Carbon, which gives the compounds CH_{4}, and CO_{2}, belongs to the fourth group of elements. The nearest element to carbon is silicon, which forms the compounds SiH_{4} and SiO_{2}; its relation to carbon is like that of aluminium to boron or phosphorus to nitrogen. As carbon composes the principal and most essential part of animal and vegetable substances, so is silicon almost an invariable component part of the rocky formations of the earth's crust. Silicon hydride, SiH_{4}, like CH_{4}, has no acid properties, but silica, SiO_{2}, shows feeble acid properties like carbonic anhydride. In a free state silicon is also a non-volatile, slightly energetic non-metal, like carbon. Therefore the form and nature of the compounds of carbon and silicon are very similar. In addition to this resemblance, silicon presents one exceedingly important distinction from carbon: namely, the nature of the higher degree of oxidation. That is, silica, silicon dioxide, or silicic anhydride, SiO_{2} is a solid, non-volatile, and exceedingly infusible substance, very unlike carbonic anhydride, CO_{2}, which is a gas. This expresses the essential peculiarity of silicon. The cause of this distinction may be most probably sought for in the polymeric composition of silica compared with carbonic anhydride. The molecule of carbonic anhydride contains CO_{2}, as seen by the density of this gas. The molecular weight and vapour density of silica, were it volatile, would probably correspond with the formula SiO_{2}, but it might be imagined that it would correspond to a far higher atomic weight of Si_{_n_}O_{2_n_}, principally from the fact that SiH_{4} is a gas like CH_{4}, and SiCl_{4} is a liquid and volatile, boiling at 57°--that is, even lower than CCl_{4}, which boils at 76°. In general, analogous compounds of silicon and carbon have nearly the same boiling points if they are liquid and volatile.[1] From this it might be expected that silicic anhydride, SiO_{2}, would be a gas like carbonic anhydride, whilst in reality silica is a hard non-volatile substance,[1 bis] and therefore it may with great certainty be considered that in this condition it is polymeric with SiO_{2}, as on polymerisation--for instance, when cyanogen passes into paracyanogen, or hydrocyanic acid into cyanuric acid (Chapter IX.)--very frequently gaseous or volatile substances change into solid, non-volatile, and physically denser and more complex substances.[2] We will first make acquaintance with free silicon and its volatile compounds, as substances in which the analogy of silicon with carbon is shown, not only in a chemical but also in a physical sense.[3]