PHOSPHORIC add. When phospho• tan undergoes combustion in oxygen gas, a great quantity of white fumes arepro duced, which are deposited in white flakes. These are phosphoric acid ; so that it is a compound of phosphorus and oxygen. The phosphoric acid was first shewn to be distinct from all other acids, in the year 1743, by Margraaff. He found that it existed in the salts which were taken from human urine, and that phos phorus could only be obtained from this acid ; as well as that it could be con verted into phosphoric acid. This acid was found to exist in some vegetable sub stances, although it was formerly suppos ed to be peculiar to animal matters. Phosphoric acid may be obtained, not on ly by the method just mentioned, but also by transmitting a current of oxygen gas. through phosphorus melted under water: The acid, as it is formed, combines with the water, from which it may be obtained in a state of purity by evaporation. The
specific gravity of this acid varies accord. ing to the different states in which it ex ists. In the liquid state it is 1.4; in the dry state it is 2.7 ; in the state of glass 2.85. It changes the colour of vegetable blues to red ; has no smell, but a very acid taste. When it is exposed to the air it attracts moisture, and is converted into a thick viscid fluid, like oil. It is very soluble in water. When in the form of dry flakes, it dissolves in a small quantity of this liquid, producing a hissing noise like that of a red-hot iron plunged into water, with the extrication of a great quantity of heat. The component parts of this acid have been accurately as. certained by Lavoisier, and it consists of, Oxygen 60 Phosphorus 40 100 It combines with the alkalies, earths, and metallic oxides, and forms salts which are denominated phosphates.