viii THERMO-ELECTRIC CRYOSCOPY 171 



dotted line is a similar graph of Raoult's freezing-point 

 determinations. The concentrations are plotted against 

 the depressions of the freezing-point. In the second graph 

 the abscissae correspond to hundredths of a degree. 



The couple on which these observations were made had 

 a length of 126 cm. interposed between the junctions, as 

 it was desired that it should give about 1 mm. deflection 

 for a temperature difference of 01. The actual 

 deflection was found to be 110*9 mm. for TOO . 



For some time after being made, the thermocouple 

 used in these observations changed its constant consider- 

 ably, owing probably to some progressive change in the 

 metals of the junction and circuit. After nine months, 

 when the constant was re-determined, it gave a deflection 

 of 130*4 mm. per 1. It had then become nearly stable, 

 and observations during the next three months showed that 

 its deflection varied between 129 *2 mm. and 133 mm. 

 per degree. The smaller fluctuations are possibly due to 

 changes in the resistance of the circuit connected with 

 changes of temperature. They show, the need of re- 

 determining the constant of the couple during each series 

 of observations, just as the zero change of the Beckmann 

 thermometer necessitates a control-experiment in the 

 thermometric method. 



With regard to the temperature of the freezing-bath, 

 it would at first sight appear of little importance in this 

 differential method, as, no matter what its temperature is, 

 it might be thought that it affects each test-tube similarly. 

 It has, however, been found to have an appreciable effect 

 on the magnitude of the deflection corresponding to the 

 freezing-point of a given solution, as will be seen from the 

 table below, in which are recorded the deflections corres- 

 ponding to the freezing-point of a solution of T5 gm. 

 sodium chloride in 100 gm. of water, having a freezing- 

 point of 0*886, when surrounded with a freezing-bath of 

 different temperatures. 



