NATURE OF THE LARGE IONS IN THE AIR. 203 



manufacture of ions of feeble mobility by the artificial 

 production of small ones has been shown in other ways 

 by M. de Broglie. 1 For help with these condensation 

 experiments I am indebted to Mr. G. W. Sinclair. 



The results just given show that ions with the charac- 

 teristic mobility of natural atmospheric large ions are 

 formed in dusty but not in dust-free air, and support the view 

 of the production of large ions previously mentioned as 

 deduced from their observations by MM. Langevin and 

 Moulin. In addition, the measurements collected in the 

 earlier portion of this paper, by establishing the existence 

 of a definite relation between the mobility of the large 

 ions and the humidity of the air, afford proof that the 

 ions are composed, in part at least, of water molecules. 

 These determinations throw considerable light on the 

 question of the structure of the natural large ion, and on 

 the whole evidence the picture most readily formed is that 

 of a small ion attached to a dust particle round which 

 water molecules are collected, even in unsaturated air, to 

 an extent depending on the humidity. 



1 M. de Broglie, Theses presentees a la Faculte des Sciences de Paris, 

 Ganthier-Villars, Paris, 1908. 



