1$64.] and other analogous Colloidal Substances, 339 



colourless. When a sulphagel is heated strongly in an open vessel, the last 

 portions of the monohydrated sulphuric acid in combination are found to 

 require a higher temperature for their expulsion than the boiling-point of 

 the acid. The whole silicic acid remains behind, forming a white, opaque, 

 porous mass, like pumice. A sulphagel placed in water is soon decomposed, 

 and the original hydrogel reproduced. No permanent compound of sul- 

 phuric and silicic acids, of the nature of a salt, appears to be formed in 

 any circumstances. A sulphagel placed in alcohol gives ultimately a pure 

 alcogel. Similar jellies of silicic acid may readily be formed with the mono- 

 hydrates of nitric, acetic, and formic acids, and are all perfectly transparent. 



The production of the compounds of silicic acid now described indicates 

 the possession of a wider range of affinity by a colloid than could well be 

 anticipated. The organic colloids are no doubt invested with similar wide 

 powers of combination, which may become of interest to the physiologist. 

 The capacity of a mass of gelatinous silicic acid to assume alcohol, or even 

 oleine, in the place of water of combination, without disintegration or altera- 

 tion of form, may perhaps afford a clue to the penetration of the albuminous 

 matter of membrane by fatty and other insoluble bodies, which seems to 

 occur in the digestion of food. Still more remarkable and suggestive are 

 the fluid compounds of silicic acid. The fluid alcohol- compound favours 

 the possibility of the existence of a compound of the colloid albumen with 

 oleine, soluble also and capable of circulating with the blood. 



The feebleness of the force which holds together two substances belonging 

 to different physical classes, one being a colloid and the other a crystalloid, 

 is a subject deserving notice. When such a compound is placed in a fluid, 

 the superior diffusive energy of the crystalloid may cause its separation from 

 the colloid. Thus, of hydrated sihcic acid, the combined water (a crystal- 

 loid) leaves the acid (a colloid) to diffuse into alcohol ; and if the alcohol 

 be repeatedly changed, the entire water is thus removed, alcohol (another 

 crystalloid) at the same time taking the place of water in combination with 

 the silicic acid. The liquid in excess (here the alcohol) gains entire pos- 

 session of the silicic acid. The process is reversed if an alcogel be placed 

 in a considerable volume of water. Then alcohol separates from combina- 

 tion, in consequence of the opportunity it possesses to diffuse into water ; 

 and water, which is now the liquid present in excess, recovers possession of 

 the silicic acid. Such changes illustrate the predominating influence of 

 mass. 



Even the compounds of silicic acid with alkalies yield to the decomposing 

 force of diffusion. The compound of silicic acid with 1 or 2 per cent, of 

 soda is a colloidal solution, and, when placed in a dialyzer over water in 

 vacuo to exclude carbonic acid, suffers gradual decomposition. The soda 

 diffuses off slowly in the caustic state, and gives the usual brown oxide of 

 silver when tested with the nitrate of that base. 



The pectization of liquid silicic acid and many other liquid colloids is 

 effected by contact with minute quantities of salts in a way which is not 



VOL, XIII, 2 c 



