PRU 
sies, &c. therefore, in order to tlie right 
management of fruit-trees, provision should 
always be made to have a sufficient quanti- 
ty of bearing wood in every part of the 
trees, and at the same time there should 
not be a superfluity of useless branches, 
which would exhaust the strength of the 
trees, and cause them to decay in a few 
years. The reasons for pruning of fruit-trees 
are, 1. To preserve them longer in a vigo- 
rous bearing-state; 2. To render them 
more beautiflil ; and, 3. To cause the fruit 
to be larger and better tasted. 
PRUNUS, in botany, bird cherry-tree, a 
genus of the Icosandria Monogynia class 
and order. Natural order of Pomaceae. 
Rosaceae, Jussieu. Essential character: 
calyx five-cleft, inferior ; petals five ; drupe 
with nut, having the sutures prominent. 
There are thirty-three species. 
PRUSSIAN blue. A rich pigment had 
been known for a considerable time under 
tlie name of Prussian blue. It is prepared 
by drying blood, and mixing three parts 
of the dried residuum with two parts of 
the potash of commerce, and calcining 
the mixture in a crucible by a red heat : 
it is then boiled in successive portions of 
water, which are afterwards mixed toge- 
ther, and concentrated by evaporation. A 
solution is prepared of one part of sul- 
phate of iron, and two parts of alum, and 
to this tlie liquor obtained from the calcined 
blood and alkali is added, as long as any 
precipitate is formed. This precipitate is 
of a green colour, but by washing it with 
a little dilute muriatic acid, it becomes of 
a dark rich blue colour. This forms the 
Prussian blue of commerce. The property 
of forming the colour depends on a peculiar 
principle combined with the alkali; that in 
the formation of the Prussian blue, this is 
transferred to the iron, and that it may be 
again abstracted from it by boiling tlie blue 
in an alkaline solution ; the properties of 
the alkali are thus changed, and it acquires 
the power of again forming the precipitate 
of Prussian blue from a solution of sulphate 
of iron. The reason the precipitate is 
thrown down green, is that the alkali is not 
entirely saturated with the colouring princi- 
ple ; the excess of alkali throws down, 
therefore, a portion of yellow oxide of iron 
from the sulphate, which mingling with 
the blue precipitate, renders it green, and 
the muriatic acid gives the deep blue co- 
lour, by dissolving, and of course removing 
this oxide. See Prussic acid, 
PRUSSIATES, in chemistry, salts form- 
PRU 
ed with the prussic acid. These salts have 
not been attentively examined, on account 
of their want of pdrmanency,, unless they 
are united with some metallic oxide ; but 
the prussiate of potash and iron, which is 
a triple salt, has been used by chemists as 
the best combination of prussic acid for 
detecting the existence of iron. In die- 
mistiy and mineralogy this is a very impor- 
tant substance, as it is capable of indicat- 
ing whether most metallic substances be 
present in any solution whatever, , and of 
pointing out the particular metal, and of 
ascertaining its quantity. This is done by 
precipitating the metals from their solution 
in consequence of the insoluble compound 
which it forms with them. The colour of 
tlie precipitate indicates the particular me. 
tal, while its quantity enables us to judge of 
the proportion of metallic oxide contained 
in any solution. This salt has obtained, at 
different times, the names of Prussian al- 
kali, phlogisticated alkali, Prussian test, &c. 
This salt, though of great importance as a 
test, is of no use whatever, if it be not quite 
pure. There are two ways in which this 
test may be rendered impure, besides the 
introduction of foreign ingredients, which 
it is needless te mention, because it is ob- 
vious that it must be guarded against. 
1. There may be a superabundance of al- 
kali 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 
prussiate of iron, for which prussiate of 
potash has also a considerable affinity. If 
the Prussian test contain a superabundance 
of alkali, two inconveniences follow. This 
superabundant quantity will precipitate 
those earthy salts which are liable to con- 
tain an excess of gcid, and which are only 
soluble by that excess. Hence alumina 
and barytes will be precipitated. Another 
inconvenience arising from the superabun- 
dance of alkali in the Prussian test is, that 
it gradually decomposes the blue prussiate 
which the test contains, and converts it 
into a yellow pru.s.siate. In what manner it 
does this will be understood, after what has 
been said, without any explanation. On 
the other hand, when the Prussian alkali 
contains a quantity of yellow prussiate of 
iron, as great inconveniences follow. This 
yellow prussiate has an affinity for prussic 
acid, which, though inferior to that of the 
potash, is still considerable; and, on the 
other hand, the potash has a stronger affi- 
nity for every other acid than for the piui- 
