Positive Ionization from Hot Salts. 675 



nature of the ions from evidence of this sort. Horton's * 

 experiment showing that carbon monoxide is evolved when 

 aluminium phosphate is heated is not of this character, but 

 it does not seem to be to the point. Apart from the objection 

 that practically everything gives off small quantities of carbon 

 monoxide when heated, there is no evidence that there is any 

 connexion between the emission of this gas and that of the 

 positive ions. On my view of course the material nature of 

 the ions would not be discovered by a spectroscopic exami- 

 nation of the gas evolved as they would collect on the 

 negative electrode and stay there. 



Ionization from Salts in a Gaseous Atmosphere. 



A large number of experiments have been made with the 

 object of measuring the magnitude of the positive emission 

 from salts under different conditions. The apparatus first 

 used was similar in principle to the arrangements employed 

 by Garrett in his experiments on aluminium phosphate, and 

 by Horton in his experiments on sodium phosphate. A 

 horizontal platinum strip was heated by an electric current, 

 and the central portion was covered with a thin layer of the 

 salt under examination. The temperature of the central 

 part of the strip was obtained by a modification of Kelvin^s 

 double-bridge method of measuring small resistances, tho 

 connexions to the strip being made by means of platinum 

 wires welded on to it, at a distance from each end of the 

 strip equal to about a quarter of the whole length. All the 

 strips used were cut on the dividing-engine, and their edges 

 were exactly parallel and 2 mm. apart. The area covered 

 by the heated salt was always about 0*2 sq. cm., and the 

 area of hot platinum which was not so covered, including 

 the back of the strip, was always about 0*6 sq. cm. The 

 salt occupied the middle centimetre of the upper surface of 

 the strip. 



In this investigation all the currents were measured by 

 means of a Dolezalek electrometer which gave a deflexion 

 of 800 mm. per volt. For currents less than 10 -8 amp. the 

 time-rate of deflexion was used, a one-microfarad subdivided 

 standard condenser being connected with the insulated 

 quadrants. For larger currents the electrometer was shunted 

 with a resistance of 1 megohm or any fraction thereof, and 

 the steady deflexions observed. This arrangement is very 

 convenient, since, by merely changing a few keys, any 



* Proc. Roy. Soc. A. vol. Ixxxiv. p. 433 (1910). 



