_Silicon chloride_, SiCl_{4}, is obtained from amorphous anhydrous silica (made by igniting the hydrate) mixed with charcoal,[6] heated to a white heat in a stream of dry chlorine--that is, by that general method by which many other chloranhydrides having acid properties are obtained. Silicon chloride is purified from free chlorine by distillation over metallic mercury. Free silicon forms the same substance when treated with dry chlorine. It is a volatile colourless liquid, which boils at 59° and has a specific gravity of 1·52. It fumes strongly in air, has a pungent smell, and in general has the characteristic properties of the acid chloranhydrides. It is completely decomposed by water, forming hydrochloric acid and silicic acid, according to the equation: SiCl_{4} + 4H_{2}O = Si(OH)_{4} + 4HCl.[7] [6] The amorphous silica is mixed with starch, dried, and then charred by heating the mixture in a closed crucible. A very intimate mixture of silica and charcoal is thus formed. In Chapter XI., Note 13, we saw that elements like silicon disengage more heat with oxygen than with chlorine, and therefore their oxygen compounds cannot be directly decomposed by chlorine, but that this can be effected when the affinity of carbon for oxygen is utilised to aid the action. When the mass obtained by the action of Mg upon SiO_{2} is heated to 300° in a current of chlorine, it easily forms SiCl_{4} (Gattermann): besides which two other compounds, corresponding to SiCl_{4}, are formed, namely: Si_{2}Cl_{6}, which boils at 145° and solidifies at -1°, and Si_{3}Cl_{8}, which boils at about 212°. These substances, which answer to corresponding carbon compounds (C_{2}H_{6} and C_{3}H_{8}), act upon water and form corresponding oxygen compounds; for instance, Si_{2}Cl_{6} + 4H_{2}O = (SiO_{2}H)_{2} + 6HCl gives the analogue of oxalic acid (CO_{2}H)_{2}. This substance is insoluble in water, decomposes under the action of friction and heat with an explosion, and should be called _silico-oxalic acid_, Si_{2}H_{2}O_{4} (_see_ later, Note 11 ^{bis}).