PROFESSOR THOMAS GRAHAM's SCIENTIFIC WORK. 193 



By means of the process of dialysis, Mr. Graham succeeded in obtaiu- 

 iug various colloid organic substances, such as tannin, albumen, gum, 

 caramel, «S:c., in a very pure state; some of them, indeed, in a state of 

 purity exceeding any in -which they had before been met with. But the 

 most curious results were obtained with different mineral substances, 

 usually thrown down from their dissolved salts in the state of gelatin- 

 ous or colloid precipitates. Most of these precipitates being soluble in 

 some or other crystalloid liquid, on submitting the so-produced solutions 

 to dialysis, the crystalloid constituents diffuse away, leaving the colloid 

 substances in pure aqueous solution. By proceeding in this manner, 

 Mr. Graham was able to obtain certain hydrated forms of silica, ferric 

 oxide, alumina, chrome, prussian-blue, stannic acid, titanic acid, tungstic 

 acid, molybdic acid, &c., &c., in the state of aqueous solution — these 

 bodies having never before been obtained in solution, save in presence 

 of strongly acid or alkaline compounds serving to dissolve them. Alto- 

 gether, the production of these colloid solutions of substances, such as 

 silica and alumina — in their crystalline state, as quartz and corundum, 

 completely insoluble — threw an entirely new light upon the conditions of 

 aqueous solution. 



The colloidal solutions, obtained as above, of substances usuall}" crys- 

 talline, were found to be exceedingly unstable. Either spontaneously, 

 or on the addition of some or other crj^stalloid reagent, even in very 

 minute quantity, they pectized or became converted into solid jellies. 

 Hence Mr. Graham was led to speak of two colloidal states ; the peptous 

 or dissolved, and the pectous or gelatinized. In addition to their power 

 of gelatinizing, their mutability, their non-erystalline habit, and their 

 low diffusibility, substances in the colloid state were found to be further 

 characterized by their chemical inertness and by their high combining 

 weights. Thus the saturating power of colloid silica was only about 

 3L of that of the ordinary- acid. 



In his supplementary paper communicated to the Chemical Society, 

 Mr. Graham showed how the pectous forms of different mineral colloids 

 could, in many cases, be reconverted into their peptous forms. He 

 further showed how the water of different peptous and pectous colloids 

 could be mechanically displaced by other liquids, as alcohol, glycerine, 

 sulphuric acid, &c. To the different classes of compounds so formed, 

 he gave distinctive names. Thus, the alcoholic solution and jelly, of 

 silicic acid for instance, he designated as the alcosol and alcogel respect- 

 ively. 



Closely associated with the passage of different liquids through mem- 

 branes is the action known as endosmose, discovered by Dutrochet. 

 Mt. Graham's principal results on this subject are recorded in a very 

 elaborate paper "On osmotic force,'' communicated to the lioyal Society 

 in 1854; * but a few further results and a statement of his linal views 

 are contained in the paper, referred to immediately above, "On liquid 



* Philosophical Transactious, 1834, p. 177. 



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