466 APPENDIX 



tilled water and allowed to stand in the water-bath for some hours 

 before making a determination. 



In order to ensure a correct determination of the potential of the 

 chain in these experiments it was always found necessary to pass 

 the gas for three hours and, in the neutral or faintly acid solutions, 

 for as much as six hours. If two successive readings on the bridge, 

 taken an hour apart, did not differ by more than 1 mm. the result 

 was considered correct. Difficulty in obtaining constant readings 

 is, of course, only encountered in the chains yielding the highest 

 potentials, in which, as the apparatus is arranged, 1 mm. on the 

 bridge-wire makes a difference of only about one-half per cent to 

 the calculated value of the potential. In the chains of low poten- 

 tial greater accuracy is desired, but no difficulty is encountered in 

 attaining it, since, in these, successive readings are nearly always 

 identical, or if they are not, the difference can invariably be traced 

 to some obvious source of error which may be eliminated in a 

 repetition of the experiment. 



Since, however, prolonged exposure of the protein solution in 

 the gas-chain to the temperature of the water-bath is essential, the 

 question arises whether the accuracy of the determinations is not 

 invalidated by hydrolysis of the casein. The answer to this ques- 

 tion is in the negative, the change in the hydrogen- or hydroxyl- 

 concentration of a protein solution, due to hydrolysis, is negligible 

 in comparison with the change in its conductivity. Moreover, it 

 has been shown* that the displacement of the neutral point on the 

 potentiometer-wire, due to hydrolysis of the casein, in an alkaline 

 solution of a caseinate is opposed in sense to the displacement due 

 to the coming to equilibrium of the electrode with the solution in 

 which it is dipped. Were appreciable change in the hydroxyl 

 concentration of alkaline solutions of casein due to hydrolysis 

 occurring, therefore, the position of the neutral point on the bridge 

 should indicate at first a diminishing and later increasing potential. 

 I have discovered no trace of this save in one or two of the most 

 alkaline solutions which I have employed (1 per cent casein in 

 0.03 NKOH; 3 per cent casein in 0.05 N KOH), in these the 

 displacement due to hydrolysis was never more than 1 mm. and 

 the minimum value of the potential was taken as the true one. It 

 is, however, to be recollected that the hydrolysis was probably 



* T. Brailsford Robertson and C. L. A. Schmidt, Journ. Biol. Chem., 5 

 (1908), p. 31. 



