664 



REVERSAL BY TRIVALENT CATIONS 



solutions were put into collodion bags previously treated with gelatin. 

 The bags were put into beakers containing the same solutions of 

 CeCls as that inside the bag but free from CaClo. Hence the Ce 

 acted only on the sign of the charge of the membrane but not on 

 the attraction of water. This latter was done by the CaCl2. When- 

 ever the level of the solution of CaCl2 rises we know the membrane 

 must be positively charged. The curve in Fig. 4 shows that the 



il jl n il M. n il n. n n m n n n 



16 32 64 128 256 512 1024 2048 4096 8192 1^38432,768 6^536^ 



Concentration o[AlCl3,CsCl3 and Al 2(504)3 



Fig. 4. Lowest concentration at which Ce and Al charge the membrane 

 positively. 



membrane already assumes a positive charge in m/32,768 CeCls and 

 probably in lower concentrations of CeCls. 



WTien the CeCls was replaced by AICI3 the membrane assumed a 

 positive charge when the concentration of AICI3 was less than m/65,536. 

 The pH of this latter solution of AICI3 was 5.0, so that the reversal 

 was due to the Al ion and not to the hydrogen ion concentration. 

 The fact that Al is more efficient than the Ce (or La) ion may be 

 connected with the fact that the ionic radius of Al is considerably 



