72 Proceedings of the Royal Irish Academy. 



gas with X-rajB, the charge oa the nadens will depend on. the intensity of 

 the ionization. In all cases, howeTer, as iax as erperiiuent has gone, the 

 mobility has the same iialne '0003. The mobility mnsfc, therefore, Ibe inde- 

 pendent of the charge, a lesnlt which is in agreement with Sir J. J. ThomsonW 

 theory of the mobility of ions in gases. 



The Mature of the- Large Ions and Condensafion-Ifudci. 



Though LangeTin's theory gives a satisfacfeozy explanation of the stable 

 existence of a reiy minate water-drop, even in nnsatorated air, such drops 

 being the nuclei for condensatiou found in the atmosphere and prodaced by 

 flames, there is yet very little to suggest an explanadon of the origin and 

 growth of the nndens itsell It has been already stated that the charge can 

 play no important part in its formation, since the number produced by the 

 flame is not affected by the removal of aU the free ions just as the gas leaves 

 the flame. On the other hand, the production of nuclei in flames is associated 

 with the production of water-vapour. Ions of the same mobility 0003, and 

 neutral nuclei, are also produced by the burning of hydrc^n : but flames such 

 as CO, sulphur, or arsenic, in which water is not produced, give no lai^ ions. 

 That the presence of water- vapour, however, is not sufficient for the production 

 of large ions and condensation-nuclei has been demonstrated by the experiments 

 of de Broglie* and Aitken on the hydrogen flame. De Broglie, using a flame 

 of hydrogen thoroughly dried and purified, and burning at the end of a lead 

 tnbe in a chamber kept cool by a water-bath, found that no large ions are 

 produced. Aitken with similar precautions as to purity found ^lat hydrogen 

 burning in thoroughly filtered air produced no condensation nuclei, but if any 

 solid imparities, such as particles of dust or rubber from the connecting tubes, 

 found their way into the flame, great numbers of nuclei were immediately 

 formed. Further experiment on the early history of the large ion, such as, 

 for example, the variation of the mobility as the gas from the flame cools, 

 may give more information on the nature and origin of the nucleus forming 

 the large ion. 



There are two other allied phenomena, which in this respect seem worthy 

 of further study. It is well known that when all condensation nuclei have 

 been removed from a mass of air, saturated with water-vapour, either by 

 repeated expansion or by filtering, an exceedinsly dense clcnd is formed 



' Sir J. J. Thomsrai : Proc of the Physical Sodecy, toL 27. Dec.. m4. 

 -de Broglie : "Comptes Bendns," toL 151, 1910, p. 67. 



