XIV. On the properties of silicic acid and other analogous colloidal substances

The prevalent notions respecting solubility have been derived chiefly from observations on crystalline salts, and are very imperfectly applicable to the ss of colloidal substances. Hydrated silicic acid, for instance, when in a soluble condition, is property, speaking a liquid body, like alcohol, scible with water in all proportions. We have no degrees of solubility speak of with respect to silicic acid, like the gelatinous condition, which is usually looked upon as destitute of solubility. The jelly of silicic acid may be more or less rich in combined water, as it is first pre-red, and it appears to be soluble in proportion to the extent of its hydragon. A jelly containing 1 per cent. of silicic acid, gives with cold water solution containing about 1 of silicic acid in 5000 water; a jelly con-ining 5 per cent. of silicic acid, gives a solution containing about 1 per acid in 10,000 water. A less hydrated jelly than the last mentioned still less soluble; and finally, when the jelly is rendered anhydrous, it ves gummy-looking white masses, which appear to be absolutely insoble, like the light dusty silicic acid obtained by drying a jelly charged with salts, in the ordinary analysis of a silicate. The liquidity of silicic acid is only affected by a change, which is permanent (namely, coagulation or pectization), by which the acid is converted into the gelatinous or pectous form, and loses its miscibility with water. The liquidity is permanent in proportion to the degree of dilution of silicic the contrary, by concentration, and by elevation of temperature. A liquid silicic acid of 10 or 12 per cent. pectizes spontaneously in a few hours at the ordinary temperature, and immediately when heated. A liquid of 5 per cent. may be preserved for five or six days; a liquid of 2 per sent. for two or three months; and a liquid of 1 per cent. or less are no doubt practically unalterable by time, and hence the possibility of soluble silicici acid existing in nature. I may add, however, that no solution, weak or strong, of silicic acid in water has shown any disposition to deposit crystals , but always appears on drying as a colloidal glassy hyalite. The formation of quartz crystals at a low temperature, of so frequent occurrence in nature, remains still a mystery. I can imagine that such crystals are formed at an inconceivably slow rate, and from solutions of silicic acid which are extremely dilute. Dilution no doubt weakens the colloidal character of substances, and may therefore allow their crystallizing tendency to gain ground and develope itself, particularly where the crystal once formed is completely insoluble, as with quartz.