I20 PHYSICAL SCIENCE 



given above supplies a foundation on which a more 

 detailed explanation may some day be built. 



The particles in solutions of colloids in water 

 generally move slowly when acted on by electric 

 forces, the direction of motion depending on the 

 nature of the colloid and on that of the solvent. 

 Hardy found that the direction of movement of 

 certain proteins could be changed by changing 

 the solvent from a very dilute acid to a very dilute 

 alkali. This reversal implied a change in the sign 

 of the charges on the colloid particles ; and, if 

 the solvent was very carefully neutralised, an iso- 

 electric point was reached at which the solution 

 became very unstable, and coagulation seemed 

 to occur spontaneously. The same observer also 

 found that, in the case of colloids travelling with 

 the current, it is the acid ion which is active in 

 causing coagulation, and not the metallic ion as 

 in the work of the older experimenters, who 

 all used colloids which travel against the electric 

 current. Thus it is always the ion possessing a 

 chargeof oppositekindto that onthecolloid particle 

 which is effective in producing coagulation. 



Burton has found a similar change in velocity 

 in an electric field when to a colloidal solution of 

 silver increasing amounts of aluminium sulphate 

 are added. The velocity of the silver decreases, 

 and vanishes at or near the coagulating point. 

 With more aluminium, coagulation is prevented 

 for a time, and the unstable colloid moves in the 

 opposite direction, showing that its electric charge 

 has been reversed by the absorption of excess of 

 aluminium ions. 



These results are of great importance from 

 the point of view of physiology, and also as 



