44 Proceedings of the Roi/al Irish Academy. 



Again, it may be mentioned that the eight groups of ions dealt witli in 

 this paper do not account for all the ionization. If the graphs are pushed 

 on, further indications are found of the existence of other groups having 

 mobilities still lower. These have not yet been fully worked out. 



If the evidence of these results be accepted, it would seem as if tlie 

 theoi"ies which regard tlie small ion as an atom or molecule must be definitely 

 abandoned. The small ion seems then to be a mi.xture, for the greater part, 

 of groups of four dilferent sizes. The unit out of whicli tliese groups are 

 built up is probably the ion of mobility 12'5, the smallest of the more 

 mobile ions found present. Now, almost any method of calculating the 

 mobility of an ion on the elastic collision hypothesis will ijivn for ihc 

 mobility of a monomolecular ion something about this value. For example, 

 in a previous paper* the author found that Sir J. J. Thomson's formula 

 would give for a monomolecular ion of water the value 12'.'-!, ami fur an ion 

 of oxygen or nilrogen the value 12. We are therefore justified in assuming 

 that this fastest ion is a single molecule, and that the other ions are groups 

 of increa.'^ing numbers of molecules. 'J'lie reasons given in the previous paper 

 for believing that the molecular unit, of wiiieh the ions are biiill ii)i, is the 

 water mi'lecule, rather than the oxygen or nitrogen molecule, still bold 

 good; in fact, the whole argument of the i)re\ious paper is strengthened by 

 the re.sults now presented. 



While the present paper ha.s been in jireparation, a paper by O. Blackwood 

 on " The Existence of liomogcneous Groups of Large Ions"' has come under 

 the writer's notice. I»lackwood claims to have proved that the existence of 

 a distinct group system of large ions, as found by the present writer and 

 others,' cannot be verified. " In other words, he finds a coivlinu^us spectrum 

 of mobilities, and not a hand spectrum." He also holds that the evidence 

 brought forward by Ihe author in favour of the existence among sjnay-ions 

 of mobilities liiglier than the normal may be interpreted in some other way, 

 without assuming such abnormal mobilities. 



Taking the latter point first, the author is of the opinion that the results 



.given in the present ])aper on ions of high mobility are a remarkable 



confirmation of those given previously. The grounds on which Ulackwood 



bases his criticism of the previous work (the validity of which criticism the 



' Proc. Royal Society, A, vol. xciv, p. 112 (1918). 



> Physical Review, Au;^.. 1920. 



> J. J. Nolan. Proc. R I.A., A, vol xxxiii, p. 9 (1916). M'ClellniKl and P. J. Nolan, 

 Proc. R.I. A., A. vol. xxxiii. y. 24 fl91i;) : vol. jtxxiv. |.. .")1 (1918) ; and vol. xxxv, ji. 1. 

 (1919). 



