A few compounds of phosphorus, arsenic, and antimony with metals have been made. They are generally obtained by direct union between the heated metal and the element. Thus, if sodium and phosphorus be heated together under xylene, a hydrocarbon boiling about 130°, a black compound is formed, Na₃P, from which excess of phosphorus can be dissolved out by treatment with carbon disulphide. Arsenide and antimonide of sodium are also obtained by heating the elements together. The formulae of these compounds are of the type AsNa₃; and with dilute acid, the corresponding hydride of phosphorus, arsenic or antimony is evolved : AsNa₃ + 3HCl.Aq = AsH₃ + 3NaCl.Aq. A mixture of calcium phosphide, Ca₃P₂, with calcium pyrophosphate is produced on throwing phosphorus into a crucible containing red-hot lime ; on treatment with water, spontaneously inflammable phosphine is evolved. The spontaneous ignition is due to its containing P₂H₄, a liquid, very unstable compound.

The phosphides, arsenides, and antimonides of the other metals are usually dark-coloured substances, with more or less metallic lustre, and therefore conductors of electricity. Some of them occur native; for example, smaltine, CoAs₂, a common ore of cobalt, forming silver-white crystals; copper-nickel, NiAs, red lustrous crystals, and one of the chief nickel ores ; speiss, a deposit formed in the pots in which smaltine and copper-nickel are fused with potassium carbonate and silica, in the preparation of smalt, a blue glass containing cobalt; its formula appears to be Ni₃As₂. Mispickel, or arsenical pyrites, is a white lustrous substance, of the formula FeSAs.

Cyanides,-The elements carbon and nitrogen form a very stable group, of which the compounds have been well investigated, termed cyanogen. Carbon and nitrogen do not unite directly ; but if a mixture of finely divided carbon with carbonate of potassium or sodium, or, better, of barium, be heated to about 1200⁰ in a current of nitrogen, combination ensues, and a cyanide is formed, KCN, NaCN, or Ba(CN)₂; BaCO₃ + 4C + N₂ = Ba(CN)₂ + 3CO. Potassium cyanide is also produced when a mixture of animal refuse (horns, hides, hair, dried entrails, etc., of animals) with potassium carbonate and iron filings is heated. The nitrogen of the animal matter and the carbon unite with the potassium of the carbonate, forming cyanide. On addition of water, this cyanide reacts with salts of iron, forming a double cyanide of iron and potassium, termed "yellow prussiate of potash," or ferrocyanide of potassium, of the formula K₄Fe(CN)_G. When this compound is heated to dull redness, it fuses ; a black mixture or compound of iron and carbon remains, and melted potassium cyanide can be poured out of the crucible. Potassium cyanide, KCN, is a very soluble salt; it crystallises well from alcohol. Its solution smells of hydrocyanic acid; this is because it is hydrolysed by water. The acid, HCN, is so very weak that the number of hydrogen ions present in its solution are comparable in number with those of ionised water ; hence the change: H|-OH + K|-CN. Aq = HCN + K|-OH.Aq. The ionised portion of the hydrocyanic acid is as usual non-volatile; but the non-ionised portion has a vapour-pressure, and can be detected by its smell (cf. p. 141).