Note 6 — Chapter XVIII · SILICON AND THE OTHER ELEMENTS OF THE FOURTH GROUP

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 SiO2 is heated to 300° in a current of chlorine, it easily forms SiCl4 (Gattermann): besides which two other compounds, corresponding to SiCl4, are formed, namely: Si2Cl6, which boils at 145° and solidifies at -1°, and Si3Cl8, which boils at about 212°. These substances, which answer to corresponding carbon compounds (C2H6 and C3H8), act upon water and form corresponding oxygen compounds; for instance, Si2Cl6 + 4H2O = (SiO2H)2 + 6HCl gives the analogue of oxalic acid (CO2H)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, Si2H2O4 (see later, Note 11bis).