[20] This is in accordance with the generally-accepted representation of the relations between salts and the hydrates of acids, but it is of little help in the study of siliceous compounds. Generally speaking, it becomes necessary to explain the property of (SiO_{2})_{_n_} to combine with (RO)_{_m_}, where _n_ may be greater than _m_, and where R may be H_{2}, Ca, &c. Here we are aided by those facts which have been attained by the investigation of carbon compounds, especially with respect to glycol. Glycol is a compound having the composition C_{2}H_{6}O_{2}, only differing from alcohol, C_{2}H_{6}O, by an extra atom of oxygen. This hydrate contains two hydroxyl groups, which may be successively replaced by chlorine, &c. Hence the composition of glycol should be represented as C_{2}H_{4}(OH)_{2}. It has been found that glycol forms so-called polyglycols. Their origin will be understood from the fact that glycol as a hydrate has a corresponding anhydride of the composition C_{2}H_{4}O, known as ethylene oxide. This substance is ethane, C_{2}H_{6}, in which two hydrogens are replaced by one atom of oxygen. Ethylene oxide is not the only anhydride of glycol, although it is the simplest one, because C_{2}H_{4}O = C_{2}H_{4}(OH)_{2} - H_{2}O. Various other anhydrides of glycol are possible, and have actually been obtained, of the composition _n_C_{2}H_{4}(OH)_{2} - (_n_ - 1)H_{2}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. These imperfect anhydrides of glycol, or _polyglycols_, still contain hydroxyls like glycol itself, and therefore are of an alcoholic character in the same sense as glycol itself. They are obtained by various methods, and, amongst others, by the direct combination of ethylene oxide with glycol, because C_{2}H_{4}(OH)_{2} + (_n_ - 1)C_{2}H_{4}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. The most important circumstance, from a theoretical point of view, is that these polyglycols may be distilled without undergoing decomposition, and that the general formula given above expresses their actual molecular composition. Hence we have here a direct combination of the anhydride with the hydrate, and, moreover, a repeated one. The formula A_{_n_}H_{2}O may be used to express the composition of glycol and polyglycols with respect to ethylene oxide in the most simple manner, if A stand for ethylene oxide. When _n_ = 1 we have glycol, when _n_ is greater than 1 a polyglycol. Such also is the relationship of the salts of hydrate of silica, if A stand for silica, and if we imagine that H_{2}O may also be taken _m_ times. Such a representation of the _polysilicic acids_ corresponds with the representation of the polymerism of silica.