The question _of the composition of silica_ was long the subject of the most contradictory statements in the history of science. In the last century Pott, Bergmann, and Scheele distinguished silica from alumina and lime. In the beginning of the present century Smithson for the first time expressed the opinion that silica was an acid, and the minerals of rocks salts of this acid. Berzelius determined the presence of oxygen in silica--namely, that 8 parts of oxygen were united with 7 of silicon. The composition of silica was first expressed as SiO (and for the sake of shortness S only was sometimes written instead). An investigation in the amount of silica present in crystalline minerals showed that the amount of oxygen in the bases bears a very varied proportion to the amount of oxygen in the silica, and that this ratio varies from 2 : 1 to 1 : 3. The ratio 1 : 1 is also met with, but the majority of these minerals are rare. Other more common minerals contain a larger proportion of silica, the ratio between the oxygen of the bases and the oxygen of the silica being equal to 1 : 2, or thereabouts; such are the augites, labradorites, oligoclase, talc, &c. The higher ratio 1 : 3 is known for a widely distributed series of natural silicates--for example, the felspars. Those silicates in which the amount of oxygen in the bases is equal to that in the silica are termed _monosilicates_; their general formula will be (RO)_{2}SiO_{2} or (R_{2}O_{3})_{2}(SiO_{2})_{3}. Those in which the ratio of the oxygen is equal to 1 : 2 are termed _bisilicates_, and their general formula will be ROSiO_{2} or R_{2}O_{3}(SiO_{2})_{3}. Those in which the ratio is 1 : 3 will be _trisilicates_, and their general formula (RO)_{2}(SiO_{2})_{3} or (R_{2}O_{3})_{2}(SiO_{2})_{9}. In these formulæ the now established composition of SiO_{2}--that is, that in which the atom of Si = 28--is employed. Berzelius, who made an accurate analysis of the composition of felspar, and recognised it as a trisilicate formed by the union of potassium oxide and alumina with silica, in just the same manner as the alums are formed by sulphuric acid, gave silica the same formula as sulphuric anhydride--that is, SiO_{3}. In this case the formula of felspar would be exactly similar to that of the alums--that is, KAl(SiO_{4})_{2}, like the alums, KAl(SO_{4})_{2}. If the composition of silica be represented as SiO_{3}, the atom of silicon must be recognised as equal to 42 (if O = 16; or if O = 8, as it was before taken to be, Si = 21).