PRUSSIAN BLUE. This salt is made by adding solution of a salt of iron to a solution of prussiate of potash. Green sul phate of iron is always employed by the manufacturer, on account of its cheapness, for mixing with solution of the ferroprus siate, in forming Prussian blue, though the red sulphate, nitrate, or muriate of iron would afford a much richer blue pigment. Whatever salt of iron be pre ferred, should be carefully freed from any cupreous impregnation, as this would give ihe pure blue a dirty brownish cast. The green sulphate of iron is the most advantageous precipitant, on account of its affording protoxide, to convert into ferrocyanide any cyanide of potassium that may happen to be present in the uncrystallized lixivium. The carbonate of potash in that iixivium might be satu rated with sulphuric acid before adding the solution of sulphate of iron ; but it is more commonly done by adding a certain portion of alum ; in which case, alumina falls along with the Prussian blue ; and though it renders it somewhat paler, yet • it proportionally increases its weight ; whilst the acid of the alum saturates the carbonate of potash, and prevents its throwing down iron-oxide, to degrade by its brown-red tint the tone of the blue. For every pound of pearlash used in the calcination, from two to three pounds of alum are employed in the pre cipitation. When a rich blue is wished for, the free alkali in the Prussian ley may be partly saturated with sulphuric acid, before adding the mingled solutions of copperas and alum. One part of the sulphate of iron is generally allowed for 15 or 20 parts of dried blood, and 2 or 8 of horn-shavings or hoofs. But the pro portion will depend very much upon the manipulations, which, if skilfully con ducted, will produce more of the cyan ides of iron, and require more copperas to neutralize them. The mixed solutions of alum and copperas should be progres sively added to the ley as long as they produce any precipitate. This is not at first a fine blue, but a greenish gray, in consequence of the admixture of some white cyanide of iron ; it becomes gradu ally blue by the absorption of oxygen from the air, which is favored by agita tion of the liquor. Whenever the color seems to be as beautiful as it is likely to become, the liquor is to be run off by a spigot or cock from the bottom of the precipitation vats, into flat cisterns, to settle. The clear supernatant fluid, which is chiefly a solution of potash, is then drawn off by a syphon ; more water is run on with agitation to wash it, which after settling is again drawn off; and whenever the washings become tasteless, the sediment is thrown upon filter sieves, and exposed to dry, first in the air of a stove, but finally upon slabs of chalk or Paris plaster. But for several purposes, Prussian blue may be best employed in the fresh pasty state, as it then spreads more evenly over paper and other sur faces.
A good article is known by the follow ing tests : it feels light in the hand, adheres to the tongue, has a dark lively blue color, and gives a smooth deep trace; it should not effervesce with acids, as when adulterated with chalk ; nor be come pasty with boiling water, as when adulterated with starch. The Paris blue, prepared without alum, with a peroxide salt of iron, displays, when rubbed, a copper-red lustre, like indigo. Prussian blue, degraded in its color by an admix ture of free oxide of iron, may be im proved by digestion in dilute sulphuric or muriatic acid, washing, and cluing. Its relative richness in the real ferroprus siate of iron may be estimated by the quantity of potash or soda which a given quantity of it requires to destroy its blue color.
Sulphureted hydrogen passed through Prussian blue diffused in water, whitens it ; while prussic acid is eliminated, sul phur is thrown down, and the sesquicy snide of iron is converted into the single cyanide. Iron and tin operate in the same way. When Prussian blue is made with two atoms of ferrocyanide of potas sium, instead of one, it becomes soluble in water. The following process deserves peculiar notice, as the first in which this interesting compound has been made to any extent, independently of animal matter. Mr. Lewis Thompson, of Lam beth, received a well-merited medal from the Society of Arts for this invention. He observed that in the common way of manufacturing prussiate of potash, the quantity of nitrogen furnished by a given weight of animal matter is not large, and seldom exceeds 8 por cent.; and of this small quantity, at least one half appears to be dissipated during the ignition. It occurred to him that the atmosphere might be economically made to supply the requisite nitrogen, if caused to act in favorable circumstances upon a mixture of carbon and potash. He has found the following prescription to answer : Take of pearlash and coke, each two parts ; iron turnings, one part ; grind them to gether into a coarse powder; place this in an open crucible, and expose the whole for half an hour to a full red heat in an open fire, with occasional stirring of the mixture. During this process, little jets of purple, flame will be observed to rise from the surface of the materials. When these cease, the crucible must be removed and allowed to cool. The mass is to be lixiviated ; the lixivium, which is a solution of ferrocyanide of potassium, with excess of potash, is to be treated in the usual way, and the black matter act aside for fresh operation with a fresh dose of pearlash. Mr. Thompson states that one pound of pearlash, containing 45 per cent. of alkali, yielded. 1,355 grains of pure Prussian blue, or ferrocyanide of iron ; or about 3 ounces avoirdupoise.