The basicity of these acids is deduced from the formulas of double salts. Thus, there are three ortho-phosphates of sodium and hydrogen; they have the formulae H2NaPO4, HNa2PO4, and Na3PO4 ; the hydrogen calcium salts are: H4Ca(PO4)2, HCaPO4, and Ca3(PO4)2. It is therefore argued that since the hydrogen atoms of ortho-phosphoric acid are replaceable in three stages by metals, there are therefore three atoms of hydrogen in the acid. These salts are made by mixture; 2H3PO4.Aq + Na3PO4.Aq = 3H2NaPO4.Aq, and so on. The acid is, therefore, said to be tri-basic. The arsenates are precisely similar ; but only simple vanadates are known, and no ortho-antimonates. A pyrophosphate is known of the formula HK2(NH4)P2O7, which demonstrates the tetra-basic character of pyrophosphoric acid, and the other pyro-acids are classified accordingly.