250 Mr. W. 0. Dampier Whetham [Feb. 16, 



ordinary viscosity. In fact, where the structural dimensions of the 

 medium which determine its viscosity are large compared with those 

 of the moving body, it is known that no such relation holds. Thus 

 there is no proportionality between the variation in viscosity of a 

 salt solution when successive quantities of gelatine are added and 

 the variation in the velocity of the ions of the salt. Here the 

 gelatinous structure is probably a sort of fibrous network, very coarse 

 compared with molecular dimensions. 



Thus, the approximate proportionality between variation of 

 viscosity with temperature and variation of ionic velocity, indicates 

 that the dimensions of the ions are probably as large as, or larger 

 than, the dimensions of the molecular structure of the solvent. We 

 may perhaps regard the ions as composed of a central charged 

 nucleus, surrounded by a group of solvent molecules. Such a view 

 is supported by Kohlrausch and by Bousfield. 



It should be noticed, however, that the solvent molecules cannot 

 remain attached to the charged nucleus throughout its whole journey. 

 The different ionic velocities of potassium, sodium, and lithium, for 

 instance, indicate differences in the amount of the watery ionic envelope. 

 The amount of water transported through a dilute solution of a 

 chloride by these three ions cannot be the same ; it cannot, in each 

 case, be equal to that transported by the chlorine ion. If the water 

 were permanently attached to the nucleus till it reached the electrode, 

 we should get changes in concentration, not contemplated by the 

 theory of Hittorf and Kohlrausch, and the migration constants 

 directly determined by Steele and Denison would not agree with those 

 measured by Hittorf. We must suppose, therefore, that the moving 

 ionic system continually sheds some of its watery envelope, and 

 continually replaces it by fresh water molecules. 



One of the most interesting properties of these charged ionic 

 systems is their power of causing the coagulation of certain solutions 

 of colloids, such as albumen. If an electrolyte be added to such a 

 solution in sufficient quantity, coagulation at once ensues, and a 

 curious relation between the coagulative power and the chemical 

 valency of the ionic nucleus enables us to obtain some Hght on the 

 mechanism of the process. Hardy has shown that colloids themselves 

 generally move through a solution when an electric field is apphed, 

 the direction of motion depending on the nature and condition of the 

 Uquid solvent. It follows that the colloid particles themselves 

 possess an electric charge, and Hardy finds that the effective ion of 

 the coagulating electrolyte is the ion with an electric charge of 

 sign opposite to that on the colloid. It seems that coagulation is 

 effected by the neutralisation of the charge on the colloid. Now a 

 very much smaller quantity of a divalent salt is able to produce 

 coagulation than is necessary in the case of a univalent salt, and the 

 coagulative power of a trivalent salt is greater again than that of a 

 divalent salt. As mean values, Linder and Picton give for the 



