Derivatives of nitrogen, phosphorus, sulphur, and even of iodine and of oxygen, containing hydrocarbon groups, are however known, which are true bases, though weak ones. If ammonia in alcoholic solution be heated with excess of methyl iodide, tetra- methyl -ammonium iodide is formed: NH₃ + 4CH3I = N(CH₃)₄I + 3HI. This iodide, digested with water and silver hydroxide, exchanges iodine for hydroxyl, and after removal of the silver iodide by filtration the solution may be evaporated to dryness. The residue is a white solid, of the formula N(CH₃)₄OH ; it is termed tetra-methyl-ammonium-hydroxide; in its reactions it shows great analogy with caustic potash, having a caustic taste, and producing precipitates with the usual salts of the metals. In solution it is more ionised than ammonium hydroxide, though less than that of potassium.

Phosphine, as remarked on p. 66, combines with hydrogen iodide, forming a salt, PH₄I, phosphonium iodide, resembling ammonium chloride. But as it is decomposed by water into phosphine, PH₃, and hydrogen iodide, an attempt to convert it into phosphonium hydroxide, PH₄OH, cannot be made. Substituted phosphonium compounds, however, are known, in which a hydrocarbon radicle, such as methyl, replaces hydrogen. Sodium and phosphorus combine when heated together under an oil called xylene, forming PNa₃ ; this body, treated with methyl iodide, yields trimethyl phosphine, P(CH₃)₃; with more methyl iodide P(CH₃)₄I is formed; and its solution in water, which is not decomposed by the solvent, yields with silver hydroxide tetra-methyl-phosphonium hydroxide, P(CH₃)₄OH, a base resembling the corresponding ammonium compound.

These compounds exist owing to the double valency of nitrogen and of phosphorus, which can function either as triad or pentad. Double valency is to be noticed also with oxygen and with sulphur, although with the former tetrad combinations are far from stable, while with the latter both dyad and tetrad compounds can be formed.