PRU 
PSI 
sic. When, therefore, the test is exposed 
to the air, the carbonic acid, which tlie at- 
mosphere always contains, assisted by the 
affinity between the yellow pmssiaie, and 
the prussic acid, decomposes the prussiate 
of potash in the test, and the yellow prtis- 
siate is precipitated in the form of Prussian 
blue, and every other acid produces the 
same etfect. A test of this kind would 
indicate the presence of iron in every mix- 
tiii'e which contains an acid (for a precipi- 
tation of Prussian blue woidd appear), and 
could not therefore be employed with any 
confidence. 
PRUiSSIC acid, in chemistry and the 
arts, is one of the most important of the 
acids. It was discovered by accident about 
the beginning of the last century by Dies- 
bach, a chemist of Berlin. This gentle- 
man, wishing to precipitate a decoction of 
cochineal with an alkali, got some potash 
on which he had distilled several times his 
animal oil, and as there was some sulphate 
of iron in the decoction, the liquor instantly 
exhibited a beautiful bine in the place of a 
red precipitate. Hence he saw the me- 
thod of producing the same substance at 
pleasure, and it soon became an object of 
commerce, and obtained the name of Prus- 
sian blue, from the place where it was dis- 
covered. This substance is now formed, 
chiefly, during the decomposition of animal 
substances in high temperatures. Three 
parts of blood, evaporated to dryness in an 
iron dish, are to be mixed with one part of 
subcarbonate of potash (common pearlash), 
and calcined in a crucible, which should be 
only two-thirds filled by the materials, and 
covered with a lid. The calcination must 
be continued, with a moderate heat, as long 
as a blue flame issues from the crucible ; 
and when it becomes faint, and likely to be 
extinguished, the process must be stopped. 
Throw the mass, when cold, into ten or 
twelve parts of water ; allow it to soak a 
few Itoisrs, and then boil them together in 
an iron kettle. Filter the liquor, and con- 
tinue pouring hot water on the mass as long 
as it acquires any taste. To tins solution 
add one composed of two parts of alum 
and one of sulphate of iron, in eight or 
ten of boiling water, and continue the mix- 
ture as long as any effervescence and preci- 
pitation ensues. Wash the precipitate seve- 
ral times with boiling water. It will have 
a green colour; but, on the addition of a 
quantity of mtiriatic acid, equal to twice 
that of the sulphate of iron which has been 
used, it will assume a beautiful blue colmif. 
Wash is again with water, and dry it in a 
gentle heat. In this state it is the pigment, 
called Prussian blue, which consists of a 
mixture of prussiate of iron with alumine. 
From prussiate of iron, the prussic acid 
may be separated by the following process : 
mix two ounces of red oxide of mercury, 
prepared by nitric acid, with four ounces 
of finely powdered Piussian blue, and boil 
the mixture with twelve ounces of water 
in a glass vessel, shaking frequently. Filter 
the solution, whicli is a prussiate of mer- 
cury, while hot, and when cool, add to it, 
in a bottle, two ounces of iron filings, and 
six or seven drachms of sulphuric acid ; 
shake these together, decant the clear li- 
quor into a retort, and distil oiF one-fourth 
of the liquor. The distilled liquor is the 
prussic acid, which combines with alkalies 
and earths, and has many of the properties 
belonging to the other acids. It has a 
sweetish taste, and a smell resembling that 
of bitter almonds ; it does not redden blue 
vegetable colours. It precipitates sulphu- 
rets, and curdles soap. It separates alu- 
mine from nitric acid. Oxygenized muri- 
atic acid entirely decomposes it. It does 
not appear to have a strong affinity for alka- 
lies, nor does it take them from carbonic 
acid, for no effervescence arises on adding 
it to a solution of alkaline carbonates ; on 
the contrary, its combinations with alka- 
lies and earths are decomposed by expo- 
sure to carbonic acid, even when highly 
diluted, as in atmospheric air. It readily 
combines, however, with pure alkalies, de- 
stroys their alkaline properties, and forms 
crystallizable salts. It does not precipitate 
iron blue, but green, and this green preci- 
pitate is soluble in acids. The rays of 
light render the green precipitate blue, as 
does also the addition of metallic iron, or sul- 
phurous acid. 
PSIDIUM, in botany, guava, a genus 
of the Icosandria Monogynia class and or- 
der. Natural order of Hesperidae. Myrti, 
Jussieu. Essential character: calyx five- 
cleft superior; petals five; berry one-cell- 
ed, many-seeded. There are eight species, 
natives of the East and West Indies. 
PSITTACUS, the parrot, in natural his- 
tory, a genus of birds of the order Piem. 
Generic character : bill hooked, upper man- 
dible moveable; nostrils round in the base 
of the bill, and sometimes covered with a 
cere ; tongue fleshy, broad, and blunt at the 
end ; head large, crown flat ; toes formed 
