460 APPENDIX 



current is supplied by an inductorium of the Ostwald type. The 

 rheostat reads correctly to within 0.01 per cent and supplies any 

 desired resistance between 0.1 ohm and 1000 ohms. The resist- 

 ance in the rheostat is always adjusted until the zero-point is 

 exactly in the middle of the bridge; the resistance in the rheostat 

 is then, of course, exactly equal to that of the conductivity-vessel 

 filled with the fluid under investigation. The arrangement of the 

 wiring is represented in the preceding diagram. 



The wires of the conductivity circuit are all of "bell-wire" so that 

 their resistance can be neglected. The wires of the potentiom- 

 eter-circuit are somewhat thinner. All of the wires are insulated 

 and carefully supported on glass, and are never allowed to touch 

 the table. Where it is necessary to carry wires through the table, 

 for example, they are run through glass tubes. The wire con- 

 nected with the slider on the bridge is encased in rubber tubing.* 



I have pointed out in Chaps. V and XII that it is difiicult to 

 obtain solutions of caseinates of much higher acidity than neutral- 

 ity to litmus by merely shaking up casein in solutions of bases, — 

 not because casein will not form such solutions, but because, 

 although it dissolves at first with considerable rapidity, after the 

 excess of alkali is neutralized, further casein dissolves with extreme 

 slowness. Now the conductivities of solutions containing proteins 

 must be measured as soon as possible after the complete solution 

 of the protein, for, otherwise, the hydrolysis which the protein 

 undergoes, the more rapidly the more alkaline its solution, intro- 

 duces a serious error into the determination. Hence it is impera- 

 tive, not only that the conductivities of these solutions should 

 be determined as soon as possible after complete solution of the 

 protein, but, also, that the preparation of the solution, after 

 the introduction of the protein, should consume as little time as 

 possible. Hence all of the solutions of casein which are acid to 

 phenolphthalein are best prepared by dissolving the casein in 

 excess of alkali and then neutralizing this excess with acid. 

 This procedure, if gas-chain measurements are being made at 

 the same time, has the further advantage that the absolute 

 conductivities of the solutions thus obtained being higher, the 

 detection of the zero-point on the bridge-wire by means of a gal- 



* More accurate conductance measurements may be made by employing 

 the method of E. W. Washburn. Cf. " The Measurement of Conductivity of 

 Electrolytes," Leeds and Northrup Co., Philadelphia, 1916. 



