306 NEWTON BALDWIN GREEN 



10 grams of gelatine in 100 cc. water, and third a mixture of 3 

 grams agar, 15 grams gelatine and 2 grams of sifted whole wheat 

 flower in 280 cc. of water. These proportions were found by 

 experimentation to give the proper consistency of gel for con- 

 venient handling, and yet be fluid enough to give fair electrical 

 conductivity. It was found that in the case of gelatine for 

 instance the conductivity varied directly with the amount of 

 water present in it. This can be accounted for on the basis of 

 increasing ionization with dilution. PhiUp^ points out that in 

 colloidal solutions the size of the aggregate molecules decreases 

 with dilution, and since it is the entire particle in the case of 

 colloidal solutions, which carries the electrical charge, dilution 

 by increasing the number of particles would increase the 

 conductivity. 



In practice a cyhnder of gel 11 x 53 mm. was employed. Its 

 diameter was identical with the diameter of the electrodes. By 

 compression in the rack its length was decreased to 50 mm. and 

 a good contact at either electrode secured. The resistance of 

 the cylinder when freshly prepared w^as first determined and it 

 was immediately immersed in the solution to be tested. Two 

 solutions and mixtures of the two were used, NaCl 0.52 M and 

 CaCL. 0.278 M. These were the concentrations employed by 

 Osterhout and each gave a conductivity equal to sea water. 

 At intervals the rack was removed from the solution, drained 

 and resistance measured. This was continued until the resist- 

 ance of the gel became identical with that of the solution under 

 consideration. Curves were then plotted with the resistance in 

 ohms as ordinates and minutes as the abscissae. The determi- 

 nations of each individual combination were repeated many 

 times and on the whole remarkably consistent results were 

 obtained. 



Results and discussion 



For the sake of convenience in the description of the results 

 the kinds of colloidal material will be designated by the numerals 

 I, II and III, corresponding to the order in which they are 



* Philip, J. C. loc. cit. 



