Nuclei produced by Action of Light on Iodine Vapour. 467 



" pressure-fall " are centimetres of mercury, and, as already 

 explained, may be taken as a measure of the expansion. For 

 the purpose of comparison the Wilson effects obtained with 

 our apparatus are first given. 



The above table shows that the presence of the iodine 

 vapour in the cloud-chamber produces a considerable increase 

 in the density of the clouds obtained, and that the effect of 

 the iodine is specially marked for pressure-fall of 18*5 cms. 



The influence of the iodine was found to be very persistent, 

 sweeping in fresh air through the by-path D for several 

 hours failed to get rid of the effect. In fact, it was found 

 necessary to take the apparatus down and wash it thoroughly 

 to make it give once more the ordinary Wilson effects. 

 Later experiments (described below) showed that the in- 

 fluence of the iodine ultimately disappears if the apparatus 

 is allowed to stand in bright light for three or four days. 



When the above effect was first obtained, we were 

 naturally led to believe that iodine, in contradistinction to 

 the other substances tried, does sublime in the form of 

 particles sufficiently large to act as condensation nuclei. 

 But this conclusion was upset by the following modification 

 of the experiment. Between the iodine reservoir and the 

 cloud-chamber a tube containing a long plug of glass-wool 

 was inserted, in order to see if the iodine particles could be 

 trapped and preA^ented from reaching the cloud-chamber. 

 After sweeping air and iodine vapour through this tube for 

 two minutes, the clouds obtained with a pressure-fall of 18*5 

 were now much denser than before. Further, the. passage 

 of the iodine vapour was observed to produce a discoloration 

 of the wool, and, finally, when the latter was coloured through 

 its whole length, heavy clouds were obtained for quite small 

 expansions corresponding to a pressure-fall of less than 

 10 cms. This last effect was at first regarded as showing 

 that when the iodine is dispersed in the form of minute 

 crystals in the interstices of the wool the mechanical action 

 of the air-current results in small solid particles of iodine 

 being dislodged from these crystals and carried over into the 

 bulb. This view, however, w r as shown to be erroneous by 

 the following subsequent observations : — 



(1) The glass-wool very soon lost its power of giving rise to 



the larger nuclei. 



(2) On expanding without previously illuminating the gas in 

 the cloud-chamber the results were just the same as the 

 Wilson effects. 



Thus the effects obtained with the iodine are not 

 mechanical at all, but photochemical* 



