8 ELECTRICAL CONDUCTIVITIES, ETC. 



was always negligible. It is quite certain that for cups made and treated as above 

 described the solubility of the glass does not stand in the way of accurate work 

 up to 65. 



Since the solubility of glass increases very rapidly with the temperature above 65, 

 it was decided not to carry these measurements of conductivity to a temperature 

 higher than 65. 



PREPARATION OF THE SOLUTIONS. 



All of the substances used were obtained from Kahlbaum. These were purified 

 by the method best adapted to each substance, and the purity of the compound 

 tested in every case. 



Whenever the nature of the compound permitted, the mother solution was pre- 

 pared by directly weighing out the amount of the pure compound desired. In other 

 cases the mother solution was standardized by the best gravimetric method available 

 for that purpose. In the case of the organic acids the mother solution was fre- 

 quently standardized by titration against a standard solution of an alkali. 



Two sets of solutions of every compound were prepared the one to be used for 

 measurements from to 35, and the other set to be studied from 35 to 65. The 

 solutions to be used over the temperature range to 35 were made up at 20, and 

 those solutions to be measured from 35 to 65 were generally made up at 50, in 

 vessels calibrated for 20 and 50 respectively. Since the coefficient of expansion of 

 water increases greatly with the temperature, it is necessary to apply the proper 

 correction to the conductivities of solutions taken at 35 and 65, when the solutions 

 were made up at 50. 



When a standard solution is cooled from 50 to 35 there is a contraction in volume 

 and a consequent increase in the concentration of the solution. The value of n v for 

 any solution would, therefore, be slightly too large. The value of /i p as found must 

 be multiplied by the factor 0.994 for results at 35 when the solutions were made up 

 at 50. The correction factor for solutions made up at 50 and used at 65 is 1.0076. 



The coefficient of expansion for distilled water is somewhat less than that for an 

 aqueous solution. However, the difference in the coefficients for water and for our 

 most concentrated solution is so small that it is negligible. 



By making use of the above correction it was necessary to prepare only one set of 

 solutions for each salt for the temperature range, 35 to 65; and, consequently, 

 much pure material and time were saved. 



By preparing one set of solutions to be used from to 35, and another set of 

 solutions from entirely new material for use from 35 to 65, we had a test of the 

 purity of the material used, the proper standardization of the solutions, and the 

 correctness of the conductivity values herein given. The two sets of solutions were 

 both measured at 35, and when discrepancies in the two sets of results, of appreci- 

 able order of magnitude, manifested themselves; as was inevitable in some cases 

 where about 40,000 measurements were made, the work was repeated over the higher 

 range in temperature, or over the lower range in temperature, or over the entire 

 temperature range. 



From these two mother solutions all of the more dilute solutions were prepared, 

 directly or indirectly, using carefully calibrated flasks and burettes. 



