The theory of the formation of this salt is, that one eq. of chlorine withdraws from two eq. of the ferrocyanide of potassium, one eq. of potassium, forming chloride of potassium, which remains in the mother liquid. The reaction is explained by the following equation: 2(K_{2}Cfy)+Cl=K_{3}Cfy_{2}+KCl. The radical ferridcyanogen, isomeric[2] with ferrocyanogen, is supposed to be formed by the coalescence of two equivalents of ferrocyanogen, and is represented by the symbol Cfdy; accordingly the formula of ferridcyanide of potassium is K_{3}Cfdy. [Footnote 2: Isomeric bodies, or substances different in properties yet identical in composition, are of constant occurrence in organic chemistry, and stand among its most peculiar features.] Ferridcyanide of potassium has found extensive application in photographic processes for intensifying negatives; those of Eder, in combination with nitrate of lead, or Selle's, with nitrate of uranium; Ander's blue intensification of gelatine negatives, Farmer's process of reducing intensity, the coloring of diapositives, the very important blue printing, and various others, are daily practiced in our laboratories. The ferrocyanide of potassium is a chemical reagent of great value, giving rise to precipitates with the neutral or slightly acid solutions of metals, like the beautiful brown ferrocyanide of copper, and that of lead. When a ferrocyanide is added to a solution of a sesquioxide of iron, Prussian blue or ferrocyanide of iron is produced. The exact composition of this remarkable substance is not distinctly stated, as various blue compounds may be precipitated under different circumstances. Berzelius gives the following account: 3 eq. of ferrocyanide and 2 eq. of sesquioxide of iron are mutually decomposed, forming 1 eq. of Prussian blue and 6 eq. of the potassa salt, which remains in solution, or 3K_{2}Cfy + 2(Fe_{2}O_{3}3NO_{3}) = Fe_{4}Cfy_{3} + 6(KO,NO_{5}). It forms a bulky precipitate of an intense blue, is quite insoluble in water or weak acids, with the exception of oxalic acid, with which it gives a deep blue liquid, occasionally used as blue ink. Ferridcyanide of potassium, added to a salt of the sesquioxide of iron, yields no precipitate, but merely darkens the reddish-brown solution; with protoxide of iron it gives a blue precipitate, containing Fe_{3}Cfdy, which is of a brighter tint than that of Prussian blue, and is known by the name of Turnbull's blue. Hence, the ferridcyanide of potassium is as excellent a test for protoxide of iron as the yellow ferrocyanide is for the sesquioxide.--_E., Photo. Times_. * * * * * FOUCAULT'S APPARATUS FOR MANUFACTURING ILLUMINATING GAS AND HYDROGEN. The illuminating gas and hydrogen apparatus, illustrated herewith, is adapted to all cases in which it is desirable to manufacture gas upon a small scale.