428 xMR. C. T. E. WILSON ON TPIE CONDENSATION NUCLEI PRODUCED IN 
into visible particles, and their persistence even in iinsaturated air, is to suppose that 
by the action of the ultra-violet light some compound is formed in solution in each 
drop. Were it not for the fact that the fogs are produced in pure oxygen as well as 
in air, one would naturally consider the combination of oxygen and nitrogen with the 
water of the incijhent drop to foiui nitric acid as the most likely reaction which could 
account for the }>henomena. Possibly when tlie clouds are produced in moist air this 
may he, in fact, the reaction which takes place. When the clouds are produced in 
oxygen, however, the only possible combination whicli can account for the phenomena 
is that of oxygen and water to form hydrogen jDeroxide. The formation of ozone 
would not enable us to explain the production of the clouds, and indeed although 
clouds are very easily produced in ozonised oxygen it is, as the experiments of 
Meissner and others show, oidy as a consequence of reactions, by which some of the 
ozone is destroyed. 
The view here taken is then, that under tlie action of the ultra-violet light small 
drops of water combine with the oxygen in contact vath them, and in consequence of 
the lowering of the equilibiium vapour pressure by the dissolved H. 2 O 2 they are ahle 
to grow, when similar drops of pure water would evaporate. 
The time taken by the nuclei to grow to any given size depends simply on the 
time required for tlie (piantity of dissolved substance produced in each drop by the 
action of the ultra-violet light to become sufficient to enable a drop of that size to be 
in equilibrium. That, for a drop containing a definite (piantity of dissolved substance, 
there is a definite size necessary foi' ecpiilibrium, is obvious from the fact that the 
lowering of vapour pressure due to the dissolved substance is proportional to the 
concentration, tliat is, inversely proportional to the volume, while the increase of 
vapour pressure due to the curvature of the surface is inversely jiroportional to the 
radius. By the growth of the drop, if initially the solution is too strong for 
equilibrium, the lowering of vapour pressure due to the dissolved substance will very 
quickly diminish till it ceases to exceed the rise of vapour pressure due to the 
curvature. 
If it is only, as is in itself ipiite likely, at the surface of separation of the gas and 
litpiid that the idtra-violet rays cause combination to take place, the maximum effect 
will be produced where, as in this case, the water is in the form of a cloud of minute 
particles ; for it is only in very small drops that any considerable proportion of the 
molecules are situated in the surface layer. 
T1 le al)sence of any effect of this kind in moist hydi'ogen is in agreement with 
the view that the growth of the drops in air or oxygen is due to the formation of 
hydrogen peroxide. 
§ G. Nuclei produced by Sunlight. 
Aitken^' has shown that many vapours when exposed to sunlight in glass vessels 
* Aitken, ‘Trans. Roy. Soc.,’ Edin., vol. 39 (1), p. 15, 1897. 
