COLLOID CHEMISTRY. ' 217 



eaergy of sui-face-tensioii agrees well with that found to produce the 

 effect, and is a suflBcient cause. The figures are perhaps less important 

 as explaining an apparent anomaly than as illustrating the smallness of 

 the forces involved in considerable changes of volume near the maximum 

 swelling. The relation of surface-tension to the drying of gels consisting 

 of aggregates of particles, as that of silica, may be here again referred to. 

 If the common tension between the surfaces of the particles is less than 

 that of water itself, as in gels is necessarily the case, they will retain a 

 large amount of water by surface attraction, the vapour-pressure of which 

 will be equal to or somewhat less than that of liquid water according to the 

 tenuity of the film. As the water is removed by evaporation the particles 

 will cohere by their mutual attraction, and if this cohesive attraction is 

 greater than the difference between the tension of water and that of the 

 common surface, or, in other words, than the difference between the 

 attraction of the particles for water and the attraction of water for itself, 

 water will not again spread between the particles, aod the drying will be 

 irreversible as in the earlier stages of the silica jell}'. 



The rate of absorption of liquid by solid colloids has been studied 

 by Hofmeister ^ and more recently by Pauli (Paschelcs).'-' The latter 

 showed, both mathematically and by experiment on agar and gelatine, 

 that if Q be the degree of swelling and M its mass, the rate of swelling 



■ at any given time is equal to (jM — Q)C, the difference between tl.e 

 etc 



actual and the maximum swelling multiplied by a constant, which 

 varies with the material and probably with the temperature. This 

 is a law common to many chemical and physical changes, but compli- 

 cated in this case by the constantly increasing thickness of the swelling 

 colloid. For very thin plates, where this can be neglected, and 

 Ijetween the times t and <, and the swelling Q and Q, the constant 



C = Id. ,- — ^ , and for agar is about 95 x 10^. 



ty-t M— Qi ° 



The swelling of gelatine jelly is much influenced by acids and alkalies, 

 both of which largely increase alike its amount and its rapidity. The action 

 of neutral salts on swelling, as regards gelatine, has been investigated by 

 Hofmeister ; ^ their effect on melting and gelatinising temperatures by 

 Pauli,* and on viscosity by von Schroeder All these effects are prac- 

 tically parallel ; salts which lessen swelling raise the melting-point of 

 jellies and increase the viscosity of the sols. The effect seems an 

 additive one in which anions and cations take part, but no definite laws 

 have been established. The order of the salts investigated, placing these 

 first which most raise the melting-point, is sulphates, citrates, tartrati s, 

 acetates, water, chlorides, chlorates, nitrates, bromides, iodides as anions ; 

 and K, Ca, Na, Sr, and Mg as cations ; those placed before water 

 raising, and those after it lowering, the melting-point. Whether the same 

 order applies to other colloids than gelatine is uncertain, but no doubt 

 the effects depend to a considerable extent on the specific affinities of salt 

 and colloid. 



The swelling, though mainly an osmotic efft^ct, is influenced by the 

 solid cohesion of the jelly, and there can be little doubt that at tho 

 moment of setting a network is formed, whether molecular or coarser, 



' Archlvf. exp. Path, loid PhoTm., ISOO, 27, 305-413. ■ Ibid., 1895, 36, luO. 



s Jbid., 1891, 28, 210-238, ^ fflager, Arckiv, 1898, 71, l-3i. 



