4. Phosphoric Acid.—This acid exists largely in nature, not only as has been already mentioned, in combination with lime, forming bone, but also in some vegetable products, and often in the bowels of the earth, combined with lime, forming a mineral which, when crystallised, is frequently called apatite. [See also PIIOSPITATITII, in IstaT. Hier. Div.) It also occurs in combination with oxide of iron, copper, lead, manganese, and uranium ; hut the quantity of these compounds is by no means large, and they are regarded chiefly as objects of curiosity.
Phosphoric Acid may be artificially formed by the direct combination of its elements. When a piece of phosphorus is inflamed, and imme diately covered by a large bell glass, white flakes of phosphoric acid are produced, which fall like snow in the vessel. When exposed for a short time to the air, the acid deliquesces. So great is its affinity for water, that when collected, and a little water added to it, it is converted Into a hydrate with explosive ebullition, owing to the heat which is evolved during combination. Phosphoric acid, when free from water, may be volatilised in a glass tube by the heat of a spirit-flame ; the hydrate is even more readily dissipated.
Phosphoric acid 13 also formed when phosphorus Is heated In mode rately strong nitric acid ; the nitric acid is decomposed, and yields oxygen to form the phosphoric acid. By evaporation in a pLatina capsule, hydrated phosphoric acid is obtained.
Phosphoric acid is colourless, inodorous, dense, extremely sour to the taste, and acts strongly on vegetable blue colours ; it does nut, however, like sulphuric acid, destroy the akin when applied to it. Phosphoric acid is peculiarly disposed to combine with different pro. portions of water, sud these compounds exhibit properties so different, that they might be supposed to be three different acids, instead of different hydrates of the same acid. When the dry acid, obtained as described from the combustion of phosphorus, is thrown into water, a mixture of the three hydrates la obtained in variable proportions ; but they may be separately obtained in a pure state from the common phosphate of soda of the shops, after it has been purified by solution and recrystallisation. Decompose a warm solution of this salt by means of a solution of acetate of lead, and wash the precipi tated phosphate of lead, and then pass a current of hydrosulphuric acid through it while suspended in water. When the excess of hydro
sulphuric acid has been expelled by heat. a very sour fluid remains, which is a terhydrafe of phosphoric add (3110, P0,1„ This terhydrated acid is unalterable by boiling its solution or keeping it for any length of time. The class of salts which it forms are phosphates, which give a yellow precipitate with nitrate of silver (3AgO, P0,). Common phosphate of soda contains, therefore, this terhydrated phosphoric acid.
PyrophoThorie Acid.—Professor Clark, of Aberdeen, first discovered that when common phosphate of soda is heated to redness, it is com pletely changed in some of its properties, and after being dissolved in water, it affords crystals of a new salt, which he named pyrophosphate of soda (2Na0,P0,). It gives a white precipitate with nitrate of silver (2AgO, P0,), instead of a yellow one. If a solution of this salt be decomposed by one of acetate of lead, and the precipitated phosphate of lead be treated with hydresulphuric acid as already described, and then evaporated in vacua, over sulphuric acid, without the application of heat, the remaining solution is pyrophosphoric acid, consisting of 2110, PO,.
lifetaphosphorie Acid.—If equivalent proportions of ordinary phos phate of soda (2Na0, HO, and terhydrated phosphoric acid (3110, P0,) be evaporated to dryness, a tribasic phosphate of soda is obtained, containing (N110,2E10,1'00, and this, on being treated as the last, gives an acid liquor, containing the metaphosphoric or monohydrated phosphoric acid (HO, P0,1.
Olacial phosphoric acid is also in general mostly metaphosphorie acid. This hydrate is characterised by producing a white precipitate in solution of albumen, and in solutions of the salts of earths and metallic oxides ; precipitates which are remarkable semifluid bodies, or soft solids without crystallisation.
Hydrogen and Phosphorus combine in three different proportions. When phosphide of calcium is thrown into hot water containing hydro chloric acid, a yellow powder precipitates, which is a solid phosphide of hydrogen, containing P,H.