, is com- 
-posed of prussic acid and peroxide of iron. 
It is a deep-blue powder, insoluble in water, 
and scarcely soluble in acids. It is com- 
posed, according to the most accurate expe- 
riments hitherto made, of equal parts of 
oxide of iron and prussic acid, it is not af- 
fected by exposure to the air. Heat decom- 
poses it by destroying the acid, and the oxide 
of iron remains behind. The Prussian blue 
of commerce, besides other impurities, con- 
tains mixed with it a great quantity of alu- 
mina. 
Yellow prussiat is composed of prussic acid 
combined with an excess of peroxide of iron : 
* it is therefore a sub-prussiat of iron. This 
prussiat is soluble in acids. It may be ob- 
tained by r digesting the alkalies or alkaline 
earths with Prussian blue. Part of the acid 
is carried off by these bodies, and the yellow 
prussiat remains in the state of a powder. 
Green prussiat, first discovered by Mr. 
Berthollet, is composed of oxyprussic acid, 
I and peroxide of iron. It is therefore in fact 
an oxy prussiat. 
4. Prussiat of barytes and iron. For the 
first accurate description of this salt, we are 
indebted to the ingenious Mr. William 
Henry. It may be formed by adding Prus- 
• sian blue to hot" barytes water till it ceases to 
be discoloured. The solution, when tiltred 
and gently evaporated, yields crystals of 
prussiat of barytes and iron. 
These crystals have the figure of rhom- 
boidal prisms : they have a yellow colour, 
I and are soluble in 1920 parts ot cold water, 
i and in about 100 parts of boiling water. In 
a red heat they are decomposed, the acid 
I being destroyed. They are soluble in nitric 
and muriatic acids : sulphuric acid occasions 
i a precipitate of sulphat of barytes. 
5. Prussiat of lime and iron. This salt was 
perhaps first mentioned by. Mr. Hagen; but 
Vox.. II, 
we are indebted to Morveau for the first ac- 
curate account of its properties and prepara- 
tion. 
Upon two parts of Prussian blue of com- 
merce, previously well washed with a suffi- 
cient quantity of boiling water to separate all 
the foreign salts, about 5fi parts of lime-water 
are to be poured, and the mixture must be 
boiled for a short time till the lime is satu- 
rated with the prussic acid, which is known 
by its no longer altering paper stained with 
turmeric : it is then to be tiltred. 
This liquid, which contains the triple prus- 
siat of lime in solution, has a greenish-yellow 
colour: its specific gravity is 1.005; audit 
has an unpleasant bitterish taste. When eva- 
porated to dryness, it yields small crystalline 
grains, soluble without alteration in water. It 
is insoluble in alcohol. 
This triple prussiat may be used with ad- 
vantage* as a test to ascertain the presence of 
metals held in solution. The only impurity 
which it contains is a little sulphat of lime. 
6. Prussiat of potass and iron. This salt, 
known also by the names of Prussian alkali, 
phlogisticated alkali, Prussian test, triple 
prussiat of potass, See. has been chosen by 
chemists as the best combination of prussic 
acid for detecting the presence of metals, and 
more especially for detecting the existence 
of iron. To chemists and mineralogists, it is 
one of the most important instruments ever 
invented ; as, when properly prepared, it is 
capable of indicating whether any metallic 
substance (platinum excepted) is present in 
any solution whatever, and even of pointing 
out the particular metal, and of ascertaining 
its quantity. This it does by precipitating 
the metals from their solution in consequence 
of the insoluble compound which it forms 
with them ; and the colour of the precipi- 
tate indicates the particular metal, while its 
quantity enables us to judge of the propor- 
tion of metallic oxide contained in any solu- 
tion. 
In order to be certain of the accuracy of 
these results, it is necessary to have a Prus- 
sian alkali perfectly pure, and to be certain 
beforehand of the quantity, or rather of the 
proportions, of its ingredients. To obtain a 
test of this kind has been the object of che- 
mists ever since the discoveries, of Macquer 
pointed out its importance. It is to the use 
of impure tests that a great part of the con- 
tradictory results of mineralogical analysis by 
different chemists is to be ascribed. 
