On a Theory of Colloidal Solution. 617 



much greater than for nickel. The tail-end of the curve — 

 like the corresponding portion of the nickel curve — is ap- 

 proximately a straight line ; its continuance as such would 

 lead to the hysteresis loss vanishing at an intensity (1050 

 (J.G.S. units) not far removed from the saturation-intensity 

 for cobalt. 



The third curve in fig. 2, with the same general character 

 as the others, has been plotted from data derived from the 

 paper already alluded to. It refers to a moderately soft 

 specimen of iron, and has been added for the sake of 

 comparison. 



Owens College, Manchester. 



LXV. A Theory of Colloidal Solution. By F. G. Donnan, 

 M.A., Ph.D., Junior Fellow, Royal University of Ireland *. 



Tl^HE main fact concerning "colloidal" solutions, a fact 

 A. which has been established by experiment and thermo- 

 dynamical reasoning, appears to be that such " solutions " 

 are in reality complexes of two phases, of which one exists 

 in a state of extremely fine division, interspersed throughout 

 the other. It must be observed, however, that it is not a 

 sufficient description to call such complexes mere " suspen- 

 sions./' for what we have here to deal with is not so much a 

 certain sort of mixture or pseud-o- solution, as rather a peculiar 

 condition of matter, namely the " colloidal " state. This 

 fundamental point was clearly emphasized by Graham, but 

 seems to have been somewhat lost sight of by some modern 

 writers. Thus Krafft f has proposed a theory of colloidal 

 solutions in which it is supposed that the molecules of col- 

 loidally-dissolved substances rotate round each other in 

 closed paths. Apart from the consideration that such a 

 theory is invalid, inasmuch as it gives no explanation why 

 these orbital systems should not possess translatory motion, 

 it is evident that such a theory must, in any case, be highly 

 unsatisfactory, for the really essential point would be the 

 explanation of how such a state of affairs came about. 



What we have to account for, in fact, is the following. A 

 solid substance C, when brought in contact with certain 

 liquid media, breaks up or disintegrates into these media, but 

 in such a manner that the disintegration process does not 

 proceed to the molecular limit. The liquid medium appears 

 then to be interspersed with minute aggregates of C, which 

 are still so much larger than molecular magnitudes that they 



* Communicated by the Physical Society : read March 8, 1901. 

 t Ber. d. d. Chem. Ges. vol. xxix. p. 1334 (1896). 



