Conductivity produced by Heating Salts. 733 



Temperature in degrees Leak with voltage sufficient to 



Centigrade. produce saturation. 



180 25 



190 70 



202 130 



204 170 



214 400 



222 500 



These are found when plotted with h for abscissae and 



\ log e ■— l°gel for ordinates to give points which lie prac- 

 tically along a straight line. Zinc iodide was tested for 

 negative ions up to temperature 255° C. with similar result. 



_Q 

 From the formula C = A6h 20 in which Q represents the 



amount of energy in calories associated with the production 

 of one gramme molecular weight of the ions, it readily follows 



that ^°gepr = o - (^ ~n) where @\ — 6 2 is only a small 



interval of temperature. 



Making use of this relation I obtained the following values 

 of Q for ions obtained from the substances named. The 

 average temperatures are stated in each case. 



For positive ions. 



Calcium fluoride (297° C.) 2-6 x 10 4 , 



Aluminium fluoride (330° C.) 2'9 x 10 4 , 



Ammonium nitrate (312° C.) 3*3 x 10 4 . 



For negative ions. 



Zinc iodide (241° C.) 2*9 x 10 4 , 



Iron chloride (355° C.) 6'1 X 10 4 , 



Ammonium chloride (352° C.) 5'0 x 10 4 , 



Calcium fluoride (346° C.) 6'0 x 10 4 , 



Ammonium nitrate (343° C.) 4*3 x 10 4 , 



Magnesium chloride (326° C.) 2*4 x 10 4 . 



It will be seen that the values of Q, obtained for the 

 negative ions are in general larger than those obtained for 

 the positive ions. A similar result for the ions produced by 

 hot platinum has been noted by Richardson *, and the values 

 here obtained are of the same order of magnitude as those 

 obtained by him. 



I found also on heating zinc iodide at a pressure of 

 2*5 mm. of mercury that the escape of positive ions could be 

 represented by a similar formula, a saturation current being- 

 obtained. 



* Phil. Trans. A. tfov. 1906, p. 61. 



