JACQUES LOEB AND ROBERT F. LOEB 191 



It can be shown that the solution of the casein chloride depends on 

 forces regulated by the Donnan equilibrium and that the rule of Hardy 

 is in this case at least only a consequence of this fact. This can be proven 

 by microscopic observation of the mechanism of the solution of solid 

 particles of originally isoelectric casein in solutions of acids of different 

 concentration. It was found that the particles of casein swell in a 

 solution of HCl, becoming more and more transparent the more 

 they swell, and that when the swelling has reached a certain stage, 

 the particles disappear — they are dissolved. When in the swollen 

 stage, slight agitation may make them fall apart. T. B, Robertson 

 had suggested such a mechanism for the solution of Na caseinate,' 

 but it was found that the mechanism of solution in this latter case is 

 different. There is no doubt, however, that the swelling of casein 

 particles is a necessary prerequisite for the solution of casein-acid 

 salts, since such particles are only dissolved when their swelling 

 exceeds a definite limit. 



The method of procedure was as follows : A small number of gran- 

 ules of isoelectric casein of the same size (going through a sieve with 

 mesh 100 but not through a sieve with mesh 120) were put into 50 cc, 

 of water containing different quantities of different acids and kept at 

 24°C. In various intervals, i.e., after 15, and 60 minutes, and 6, 

 and 24 hours, the diameter of about 15 grains was measured with a 

 micrometer in a microscope and the average diameter calculated. 

 The particles were not stirred, and care was taken to avoid their 

 breaking into smaller fragments. The averages after 1 hour are 

 plotted in Fig. 1. The abscissae are the logarithms of the concentra- 

 tions of acid of the watery solution, the ordinates are the average 

 diameters of the particles. It is obvious that the average diameter 

 of the particles increases at first with the increase of the concentra- 

 tion of the acid, reaching a maximum at about pH 2.0 of the out- 

 side solution, and with a further increase in the concentration of the 

 acid the swelling becomes less again. 



Fig. 2 gives the measurements of the same particles after 24 hours. 

 At this time all the particles in the region of greatest solubility for 

 HCl and for HsPO*, i.e., between pH of the outside solution of 1.8 

 and 2.9, had completely dissolved and could no longer be measured. 



^Robertson, T. B., The physical chemistry of the proteins, London, Bombay, 

 Calcutta, and Madras, 1918, 275 ff. 



