PH_{3} PCl_{3} PCl_{5} P_{2}O_{3} P_{2}O_{5} AsH_{3} AsCl_{3} AsCl_{5} As_{2}O_{3} As_{2}O_{5} SbH_{3} SbCl_{3} SbCl_{5} Sb_{2}O_{3} Sb_{2}O_{5} .... BiCl_{3} BiCl_{5} Bi_{2}O_{3} Bi_{2}O_{5} In the case of phosphorus, arsenic, and antimony the oxides are acid anhydrides. Salts of at least four acids of each of these three elements are known, the free acid in some instances being unstable. The relation of these acids to the corresponding anhydrides may be illustrated as follows, phosphorus being taken as an example: P_{2}O_{3} + 3H_{2}O = 2H_{3}PO_{3} (phosphorous acid). P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4} (phosphoric acid). P_{2}O_{5} + 2H_{2}O = H_{4}P_{2}O_{7} (pyrophosphoric acid). P_{2}O_{5} + H_{2}O = 2HPO_{3} (metaphosphoric acid). PHOSPHORUS ~History.~ The element phosphorus was discovered by the alchemist Brand, of Hamburg, in 1669, while searching for the philosopher's stone. Owing to its peculiar properties and the secrecy which was maintained about its preparation, it remained a very rare and costly substance until the demand for it in the manufacture of matches brought about its production on a large scale. ~Occurrence.~ Owing to its great chemical activity phosphorus never occurs free in nature. In the form of phosphates it is very abundant and widely distributed. _Phosphorite_ and _sombrerite_ are mineral forms of calcium phosphate, while _apatite_ consists of calcium phosphate together with calcium fluoride or chloride. These minerals form very large deposits and are extensively mined for use as fertilizers. Calcium phosphate is a constituent of all fertile soil, having been supplied to the soil by the disintegration of rocks containing it. It is the chief mineral constituent of bones of animals, and bone ash is therefore nearly pure calcium phosphate. ~Preparation.~ Phosphorus is now manufactured from bone ash or a pure mineral phosphate by heating the phosphate with sand and carbon in an electric furnace. The materials are fed in at M (Fig. 70) by the feed screw F. The phosphorus vapor escapes at P and is condensed under water, while the calcium silicate is tapped off as a liquid at S. The phosphorus obtained in this way is quite impure, and is purified by distillation. [Illustration: Fig. 70] ~Explanation of the reaction.~ To understand the reaction which occurs, it must be remembered that a volatile acid anhydride is expelled from its salts when heated with an anhydride which is not volatile. Thus, when sodium carbonate and silicon dioxide are heated together the following reaction takes place: Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}. Silicon dioxide is a less volatile anhydride than phosphoric anhydride (P_{2}O_{5}), and when strongly heated with a phosphate the phosphoric anhydride is driven out, thus: Ca_{3}(PO_{4})_{2} + 3SiO_{2} = 3CaSiO_{3} + P_{2}O_{5}.