1908.] 



Electrolytes and Colloids. 



41 



On this view the formation of the hydrosol of gluten is due to the 

 development of electric charges round the particles of the protein owing to 

 chemical interaction between the protein, the acid or alkali and the water ; 

 and the tenacity, ductility, and water-content of a solid mass of moist gluten 

 depends upon the total or partial disappearance of these electric double 

 layers, and the reappearance of what is otherwise obscured by them, namely, 

 the adhesion or "idio attraction," as Graham called it, of the colloid particles 

 for each other, which makes them cohere when they come together. 



It is possible to put this hypothesis to the proof. We can measure the 

 potential difference between the water face and the protein face of each 

 particle in the hydrosol of gluten by determining the rate of transport of 

 the particles in a uniform electric field. The method adopted has been 

 described by one of us.* Briefly it consists in the use of a graduated 

 U-tube, the opalescent hydrosol is introduced as the lower layer, the upper 

 layer in each limb being a clear solution of the same electrical resistance. 

 Electrodes are immersed, in the upper layer, a field established, and the rate 

 of movement of the boundaries between upper and lower layers observed. 



The hydrosol was prepared either by washing gluten in distilled water 

 containing carbonic acid, a process which occupied at least two days, or in a 

 few hours by washing in a few changes of O0001 normal sulphuric acid. It 

 was freed from all starch by centrifuging. To successive lots of the hydrosol, 

 acid was added in varying amounts, and water when necessary, so that the 

 concentration of protein was constant, while the concentration of acid varied. 

 Finally the resistance was measured, and a fluid to form the upper layer 

 was prepared either by adding the same acid to water or by adding sodium 

 chloride. Hydrochloric, sulphuric, and acetic acids were used, and the results 

 were in all cases the same. The figures for hydrochloric acid are plotted in 

 the following curve, the ordinates being specific conductivity of the solution 

 x 10 -6 , the abscissa? the specific velocity in centimetres per second for unit 

 potential gradient x 10~ (fig. 2). 



The curve agrees in form with those already given for the effect of salt 

 upon cohesion, and we may therefore conclude that acids, and by inference 

 alkalis also, destroy the cohesion of gluten by forming double electric layers 

 round the particles, and that the potential difference between these layers 

 rises with increasing concentration of acid to a maximum, and then falls. 



Action of Alkalis. — The action of alkali in destroying the cohesion of 

 gluten is essentially similar to that of acid, except that the electric sign is 

 reversed. In a hydrosol of gluten formed by carbonic acid or any other acid 



* Hardy. ' Journ. Physiol.,' vol. 33, p. 251, 1905. 