The great object of chemists at first was 
to obtain this prussiat entirely free from iron ; 
but their .attempts uniformly failed, because 
the oxide of iron is one of its necessary' com- 
ponent parts. This was first properly pointed 
out by Morveau. 
There are two ways in which this test may 
be rendered impure, besides the introduction 
of foreign ingredients, which it is needless to 
mention, because it is obvious that it must 
be guarded against. 1. There may be a su- 
perabundance of alkali present, or, which is 
the same thing, there may be mixed with the 
Prussian test a quantity of pure alkali : or, 
2. There may be contained in it a quantity 
of yellow prussiat of iron, for which prussiat 
of potass has also a considerable affinity. 
If the Prussian test contains a superabun- 
dance of alkali, two inconveniences follow. 
This superabundant quantity will precipitate 
3 I 
those earthy salts which are liable (o contain an 
excess of acid, and which are only soluble by 
that excess: hence alumina and barytes will 
be precipitated. It is to the use ot impure 
tests of this kind that we owe the opinion, 
that barytes and alumina are precipitated by 
the Prussian alkali, and the consequent theo- 
ries of the metallic nature of these earths. 
This mistake was first corrected by Mey er of 
Steint. 
Another inconvenience arising from the 
superabundance of alkali in the Prussian test 
is, that it gradually decomposes the blue 
prussiat which the test contains, and converts 
it into yellow prussiat. In what manner it 
does this will lie understood, after what has 
been said, without any explanation. 
On the other hand, when the Prussian al- 
kali contains a quantity of yellow prussiat of 
iron, as great inconveniences follow. This 
yellow prussiat has an affinity for prussic acid, 
which, though inferior to that of the potass,, 
is still considerable ; and, on the other hand, 
the potass has a stronger affinity for every 
other acid than for the prussic. When, there- 
fore, the test is exposed to the air, the car- 
bonic acid which the atmosphere always con- 
tains, assisted by the affinity between the 
yellow" prussiat and the prussic acid, decom- 
poses the prussiat of potass in the test, and 
the yellow' prussiat is precipitated in the form 
ot Prussian blue; and every other acid pro- 
duces the same effect. A test of this kind 
w'ould indicate the presence of iron in every 
mixture which contains an acid (for a preci- 
pitation of Prussian blue would appear), and 
could not therefore be employed with any 
confidence. 
To describe the various methods proposed 
by chemists for preparing this salt would be 
unnecessary, as the greater number do not 
answer the purpose intended. The method 
practised by Klaproth, first made known to 
chemists by Westrum, and afterwards de- 
scribed in our language by Kirwan, is con- 
sidered as one of the best. It is as follows: 
Prepare pure potass, by gradually project- 
ing into a large crucible, heated to whiteness, 
a mixture of equal parts of purified nitre and 
crystals of tartar; when the whole is injected, 
let it be kept at a white heat for half an hour, 
to burn off the coal. Detach the alkali thu* 
obtained from the crucible, reduce it to pow- 
der, spread it on a inutile, and expose it to a 
white heat for half an hour. Dissolve it in 
six times its weight of water, and nitre the 
solution while warm. Pour this solution int<* 
a glass receiver, placed in a sand-furnace 
heated to 170 ;1 or 180°; and then gradually 
add the best Prussian blue in powder, inject- 
ing new portions according as the former be- 
come grey, and supplying water as fast as it 
evaporates ; continue until the added por- 
tions are no longer discoloured, then increase 
the heat to 212°, and continue it for half an 
hour. Filtre the ley thus obtained, and sa- 
turate it with sulphuric acid moderately di- 
luted ; a precipitate wilt appear : w r hen' this 
ceases, filtre off the whole, and wash the pre- 
cipitate. Evaporate the filtred liquor to 
about one quarter, and set it by to crystal- 
lize : after a few days, yellowish crystals of a 
cubic or quadrangular form will be found, 
mixed with some sulphat of potass and oxide 
of iron ; pick out the yellowish crystals, lay 
them on blotting-paper, and re-dissolve thG* 
