86 Proceedings of the Royal Irish Academy. 



from experiments perfonned by as on the effect on the reproduction of the 

 larwe ions of filtering the air through cotton-wool. The air was drawn into 

 the reservoir through a plug of cotton-wool tightly packed and of considerable 

 length. It was allowed to remain in the gas-holder for about an hour and a 

 half, corresponding to the time for maximum reproduction of large ions in 

 deionised air, and was then examined by passing it through the electric field 

 and taking the saturation current. 'ITie saturation current obtained was only 

 "14 of that of the free air. The filter became more and more effective in pre- 

 venting the reproduction of the large ions as the quantity of cotton-wool was 

 increased, and as it was more closely packed, so that it seems likely that by 

 very perfect filtering the air could be reduced to a state in which no lai^e 

 ions would be reproduced. This is what would happen if all the nuclei pre- 

 sent in the air and capable of forming large ions were removed, and corresponds 

 with the fact that no cloud is produced by small expansions when the air is 

 filtered through cotton-wool before being allowed into the condensation 

 chamber. There is, therefore, strong evidence for considering these large 

 ions as being formed by an uncharged centre attracting to it a small ion 

 present in the air. 



HaA"ing arrived at this conception of the large ions, we must examine 

 more closely the nature and properties of the nuclei necessary for their 

 production. What is probably the most striking characteristic of the large 

 ions is the feet that they are all of the same size. If, therefore, the lai^e ion 

 consists of a small ion and a nucleus, the nuclei must all be of the same 

 size. Ghauveau* in examining the nature of the nuclei necessary for the 

 condensation experiments of Aitken considers the sources of " dust " in the 

 form of fine solid particle suspended in the atmosphere ; for example, the 

 raising of considerable quantities of matter from the surface of the ground 

 by upward air-currents, especially in desert countries, and the occasional 

 volcanic eruptions which project immense quantities of finely divided matter 

 to a great distance. Though admitting the difficulty of certain proof, he 

 gives r^sons for considering it doubtful if in normal air there is a sufficient 

 quantity of solid matter in a fine state of division to account for the 

 experiments of Aitken, who quotes 100,000 per cc. as the number of these 

 nuclei frequently obtained near the ground. There is an incomparably 

 greater difficulty in accepting these soUd dust particles as being the nuclei 

 concerned in the formation of large ions : for it is not conceivable that the 

 particles produced without any apparent regular law could be all of the same 



« Le Radium, avrO, 1912, pp. 161-169. 



