1897.] on the condensation of Water Vapour. 337 



The Uranium salt shows a strong effect on the condensation, 

 even when the thin glass vessel containing it is completely 

 wrapped in tinfoil during the experiment and for many hours 

 previously ; showing both that this material is transparent to 

 the agent which influences the condensation, and that the 

 Uranium salt continues to be active when kept in the dark. 

 There can be little doubt therefore that the effects on the con- 

 densation are really due to the radiation studied by Becquerel. 



The nuclei introduced by the action of the Uranium rays, like 

 those formed under the influence of the Z-rays, persist for some 

 seconds after the radiation has been cut off; for a comparatively 

 dense shower can be obtained, by expansions of suitable amount, 

 five or ten seconds after the removal of the tube containing the 

 Uranium salt. If, however, an interval of one minute be allowed 

 to elapse after cutting off the radiation, the drops produced on 

 expansion are not appreciably more numerous than they would 

 be, were the air not exposed to the radiation at all. 



The nuclei produced in moist air by Uranium rays appear, 

 from the experiments described above, to be identical with those 

 formed under the action of Rontgen rays, as well as with those 

 always present in very small numbers in the moist gas. They 

 require the air, originally saturated, to suffer sudden expansion 

 such that v 2 /v 1 exceeds 1*25 in order that condensation may take 

 place upon them ; the density of the vapour at the moment when 

 the expansion is completed being then approximately four times 

 that of vapour in equilibrium over a flat surface of water at the 

 same temperature (v. Phil. Trans, loc. cit). 



The electrical properties of gases under the action of "Rontgen 

 and Uranium rays point to the presence of free ions. It is natural 

 to identify with these the nuclei made manifest in the gas under 

 the same conditions by the condensation phenomena described 

 in this and previous papers. As early as 1887 R. v. Helmholtz 

 (Wied. Ann., vol. 32, p. 1) and later the same author and Richarz 

 (Wied. Ann., vol. 40, p. 161, 1890) accounted for the effect of 

 electrification and chemical action in bringing about dense con- 

 densation in a steam-jet by supposing free atoms or ions to be 

 produced by these processes. Prof. J. J. Thomson (Phil. Mag., 

 vol. 36, p. 313, 1893) has explained how we should expect free 

 ions, in virtue of the charge they carry, to have such an effect 

 in helping condensation. Richarz has more recently (Wied. Ann., 

 vol. 59, p. 592, 1896) found that a steam-jet reacts also with 

 Rontgen rays, and again explains the phenomenon by the hypo- 

 thesis of free ions. 



The great advantage of expansion experiments over those 

 made upon steam-jets is that we are able in the former to dis- 

 tinguish between various kinds of nuclei, by means of the different 



