CYANOGEN COLOURS .

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potassium, 35*58; cyanogen (carbon, 21*63; nitrogen, 25*54); iron, 17*29.This substance is considered to be either a combination of ferricyanogen( 2 Fe + 6C 2 N = 2 Cfy) and potassium, 2Cfy,3K, or as a combination of cyanideof iron (2Fe,3C 2 N = Fe 2 Cy 3 ) with cyanide of potassium, Fe 2 Cy 3 -j-KCy. It isprepared by submitting the yellow prussiate, either in the state of dry powderor in solution in water (the former is preferred), to the adtion of chlorine gas.As soon as the latter ceases to be absorbed the operation is finished. Thesaline mass is then dissolved in the smallest possible quantity of water, andthe red prussiate crystallises out, chloride of potassium being left:—

2(2KCy + FeCy) + Cl = C 1 K + (Fe 2 Cy 3 + 3KCy).

Yellow Prussiate. Chlorine. Chloride of Red Prussiate.

Potassium.

This salt, which forms beautifully ruby-coloured crystals, its powder beingorange-coloured, is soluble in twice its weight of cold, and about its ownweight of warm water ; it is insoluble in absolute alcohol, and only veryslightly soluble in weaker spirits. The crystals decrepitate on being heated,and yield, on being ignited in closed vessels, cyanogen and a little nitrogen ;while the residue consists of cyanide of potassium, ferrocyanide of potassium,paracyanogen, and carbide of iron. This salt is an energetic oxidising agent,especially in the presence of alkalies ; it converts, for instance, the protoxidesof lead and manganese into peroxides; destroys indigo, converting it intoisatine, on which account it is sometimes used as a discharge (Mercer’sprocess).* Red prussiate is used by dyers for obtaining peculiar shades ofblue. If cotton cloth be passed in the usual way through nitrate of iron, andrinsed, it will make no blue when put into red prussiate ; but if it is afterwardspassed into muriate of tin liquor, it strikes a blue dire&ly, which has a goodshade. Red prussiate is not much used in printing. It is found in somerecipes for dark steam blues, and for a few other colours, such as myrtle andchocolates.

As regards the colours known as Prussian blue we have to distinguish—(1). Neutral Berlin, or Prussian blue, which is obtained when a solution of theferrocyanide of potassium is poured into any solution of a salt of peroxide ofiron, preferably the perchloride ; a very bulky deep blue precipitate is formed,which it is very difficult to free from the ferrocyanide by washing, and tena-ciously retains a portion of that salt. (2). The basic Berlin blue is obtainedwhen a solution of a proto-salt of iron is precipitated by means of a solutionof the ferrocyanide ; the precipitate is at first white, but gradually oxidationtakes place, which may also be induced by oxidising agents, and the result isthe formation of a blue substance, stated to be soluble in water ; this solubility,however, is due to the presence of ferrocyanide of potassium. The Berlinblue of commerce is always a mixture of basic and neutral blue, or, in chemicalterms, a ferrocyanide of iron ; but the material as met with in the trade con-tains variable quantities of inert mineral matter, added for the purpose ofdiluting the colour, or for giving weight, and also as dryers. The substancesso employed are chalk, pipe-clay, and the sulphates of lime and baryta. Ac-cording to M. Pohl, there is met with in the trade a kind of Berlin blue that isadulterated with starch coloured blue by means of iodine.

* Although theoretically an excellent plan, it is not much used in practice, for two reasons,—one of these being its too quick adtion and the difficulty of properly thickening, and theother its expense.

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