When a solution of trisodium phosphate is added to a solution of a magnesium salt it gives a white precipitate of the normal orthophosphate Mg_{2}(PO_{4})_{2},7H_{2}O. If the trisodium salt be replaced by the ordinary salt, Na_{2}HPO_{4}, a precipitate is also formed, and MgHPO_{4},7H_{2}O is obtained. It might be thought that the normal salt Mg_{3}(PO_{4})_{2} would be precipitated if disodium phosphate was added to ammonia and a salt of magnesium, but in reality _ammonium magnesium orthophosphate_, MgNH_{4}PO_{4},6H_{2}O, is precipitated as a crystalline powder, which loses ammonia and water when ignited, and gives a pyrophosphate, Mg_{2}P_{2}O_{7}. This salt occurs in nature as the mineral struvite, and in various products of the changes of animal matter. If we consider that the above salt parts with ammonia with difficulty, and that the corresponding salt of sodium is not formed under the same conditions (MgNaPO_{4},9H_{2}O is obtained by the action of magnesia on disodium phosphate), if we turn our attention to the fact that the salts of calcium and barium do not form double salts as easily as magnesium, and remember that the salts of magnesium in general easily form double ammonium salts, we are led to think that this salt is not really a normal, but an acid salt, corresponding with Na_{2}HPO_{4}, in which Na_{2} is replaced by the equivalent group NH_{3}Mg.