[1] Chloroform, CHCl_{3}, boils at 60°, and silicon chloroform, SiHCl_{3}, at 34°; silicon ethyl, Si(C_{2}H_{5})_{4}, boils at about 150°, and its corresponding carbon compound, C(C_{2}H_{5})_{4}, at about 120°; ethyl orthosilicate, Si(OC_{2}H_{5})_{4}, boils at 160°, and ethyl orthocarbonate, C(OC_{2}H_{5})_{4}, at 158°. The specific volumes in a liquid state--that is, those of the silicon compounds--generally are slightly greater than those of the carbon compounds; for example, the volumes of CCl_{4} = 94, SiCl_{4} = 112, CHCl_{3} = 81, SiHCl_{3} = 82, of C(OC_{2}H_{5})_{4} = 186, and Si(OC_{2}H_{5})_{4} = 201. The corresponding salts have also nearly equal specific volumes; for example, CaCO_{3} = 37, CaSiO_{3} = 41. It is impossible to compare SiO_{2} and CO_{2}, because their physical states are so widely different. [1 bis] But silica fuses and volatilises (Moissan) in the heat of the electric furnace, about 3000°, SiO_{2} is also partially volatile at the temperature attained in the flame of detonating gas (Cremer, 1892). [2] A property of intercombination is observable in the atoms of carbon, and a faculty for intercombination, or polymerisation, is also seen in the unsaturated hydrocarbons and carbon compounds in general. In silicon a property of the same nature is found to be particularly developed in silica, SiO_{2}, which is not the case with carbonic anhydride. The faculty of the molecules of silica for combining both with other molecules and among themselves is exhibited in the formation of most varied compounds with bases, in the formation of hydrates with a gradually decreasing proportion of water down to anhydrous silica, in the colloid nature of the hydrate (the molecules of colloids are always complex), in the formation of polymeric ethereal salts, and in many other properties which will be considered in the sequel. Having come to this conclusion as to the polymeric state of silica since the years 1850-1860, I have found it to be confirmed by all subsequent researches on the compounds of silica, and, if I mistake not, this view has now been very generally accepted. [3] It was only after Gerhardt, and in general subsequently to the establishment of the true atomic weights of the elements (Chapter VII.), that a true idea of the atomic weight of silicon and of the composition of silica was arrived at from the fact that the molecules of SiCl_{4}, SiF_{4}, Si(OC_{2}H_{5})_{4}, &c., never contain less than 28 parts of silicon.