Prof. Graham on the Properties of Silicic Acidfyc. 315 



The liquidity of silicic acid is only effected by a change which is per- 

 manent (namely, coagulation or pectization), by which the acid is con- 

 verted 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 tem- 

 perature. 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, maybe 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 0*1 per cent, or less are no doubt practically unalter- 

 able 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 there- 

 fore allow their crystallizing tendency to gain ground and develope 

 itself, particularly where the crystal once formed is completely inso- 

 luble, as with quartz. 



The pectization of liquid silicic acid is expedited by contact with 

 solid matter in the form of powder. By contact with pounded gra- 

 phite, which is chemically inactive, the pectization of a 5 per cent, 

 silicic acid is brought about in an hour or two, and that of a 2 per 

 cent, silicic acid in two days. A rise of temperature of 1°*1 C. was 

 observed during the formation of the 5 per cent, jelly. 



The ultimate pectization of silicic acid is preceded by a gradual 

 thickening in the liquid itself. The flow of liquid colloids through a 

 capillary tube is always slow compared with the flow of crystalloid 

 solutions, so that a liquid-transpiration-tube may be employed as a 

 colloidoscope. With a colloidal liquid alterable in viscosity, such as 

 silicic acid, the increased resistance to passage through the colloi- 

 doscope is obvious from day to day. Just before gelatinizing, silicic 

 acid flows like an oil. 



A dominating quality of colloids is the tendency of their particles 

 to adhere, aggregate, and contract. This idio-attraction is obvious in 

 the gradual thickening of the liquid, and when it advances leads to 

 pectization. In the jelly itself, the specific contraction in question, 

 or synceresis, still proceeds, causing separation of water, with the divi- 

 sion into a clot and serum ; and ending in the production of a 

 hard stony mass, of vitreous structure, which may be anhydrous, or 

 nearly so, when the water is allowed to escape by evaporation. The 

 intense synseresis of isinglass dried in a glass dish over sulphuric 

 acid in vacuo, enables the contracting gelatine to tear up the surface 

 of the glass. Glass itself is a colloid, and the adhesion of colloid to 

 colloid appears to be more powerful than that of colloid to crystalloid. 



Y2 



