Electrolytes and Colloids. 



39 



dispersion increases to a maximum and then falls to zero at the critical 

 point. In other words, the power of destroying the cohesion and dispersing 

 the gluten as a cloud varies with the concentration of the acid, so that the 

 relation can be shown by a curve. The form of the curve will be seen later. 



The dispersion of the gluten is not due to a change in the protein molecule 

 of the nature, for instance, of hydrolysis, since it can be recovered as a 

 tenacious stringy precipitate by neutralising the acid or by the addition 

 of salt. 



The following table gives the mean of several determinations of the 

 concentration at which the gluten retains its coherence. The exact point is 

 the concentration at which gluten just breaks under its own weight when 

 suspended in the solution of acid ; and the results obtained in different 

 experiments are fairly consistent. It is remarkable that there should be no 

 simple relation between the observed concentrations and the strengths of 

 acid used as measured by electric conductivity. The conductivity of the 

 solutions after the gluten had been immersed in them was measured, and the 

 results are given in the second column of figures, the value of the sulphuric 

 acid solution being taken as unity : — 



Table I. 



Normality of critical Relative 



Acid. concentration. conductivity. 



H 2 S0 4 0-017 1-0 



Camphorsulphonic 0'02 T59 



HN0 3 0-03 1-9 



HC1 0-05 3-8 



Oxalic 0-15 3-8 



H3PO4 2-00 — 



Action of Distilled Water. — Gluten breaks up when washed very 

 thoroughly in many changes of ordinary distilled water. The distilled 

 water used was acid to litmus owing to the presence of carbonic acid ; and 

 the dispersion of the protein is due to this acidity, since (1) it is precipitated 

 by the addition of a trace of alkali, and (2) the protein when dispersed is- 

 electro-positively charged — that is to say, it displays the characteristic 

 relation of protein to acid. 



The Influence of Salts. — Salt in small concentration precipitates a hydrosol 

 of gluten whether it be formed by acid or by alkali. Therefore, salts lessen 

 the power which acids or alkalis possess of destroying the cohesion of 

 gluten, and, in sufficient concentration, completely neutralise it. The 

 concentration of salt necessary completely to nullify the dispersive power 



