1864.] Ml*. Graham on the Properties of Silicic Acid. 335 



XIV. On the Properties of Silicic Acid and other analogous Col- 

 loidal Substances.'' By Thomas Graham, F.R.S., Master of 

 the Mint. Received June 16, 1864. 



(Preliminary Notice.) 

 The preralent notions respecting solubility have been derived chiefly from 

 observations on crystalline salts, and are very imperfectly applicable to the 

 class of colloidal substances. Hydrated silicic acid, for instance, when in 

 the soluble condition, is properly speaking a liquid body, like alcohol, 

 miscible with water in all proportions. We have no degrees of solubility 

 to speak of with respect to silicic acid, like the degrees of solubility of a 

 salt, unless it be with reference to silicic acid in the gelatinous condi- 

 tion, 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- 

 pared, and it appears to be soluble in proportion to the extent of its hydra- 

 tion. A jelly containing 1 per cent, of silicic acid, gives with cold water 

 a solution containing about 1 of silicic acid in 5000 water ; a jelly con- 

 taining 5 per cent, of silicic acid, gives a solution containing about 1 part 

 of acid in 10,000 water. A less hydrated jelly than the last mentioned 

 is still less soluble ; and finally, when the jelly is rendered anhydrous, it 

 gives gummy-looking white masses, which appear to be absolutely inso- 

 luble, 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 perma- 

 ment (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 

 acid, and appears to be favoured by a low temperature. It is opposed, on 

 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 cent, for 

 two or three months ; and a liquid of 1 per cent, has not pectized after two 

 years. Dilute solutions of O'l per cent, or less are no doubt practically 

 unalterable by time, and hence the possibility of soluble silicic acid ex- 

 isting 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 only 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. 



