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II. "Liquid Diffusion applied to Analysis." By THOMAS 

 GRAHAM, Esq., F.R.S., Master of the Mint. Received 



May 8, 1861. 



(Abstract.) 



The unequal diffusibility of different substances in water appears 

 to present means of separation not unlike those long derived from 

 unequal volatility. For as regards diffusion, there exists a " volatile " 

 and also a " fixed " class of substances ; and these distinctions appear 

 to correspond with differences in molecular constitution of a funda- 

 mental nature. Much value is attached to diffusion, as affording 

 the means of bringing out clearly, and subjecting to numerical 

 expression, the distinctive properties of what appear to be two great 

 divisions of chemical substances. 



The first, or diffusive class of substances, are marked by their 

 tendency to crystallize, either alone or in combination with water. 



When in a state of solution they are held by the solvent with a 

 certain force, so as to effect the volatility of water by their presence. 

 The solution is generally free from viscosity, and is always sapid. 

 Their reactions are energetic and quickly effected. This is the class 

 of crystalloids. 



The other class, of low diffusibility, may be named colloids, as they 

 appear to be typified by animal gelatine. They have little if any 

 tendency to crystallize, and they affect a vitreous structure. The 

 planes of the crystal with its hardness and brittleness are replaced in 

 the colloid by rounded outlines with more or less softness and tough- 

 ness of texture. Water of crystallization is represented by water of 

 gelatination. Colloids are held in solution by a feeble power, and have 

 little effect on the volatility of the solvent. They are also preci- 

 pitated from their solution by the addition of crystalloids. The solu- 

 tion of colloids has always a certain degree of viscosity or gumminess, 

 when concentrated. They appear to be insipid or wholly tasteless, 

 unless when they undergo decomposition upon the palate and give 

 rise to sapid crystalloids. Their solid hydrates are gelatinous bodies. 

 They are united to water with a force of low intensity ; and such 

 is the character of the combinations in general between a colloid and 

 a crystalloid, even although the latter may be a powerful reagent 

 in its own class, such as an acid or an alkali. In their chemical re- 



