Nuclei produced by Action of Light on Iodine Vapour. 471 



Influence of the Gas in the Expansion Apparatus. 

 Air, hydrogen, carbon dioxide, coal-gas, and oxygen were 

 tried in tarn in the expansion apparatus. In each case the 

 apparatus was first calibrated before introducing any iodine 

 in order to study the normal Wilson effects in these gases. 

 With hydrogen, C0 2 , and coal-gas, the introduction of the 

 iodine produced absolutely no change. On the other hand, 

 in the case of oxygen, the iodine gave rise to clouds similar 

 to (possibly a little denser than) those obtained with air. 

 Thus the presence of oxygen is necessary to the formation of 

 the nuclei. 



Alcohol in the Expansion Apparatus. 

 Some experiments were tried in which the water in the 

 expansion apparatus and in the cloud-chamber was replaced 

 by alcohol. With this liquid the normal Wilson effects 

 begin at a pressure-fall of 10 cms., and fogs are obtained 

 for a pressure-fall of about 12 cms. No increased effect 

 could, however, be detected on admitting iodine. In fact, 

 when the iodine had been in the apparatus for a day or two 

 so that the alcohol had developed a bright yellow colour by 

 the solution in it of some iodine vapour, the Wilson effects 

 at any given expansion were then distinctly smaller than 

 they were initially. 



Behaviour of the Nuclei in an Electric Field. 

 In order to ascertain if the nuclei are charged, a cloud- 

 chamber containing a horizontal platinum disk was con- 

 structed, the distance between the disk and the water surface 

 being about 1*5 cms. A potential difference of 230 volts 

 could be established when desired between the disk and the 

 water. We could, however, detect no evidence of any 

 motion of the nuclei under the electric force, for their rate 

 of disappearance in the dark was the same with the field on 

 as without. It may be concluded, therefore, that the nuclei 

 do not carry an electric charge. 



Diminution of the Effect with Time. 

 We noticed early on in the course of the experiments 

 that the coloured clouds obtained with the iodine in the 

 apparatus become less and less dense as time goes on. By 

 the second day the result of an expansion of 18'5 is only a 

 kl di>n*e shower/' This decay continues from day to day, 

 until by the fourth or fifth day the effect obtained is actually 

 smaller than the normal Wilson effect i'ov the same ex- 

 pansion. It was found, ill addition, thai the effect of the 



