Nuclei produced by Action of Light on Iodine Vapour. 4:13 



(7>) A small quantity o£ radium was placed near the 

 cloud-chamber and the minimum expansion required 

 to catch the ions so produced carefully determined, 

 first before any iodine has been introduced, and after- 

 wards when the water in the apparatus had become 

 strongly coloured by the iodine. The condensation 

 on the ions was found to start at exactly the same 

 expansion in both cases*. Hence the vapour-pressure 

 of the water is not appreciably altered by the iodine. 

 (t>) It has already been mentioned that the clouds after their 

 disappearance are brought back by rinsing the apparatus 

 out afresh with distilled water. We have also stated 

 that admitting fresh water into the cloud-chamber 

 through the tap D (fig. 2) is without effect. The two 

 statements may appear contradictory. But an inspection 

 of fig. 2 shows that there is a difference between the 

 two operations. As may be seen from the figure, the 

 water enters the apparatus through a nozzle F projecting 

 into the bulb, and consequently settles in the bulb 

 without flowing down the walls. With an earlier 

 form of cloud-chamber, in which the nozzle F was 

 absent, the water on admission ran down the sides of 

 the bulb, and in this case the clouds were found to have 

 been partly brought back. But the experiments with 

 the cloud-chamber of tig. 2 show that this increase in 

 the effect was due not so much to the changing of the 

 water in the apparatus as to the rinsing of the glass 

 walls by the water as it flowed into the bulb. 



* Taking- as the measure of the expansion the ratio of the final to the 

 initial volume of the gas, we found that in our apparatus the minimum 

 expansion required to catch the ions produced by radium was 1-22. 

 C. T. R. Wilson (Camb. Phil. Soc. Proc. vol. ix.) gives the same value 

 for the same form of apparatus. It lias already been mentioned that the 

 ordinary Wilson effect was observed in our experiment to be smaller 

 when the apparatus had had iodine in it for some days than it is for the 

 same expansions in an apparatus free from iodine. And yet the experi- 

 ment with the radium shows that the ionic condensation commet/ces at 

 the same point in the two cases. This fact suggests that the sjtontaneous 

 ionization in a closed space is reduced by saturating the space with 

 iodine vapour. A similar effect was noticed in the experiments with 

 alcohol. We propose investigating this point further. It is possible 

 that there may be a connexion between this decrease in the Wilson 

 effects and the result obtained by Henry (Proc. Camb. Phil. Soc. L897J 

 in his experiments on the " Effect of Ultra-Violet Light on the Con? 

 ductivity of Iodine Vapour." Henry found that the discharge of ions 

 from a metal plate when illuminated by ultra-violet light was greatly 

 reduced by admitting iodine vapour into the ionizati on-chamber, but he 

 was uncertain as to whether this was a real effect or a spurious one due 

 to the weakening of the light in its passage through the vapour. 

 Phil. Mag. S. 6. Vol. 21. No. 124. April 1911. 2 I 



