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 
Horizontal Section through Electrolytic Cell. 
two lateral expansions for the solution under examination. A large 
cavity C was left in the paraffin to permit artificial cooling by 
means of a water circulation. For experiments with Rontgen 
Rays 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. 
50 
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 
