40 



Prof. T. B. Wood and Mr. W. B. Hardy. [Oct. 24, 



of particular acids was investigated in the manner already described, namely, 

 by suspending approximately equal pieces of gluten in varying concentra- 

 tions of acid and salt, and noting the point at which cohesion was so far 

 reduced as to allow the protein to flow off the rod. The relations appear in 

 ithe following curves (fig. 1), which show that for all strong acids and for all 

 .salts the concentration of the latter needed to balance the former increases to 

 a maximum as the concentration of acid increases, and then declines to 

 zero at the point where the acid alone is sufficient to maintain cohesion. 

 The curves all agree, therefore, in showing that, measured by the concentra- 

 tion of salt needed to prevent dispersion, the dispersive power of an acid 

 increases with increasing concentration, and then falls until the critical 

 concentration is reached, where dispersive action is nil. 



!800 1700 1800 1500 14,00 1300 1200 110(1 1000 900 800 700 BOO SJO 400 ;joo 200 100 



Fig. 1. 



These curves are so characteristic that they afford a means of testing 

 a point of general theoretical interest. One great class of colloidal solutions, 

 the aqueous solutions of characteristically insoluble bodies such as metals, 

 some proteins, sulphides, and gums, are characterised by the fact that round 

 each particle of the solute there is an electric double layer, and on the 

 potential difference between which the stability of the solution depends. 

 Coagulation or precipitation of such a solution is approximately coincident 

 with the reduction of the potential difference to zero, the most complete 

 coagulation, i.e., mechanically the densest and most coherent coagulum, being- 

 formed at the isoelectric point.* 



* Hardy, 'Roy. Soc. Proc.,' vol. 66, p. 110, 1900; Picton and Linder, 'Chem. Soc. 

 Trans.,' 1905—1906, vol. 87; Perrin, 'Journ. de Chim. Physique,' vol. 2, p. 601, 1904; 

 vol. 3, p. 50, 1905. 



