Nuclei produced by Action of Light on Iodine Vapour. 477 



that ultra-violet light caused clouds to form without ex- 

 pansion in air containing traces of amyl nitrite, iodide of 

 allyl, and other vapours. 



Aitken found that clouds are produced in NH 3 , H 2 SO s , 

 H 2 S, HC1, and Cl 2 when expanded after exposure to sun- 

 light. 



C. T. R. Wilson showed that the action of ultra-violet 

 light on pure moist air or oxygen produced condensation 

 nuclei which grew under the influence of the light, and with 

 very intense light he obtained clouds without any expansion 

 at all. Wilson suggested that the clouds obtained by him 

 are due to the formation of H 2 2? winch, by dissolving in the 

 drops as they form, lowers the vapour-pressure and thus 

 makes it possible for the drops to grow where drops of pure 

 water would evaporate. 



Bevan * again, in his experiments on the u Combination of 

 Hydrogen and Chlorine under the Influence of Light/'' found 

 that some substance is produced by the action of the light 

 on the chlorine and water-vapour which acts as condensation 

 nuclei, and that the formation of this intermediate substance 

 is necessary for the production of the HC1. 



With regard to the nuclei described in the present paper, 

 it is possible that the reactions going on under the influence 

 of the light are as follows : — 



H 2 + I 2 — »HI + HIO, 

 HIO + 0,.— HI + O,; 



or it may be a reaction in which the iodine is oxidized 

 directly. Moreover, it is difficult to say which of the products 

 of such reactions would actually form the nuclei. Ozone, of 

 course, is known to give rise to clouds, but, according to 

 Meissner f , only as a consequence of reactions by which 

 some of the ozone is destroyed. 



The above reactions, being reversible, would cease when 

 a certain amount of the products had been formed. This 

 fact would explain why no clouds are obtained when the 

 iodine has been for some days in the apparatus. It would 

 also account for the reappearance of the clouds on washing 

 the apparatus, for such treatment would remove the products 

 of: the reaction and allow it to go on once more. From this 

 point of view the effect of introducing a plug of glass-wool 

 into the cloud-chamber is readily understood, for the fibres 



•' * Bevan, Phil. Trans, vol. ccii. A. p. 347 (1903). 

 t Quoted by C. T. R. Wilson, Phil. Trans, vol. cxcii; p. 42S (1899). 



