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These crystals, by careful analysis, were shown, first by Hilgenstock, to consist of a form of phosphate of lime hitherto unknown, in which four equivalents of lime were combined with one equivalent of phosphoric acid, and which was therefore called "tetrabasic phosphate."

The discovery that its phosphoric acid existed, as has been already explained, as a tetrabasic phosphate of lime, has strengthened the opinion that this is the best method of application.

Lastly, there is a fourth compound of lime and phosphoric acid, which only occurs in one phosphatic manure—viz., phosphatic slag, in which indeed it was first discovered—which consists of four equivalents of lime to one of phosphoric acid, to which the name tetrabasic phosphate of lime or tetracalcic phosphate has been given.

Similarly, we may have tribasic and tetrabasic acids.