Conductivity produced by Heating Salts. 739 



at higher pressures and higher potentials. Under these 

 conditions, with leaf positively charged, the time with 

 magnet off to the time with magnet on was in ratio 17 : 20. 

 With negatively charged leaf, under similar conditions, the 

 magnet produced no effects. 



In the case of the sudden discharge, which was brought 

 about by the application of the magnetic field, it is possible 

 that the field produces a concentration of the paths of the 

 ions ; and hence a much more marked effect, owing to 

 the formation of ions by collision, is the result. The effect 

 on the positive ions may be a secondary one, since it is 

 known that very strong magnetic fields are required to 

 deflect positive ions. 



(c) Leah stopped by aluminium-foil. — Some zinc iodide was 

 placed in a small copper dish, and covered with two layers of 

 aluminium foil, each *0004 cm. in thickness. Some wax 

 was placed round the edges of the dish, and the foil was 

 placed on the wax so as to reduce the amount of diffusion to 

 a minimum. This was placed in the electroscope, and the 

 pressure reduced to 2*8 mm. of mercury. No leak could 

 now be detected, when the leaf was either positively or 

 negatively charged. Hence it appears that the ions pro- 

 ducing the leak must be very easily absorbed, as their action 

 could be stopped by -0008 cm. of aluminium. 



(d) It was repeatedly found that if zinc iodide was left in 

 the apparatus (fig. 3) for two or three days, a very large leak 

 due to negative ions occurred when the apparatus was first 

 connected with the battery, but that the leak quickly died 

 away. In four minutes the leak was less than one- half its 

 original value. In order to see if the presence of the 

 zinc iodide was the actual cause of this increased con- 

 ductivity of the enclosed air, the tube was left empty for 

 five days under similar conditions. At the end of that time 

 no large leak was noted. 



Hence it appears that the zinc iodide has the power of 

 emitting negatively charged particles at ordinary tempe- 

 ratures. 



VIII. Experiments on the Velocity of the Ions. 



In the conditions in which the ions are produced, the 

 choice of the method used to determine their velocity was 

 limited. 



Prof. J. J. Thomson has shown, in his ' Conduction of 

 Electricity through Gases,' first edition, pp. 74-78, that 

 if the ionization is confined to a very thin layer compared 



3D 3 



