The formula of the third anhydride of the di-acid, H₂Z₂O₆, given on the preceding page, is a multiple by two of that of the meta-acid, and it is evident that the fourth anhydride of the tri-acid, the fifth of the tetra-acid, and so on, will all be multiples of the simpler formula of the meta-acid. These acids and some of their salts appear to be known in the case of the phosphoric acids, and what is usually termed " meta-phosphoric acid/' and given the formula HPO₃, is probably the seventh anhydride of hexa-phosphoric acid, H₆P₆O₁₈, for one of its double salts has the formula Na₂Ca₅(P₆O₁₈)₂.

Complex salts are known in the case of almost all these elements. Among such compounds are: H₂N₄O_n, Ag₆P₄O₁₃, Ag₁₂P₁₀O₃₁, Na₆V₆O₁₇; while niobates and tantalates are particularly prone to form such complex salts. Compounds of a complicated kind, too, in which one of these elements replaces another partially, have been made; as an example, K₍.(P₈V₁₂)O₅₀.2iH₂O may be instanced. They are made by mixture.

The source of the nitrates is the attack of ammonium salts resulting from the decomposition of urea (the chief form in which all animals part with the nitrogen they absorb as a constituent of their food) by a bacterium named the ^M nitrate ferment." This organism exists only in the dark ; it is an inhabitant of the soil, and causes the oxidation of ammonia, from whatever source, into a nitrate. As potash and lime are the commonest bases in the soil, nitrates of potassium and calcium are the chief compounds formed. Vast tracts of country in Chili and Peru contain beds of sodium nitrate, possibly formed by the attack of the debris of previous generations of animal organisms by the nitrate ferment. By distilling a mixture of sodium or potassium nitrate with sulphuric acid in glass vessels, or, on a large scale, in iron vessels on which concentrated nitric acid is without action, nitric acid comes over into the receiver: NaNO₃ + H₂SO₄ - HNO₃ + HNaSO₄. It is not economical to use the equivalent quantity of sulphuric acid, for the reaction between hydrogen sodium sulphate and sodium nitrate takes place at a temperature so high that much of the nitric acid is decomposed : 4HNO₃ = 2H„0 + 4NO₂ + O₂. Nitric acid is a colourless fuming liquid, with very corrosive properties. In aqueous solution it is one of the strongest acids, for it is highly ionised. It is also a powerful oxidising agent; this has been referred to on p. 97. Its anhydride, N₂O₅, is produced on distilling a mixture of nitric acid with phosphoric anhydride; the distillate separates into two layers ; the upper one consists mainly of N₂O₅, and the anhydride separates in crystals when it is cooled; the lower layer contains H₂N₄O_n, a liquid solidifying at - 5⁰; it is the last anhydride of tetra-nitric acid. The anhydride decomposes spontaneously in a few days into the peroxide, N₂O₄, and oxygen.