432 Mr Cunningham, On an Attempt to detect the Ionisation 



paraffin until it reached these holes, so that when the quartz 

 plate QQ was fastened over this recess it left a flat chamber with 



D 



Hoeizontal Section through Electrolytic Cell. 



two lateral expansions for the solution under examination. A large 

 cavity G was left in the paraffin to permit artificial cooling by- 

 means of a water circulation. For experiments with Rontgen 

 Ra}'s a thin sheet of ebonite was substituted for the quartz 

 plate. 



The source of light chiefly employed was an alternating arc 

 between carbon terminals in the cores of which thick iron wires were 

 inserted. By means of two quartz lenses the light from this arc 

 was made to converge upon the film of solution under investiga- 

 tion. In some cases the cell was simply exposed to a bright sky 

 at the Laboratory window. For Rontgen radiation an ordinary 

 X-Ray tube driven by a small induction coil was employed. 



The resistance of any given solution was measured by Mr 



Fitzpatrick's modification of Kohlrausch's method (Brit. Ass. 



Report, 1886, p. 328). The cell was standardised by making a 



N 

 measurement of its resistance when filled with — KC1 solution. 



oO 



After some rather rough preliminary work, all the solutions 

 were made up and kept in a dark-room. The cell was also filled 

 there, and placed in a carefully constructed light-tight box, 

 through which projecting wires were permanently sealed. These 

 were readily connected with the electrodes of the cell by means 

 of mercury cups. 



Even with this last most improved form of cell and with 

 the assistance of a water circulation maintained only a few milli- 

 metres behind the illuminated layer (and the paraffin prepared 

 as above is very transparent to ordinary visible light), it was 

 found impossible to keep the temperature constant before and 

 after turning on the light. The temperature of the solution was 

 measured by the change in resistance of a very thin platinum 



