GASES BY THE ACTION OF EONTGEN RAYS AND OTHER AGENTS. 
453 
combination at once takes place with the formation of a cloud. If the moist gases 
contain minute water drojDS, it is evident that combination must take place within the 
drops, for HCl and NHg at once combine when in solution. It appears to me natural 
to suppose that the fogs produced by this and similar reactions are to be explained by 
the products of the reaction being formed in solution in incipient drops, in quantities 
sufficient to counterbalance the effect of the curvature of the surface on the vapour 
pressure. The drops then grow so long as the products of the reaction continue to 
accumulate within them. The original droplets may be formed by the action of the 
ions ; but it is quite possible that even in the absence of any ions, minute drops are 
continually being formed, and on account of surface tension at once evaporating 
again, unless made permanent by the formation within them of some other substance 
than water. 
Aitken"^ found that when proper precautions were taken, no condensation nuclei 
were produced by the combustion of hydrogen. In this case (if we assume the 
product of combustion to be pure water only) any growth of the droplets through the 
lowering of the vapour pressure by a dissolved substance is out of the question. 
Although these experiments of Aitken show that in this case there is no production 
of comparatively large nuclei, such as would be capable of promoting condensation 
with slight supersaturation, or of travelling a considerable distance along a narrow 
tube without being removed, they do not prove that no free ions are produced by 
the combustion, or that these would not act as centres of condensation if the degree 
of supersaturation were reached, which the experiments described in this paper show 
to be in general required to cause condensation on the ions. 
There is, I think, no evidence that the ions alone, in the absence ot other influences, 
ever act as centres of condensation unless the above-mentioned comparatively great 
degree of supersaturation (approximately fourfold) be exceeded. 
In conclusion, I wish to acknowledge how greatly I am indebted to Professor 
Thomson for his suggestions and encouragement during the course of this work. 
* Aitkex, ‘ Trans. Roy. Soc., Edin.,’ vol. 39 (1), p. 15, 1897. 
