3. _Pyrophosphoric acid_ (H_{4}P_{2}O_{7}). On heating orthophosphoric acid to about 225 deg. pyrophosphoric acid is formed in accordance with the following equation: 2H_{3}PO_{4} = H_{4}P_{2}O_{7} + H_{2}O. It is a white crystalline solid. Its salts can be prepared by heating a secondary phosphate: 2Na_{2}HPO_{4} = Na_{4}P_{2}O_{7} + H_{2}O. 4. _Metaphosphoric acid (glacial phosphoric acid)_ (HPO_{3}). This acid is formed when orthophosphoric acid is heated above 400 deg.: H_{3}PO_{4} = HPO_{3} + H_{2}O. It is also formed when phosphorus pentoxide is treated with cold water: P_{2}O_{5} + H_{2}O = 2HPO_{3}. It is a white crystalline solid, and is so stable towards heat that it can be fused and even volatilized without decomposition. On cooling from the fused state it forms a glassy solid, and on this account is often called glacial phosphoric acid. It possesses the property of dissolving small quantities of metallic oxides, with the formation of compounds which, in the case of certain metals, have characteristic colors. It is therefore used in the detection of these metals. While the secondary phosphates, on heating, give salts of pyrophosphoric acid, the primary phosphates yield salts of metaphosphoric acid. The equations representing these reactions are as follows: 2Na_{2}HPO_{4} = Na_{4}P_{3}O_{7} + H_{2}O, NaH_{2}PO_{4} = NaPO_{3} + H_{2}O. ~Fertilizers.~ When crops are produced year after year on the same field certain constituents of the soil essential to plant growth are removed, and the soil becomes impoverished and unproductive. To make the land once more fertile these constituents must be replaced. The calcium phosphate of the mineral deposits or of bone ash serves well as a material for restoring phosphorus to soils exhausted of that essential element; but a more soluble substance, which the plants can more readily assimilate, is desirable. It is better, therefore, to convert the insoluble calcium phosphate into the soluble primary phosphate before it is applied as fertilizer. It will be seen by reference to the formulas for the orthophosphates (see page 244) that in a primary phosphate only one hydrogen atom of phosphoric acid is replaced by a metal. Since the calcium atom always replaces two hydrogen atoms, it might be thought that there could be no primary calcium phosphate; but if the calcium atom replaces one hydrogen atom from each of two molecules of phosphoric acid, the salt Ca(H_{2}PO_{4})_{2} will result, and this is a primary phosphate. It can be made by treatment of the normal phosphate with the necessary amount of sulphuric acid, calcium sulphate being formed at the same time, thus: Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = Ca(H_{2}PO_{4})_{2} + 2CaSO_{4}. The resulting mixture is a powder, which is sold as a fertilizer under the name of "superphosphate of lime." ARSENIC