CONDUCTIVITY OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. 299 



intervals of 20 C. on the ascending scale and 10 C. on the descending scale of 

 temperatures. It was found that under these conditions the heating and cooling 

 occupied almost exactly the same period of time, and we were therefore able to apply 

 a simple correction for the loss* of conductivity due to the neutralisation of the 

 alkali by the silica of the glass, by assuming that the fouling took place to an equal 

 extent during the periods of heating and cooling. The temperatures were measured 

 by means of a pair of thermometers graduated in tenths on an open scale and reading 

 from C. to 50 C. and from 50 C. to 100 C. ; these thermometers had been 

 recently calibrated at the Reichsanstalt, Charlottenburg, and the errors were known 

 within 0'01 C. In the lower ranges an exact adjustment of temperature was 

 possible, but in the higher ranges it was not found possible to avoid fluctuations of 

 several hundredths on either side of the steady temperature at which it was desired 

 to determine the conductivity. 



(3.) Determination of Temperature Coefficients with the Help of Platinum 

 Apparatus. During the later part of the investigation we had at our disposal 

 a silver-encased platinum bottle of about 500 cub. centims. capacity, in which we 

 were able to carry out conductivity measurements between C. and 100 C. without 

 appreciable fouling of the solutions. The bottle and the electrodes used with it had 

 the form shown in fig. 9. The resistance capacity was dependent upon the form of 

 the glass tube surrounding the inner electrode ; the electrodes principally used 

 for this series of observations had the capacities 16-371 and 3 '293 cm." 1 . The 

 area of glass was much less than in the electrode designed for measurements 

 at 18 C. , and the electrode was lifted out of the solution and allowed to drain when 

 not actually in use. The solution within the tubes was displaced from time to time 

 by forcing air free from carbon dioxide through the side tube of the electrode. The 

 smaller neck of the bottle was used to admit a thermometer, by which the actual 

 temperature of the solution could be determined ; the four thermometers used for 

 this purpose were graduated in tenths from C. to 25 C., 25 C. to 50 C., 50 C. 

 to 75 C. and 75 C. to 100 C., and had recently been calibrated at the Reichs- 

 anstalt, the errors being given to 0'02 C. at the lower and 0'05 C. at the higher 

 temperaturea The platinum pot, described in Part. I. of the paper, was also used in 

 a modified form for some of the latest temperature-conductivity measurements. In 

 these measurements the solution was initially heated to 100 C., and the conductivity 

 determined at a series of temperatures from 100 C. to C. ; the density of the 

 solution was then determined and its exact concentration deduced ; any change of 

 concentrations that might have been produced by the distillation of water from the 

 solution during the initial heating was thus allowed for. 



(4.) Accuracy of the Observations. As has already been pointed out, the chief 

 errors in determining the temperature coefficients of conductivity are (1) those due 

 to temperature adjustment, and (2) those due to fouling of the solutions, but in 

 * The most dilute solutions gained in conductivity by the dissolution of the glass. 



2 Q 2 



