Theory of Colloidal Solution. G5 L 



medium by chemical means, its aggregates will grow to 

 this critical size and stop there, i. e., will remain in colloidal 

 solution. 



Fig. 2. 



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If a colloid be denned as a substance which forms those 

 pseudo-solutions termed colloidal, it follows from the above 

 that this description is not sufficient unless the other medium, 

 i. e. the pseudo-solvent, be specified. This point is well illus- 

 trated by the result obtained by KrafFt * with the soaps ; 

 these substances give true solutions with alcohol, and colloidal 

 solutions with water. 



There is nothing to prevent a substance being partly 

 colloidally distributed throughout a given medium and partly 

 in true molecular solution f. Hence all gradations may 

 occur. The foregoing considerations render intelligible 

 many of the peculiarities exhibited by colloidal solutions, 

 namely, their small diffusibility and small osmotic pressure, 

 their frequent want of optical transparency, their " solidifica- 

 tion " to jellies, their precipitation by changes of temperature 

 or by the addition of comparatively small quantities of certain 

 substances, &c, &c. It is necessary, however, to remark that 

 only reversible changes admit of explanation on the above 

 theory. The irreversible phenomena so frequently exhibited 

 by organic colloids can only be explained by intra-molecular 

 changes. As regards the nature of the " colloidal " state, as 

 distinct from a colloidal solution, the present theory lends 

 support to the view that in matter in the colloidal state the 

 intermolecular forces are in general smaller than in the case 



* Ber d. d. chem. Ges. xxvii. p. 1747 ; xxviii. p. 2556; xxix. p. 1328. 

 t So far as I am aware, this possibility was first mentioned by Mr. W. 

 B. Hardy. 



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