PHYSICAL CONSTANTS OF SATURATED SOLUTIONS. 



197 



electric conductivity of the solution while it is becoming saturated, and I have made 

 a few somewhat unsatisfactory experiments on the method, but hope to be able to 

 return to it shortly. 



Measurement of Temperature. 



Thermometers whose graduations were sufficiently open to allow of an estimation 

 to 0'01 C. were used. They were standardized at Kew, and the rise of the zero 

 point was determined after an interval of 18 months. The rise of the zero point 

 was assumed to be proportional to the elapsed time, and all observations are corrected 

 on that assumption. Corrections were also applied for the emergent column by 

 suspending an auxiliary thermometer half way up the exposed stem and calculating 

 in the usual manner ; in no case did this correction amount to more than 0- 37 C. 



The temperatures given in the tables are corrected to the hydrogen scale of the 

 Bureau International at Paris. 



Experimental Errors. 



On page 192 I have already given an example of the results obtained in determining 

 the capacities, and it will be seen that the largest difference between any two 

 observations at the same temperature is '0012 cub. centim., which is roughly '01 per 

 cent. To give an idea of the order of accuracy of the observations of density and 

 solubility, the actual figures obtained with NaCl are appended below. NaCl was 

 selected because the differences between the two sets of densities are fairly typical, 

 while, on account of the small change in the solubility of the salt, those between the 

 solubilities, besides being typical, show the experimental errors without the necessity 

 of correcting for small changes of temperature. 



The largest difference between two densities at the same temperature is '00038 at 

 91 C., which, if the observation at 90'5 C. be corrected to 91'25 C., is reduced to 

 a difference of '00020, and this corresponds to an error of about 0'02 per cent. In 

 solubilities, however, the largest difference is '06 at 75 C., and this corresponds to 



