43 Note 33: conductances of solutions of common salt 



When the temperature is near 18 C., we may use the equation 



= 2015 + 45-1 (/ - 18). 



Saturated solution at 18 contains according to Kohlrausch 26-4 per cent, 

 of salt. Cavendish's saturated solution contained ^ of salt, which is equivalent 

 to 26-45 per cent. 



Kohlrausch finds that saturated solution of salt is one of the best standard 

 substances for the comparison of the resistance of other electrolytes. Its con- 

 ductivity seems to be sensibly the same, whether it is made with chemically 

 pure salt or with the ordinary salt of commerce. The temperature coefficient 

 is also smaller than that of many other electrolytes. 



For other solutions of sodium chloride he finds that at 18 



IO*k = 13650^) 227OO/) 2 , 



where p is the proportion, by weight, of the salt to the whole solution. 

 For the particular solutions examined by Cavendish we have 



resistance in resistance found 

 p io*A terms of sat. sol. by Cavendish 



3^ 2015 i i sat. sol. 



t \ 980 2-56 1-91 salt in ii 



rV 430 4' 6 9 3-97 salt in 2 9 



7 V 190 .10-58 8-8 salt in 69 



TT 94 21-44 15-75 salt in 142 



TTTJ 9 22-39 20-05 salt in 149 



13-65 147-6 93-02 salt in 999 



4-55 442-9 340-85 salt in 2999 



1 



NOTE 34, ART. 626. 

 Conductivity of other Solutions. 



The substances mentioned by Cavendish are easily identified, with the 

 exception of "calc. S. S. A." and "f. alk. d." The weights of the quantities 

 furnish no indication, for they are so large as to show that a dilute solution 

 was used. The letters A and D probably indicate the bottles in which the 

 solutions were kept. 



The expression f. alk. or fixed alkali occurs in several parts of Cavendish's 

 writings, especially in the manuscripts lithographed by Mr Vernon Harcourt 

 in the Report of the British Association for 1839. ^ certainly means pearl ashes 

 or carbonate of potash. The full title seems to have been alkali fixum vcgetabilc, 

 as distinguished from alkali fixum fossile, which is sodic carbonate, and other 

 writers seem to have used the expression fixed alkali for either of these, but 

 Cavendish always uses the expression as a synonym for pearl ashes, and dis- 

 tinguishes potassic hydrate by the name of "sope leys." 



