318 Royal Society : — 



Glycerine 87*44 



Water 378 



Silicic acid 8'95 



100-17 



The glycerogel has somewhat less bulk than the original hydrogel. 

 When a glycerine jelly is distilled by heat, it does not fuse, but the 

 whole of the glycerine comes over, with a slight amount of decom- 

 position towards the end of the process. 



The compound of sulphuric acid, sulphagel, is also interesting from 

 the facility of its formation, and the complete manner in which the 

 water of the original hydrogel is removed. A mass of hydrated 

 silicic acid may be preserved unbroken if it is first placed in sulphu- 

 ric acid diluted with two or three volumes of water, and then trans- 

 ferred gradually to stronger acids, till at last it is placed in concen- 

 trated oil of vitriol. The sulphagel sinks in the latter fluid, and may 

 be distilled with an excess of it for hours without losing its transpa- 

 rency or gelatinous character. It is always somewhat less in bulk 

 than the primary hydrogel, but not more, to the eye, than one-fifth 

 or one-sixth part of the original volume. This sulphagel is transparent 

 and colourless. When a sulphagel is heated strongly in an open ves- 

 sel, the last portions of the monohydrated sulphuric acid in combi- 

 nation 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 sul- 

 phagel placed in water is soon decomposed, and the original hydrogel 

 reproduced. No permanent compound of sulphuric 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 monohydrates 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 in- 

 vested 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 alteration of form, may per- 

 haps afford a clue to the penetration of the albuminous matter of mem- 

 brane 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 com- 

 pound is placed in a fluid, the superior diffusive energy of the crystal- 

 loid may cause its separation from the colloid. Thus, of hydrated 



