viii THERMO-ELECTRIC CRYOSCOPY 169 



due to the slow cooling of the support of the junction 

 in the solution. It will be found that readings after the 

 junctions have been in the freezing-chamber about 15 min. 

 do not tend to be greater than the preceding ones. In 

 the natural routine the second pair of readings are made 

 about 15 mins. after the junctions have been put in posi- 

 tion. A third pair of readings made in a similar manner 

 will plainly show whether the apparatus has reached a 

 steady state. If the observations have been satisfactory, 

 they should not diverge from one another by more than 

 one-half per cent., and with care greater accuracy may 

 be obtained. 



To calibrate the apparatus, sodium-chloride solutions of 

 known strength are introduced into one of the test-tubes. 

 The deflection produced by the depression of the freezing- 

 point of each is observed. These depressions being known 

 by the work of Raoult, Loomis, Nernst, and Abegg, we 

 obtain the value of a millimetre deflection of the light- 

 spot in degrees. 



Table 19 (p. 170) exhibits the figures of one of these 

 calibrations. The individual readings are recorded to 

 give some idea of the accuracy of the arrange- 

 ment. 



The scale reads continuously from left to right : 250 mm. 

 marks its middle point. 



In this table are recorded the two positions of the spider- 

 line in the spot of light on the scale for three successive 

 observations of the freezing-point of each solution. The 

 deflection corresponding to this freezing-point is obtained 

 by subtracting the second from the first, and halving the 

 result. It will be seen that deflections obtained in this 

 way diverge only slightly from the mean, which is given 

 in the last column but one. The greatest divergence is not 

 one-half per cent. In the final column are given the actual 

 freezing-points of the solutions derived from Raoult's 

 results quoted from Hamburger. 



