Ionization produced by Heated Salts. 371 



powers * and certain conclusions drawn which are indicated 

 below. The relation f between the values o£ the currents 

 and the temperature is generally that given by Richardson's % 

 formula 



i = k.e h e 0. 



It seems likely that this formula always holds good, provided 

 the chemical nature of the source of the ionization is un- 

 affected by the temperature changes. The positive current- 

 time relations show a rise to a maximum value of the current 

 followed by decay with time, and are satisfactorily re- 

 presented by a formula § of the type 



i = K{e- K ^ — e- klt ), 



The only exceptions to this statement of which the writer is 

 aware are the results of Schmidt's || experiments with haloids 

 of cadmium. Some substances apparently give copious 

 positive and others abnormally large negative ionizationslf. 

 Most of these experiments have been made with salts in 

 contact with metallic plates, usually platinum. Garrett and 

 Willows **, in repeating the work of Beattie ft upon the 

 conductivity produced by heating common salt and iodine 

 on an insulated zinc plate, concluded " that the metal is a 

 necessary part to the changes causing the conduction." This 

 is likewise the view held by W. Wilson %% in some recent 

 investigations with aluminium phosphate. Furthermore, 

 Horton §§ has advanced the view that molecules of CO gas, 

 liberated from the glass walls of the containing vessel and 

 diffusing into the surface of the metal, finally emerge in an 

 ionized condition. It is important, then, that the ionization 



* Some of the substances which have been found to produce ionization 

 at temperatures of about 400° 0. are listed in papers by Garrett (Phil. 

 Mag. [6] xiii. p. 729, 1907) and Schmidt {Ann. der Phys. [4] xxxv. 

 p. 404, 1911). 



t Garrett & Willows (Phil. Mag. [61 viii. p. 437, 1904). A. E. 

 Garrett (Phil. Mag. [6] xiii. p. 728, 1907)r 



% Proc. Camb. Phil. Soc. xi. p. 286, 1902, and Phil. Trans. 201, p. 516, 

 1903. 



§ Loc. cit. under f and also A. E. Garrett (Phil. Mag. [6] xx. pp. 573- 

 591, 1910). 



|| Ann. der Phys. [4] xxxv. p. 401 (1911). 



If A. Wehnelt (Phil. Mag. [6] x. p. 80, 1905). J. J. Thomson (Camb. 

 Phil. Soc. Proc. xiv. p. 105, 1907). 



•3£$ff Loc. cit. 



ft Phil. Mag. [5] xlviii. p. 97 (1899) and [6] i. p. 442 (1901). 



XX Phil. Mag. [6] xxi. p. 634 (1911). 



§§ Proc. Roy. Soc. ser. A. vol. lxxxiv. p. 433 (1910) ; Camb. Phil. Soc. 

 Proc. xvi. p. 8*9 (1911). 



