ON THE PROPERTIES OP SILICIC ACID. 
63 
after several days’ unsuccessful searching for it, I am afraid I must conclude it is not here; 
hut I will go to Bojorques in another day or two, and perhaps I may find it there. 
The Rhatany, I was told at Barranquilla, came from the neighbourhood of Bucara- 
manga, and as I intend to go up the river Sogamoza to that place when I leave Bojorques, 
I hope to be able to procure specimens of the plant that produces it there. 
Barranca Vermeija, on the River Magdalena , New Granada, 
January 13 th, 1864. 
ON THE PROPERTIES OF SILICIC ACID AND OTHER ANALOGOUS 
COLLOIDAL SUBSTANCES. 
BY THOMAS GRAHAM, F.R.S., MASTER OF THE MINT. 
(Preliminary Noticed) 
The prevalent notions respecting solubility have been derived chiefly from observa¬ 
tions 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 pro¬ 
perly 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 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 prepared, and it appears to be soluble 
in proportion to the extent of its hydration. 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 containing 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 insoluble, 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 pec- 
tous form, and loses its miscibility with water. The liquidity is permanent in propor¬ 
tion to the degree of dilution of silicic acid, and appears to be favoured by a low r tem¬ 
perature. It is opposed, on the contrary, by concentration, and by elevation of tempera¬ 
ture. 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 0T 
per cent, or less are no doubt practically unalterable by time, and hence the possibility 
of soluble silicic 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 
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 com¬ 
pletely insoluble, as with quartz. 
The pectization of liquid silicic acid is expedited by contact with solid matter in the 
form of powder. By contact with powdered graphite, 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°T C. w r as observed 
duriug 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 com¬ 
pared 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 colloidoscope is obvious from 
day to day. Just before gelatinizing, silicic acid flows like an oil. 
