1889-90.] 
Mr J. Aitken on Dust Particles. 
225 
seems only to begin when the air is near the point of saturation. 
There seems no definite degree of humidity necessary for the first form 
of condensation to begin, or if there is, it begins while the air is 
very dry, and by such imperceptible degrees, it has not been noticed. 
There is no hard and fast line between what we call clear air and 
thick haze. The clearest air has some haze, and the effect of in- 
creasing humidity is to increase the thickness of the air. But from 
clear air up to cloudy condensation there is no real difference in 
kind, only in the amount, of the thickening. But when cloudy 
condensation begins, a real change in the nature of the condensation 
takes place. The dust particles have now no tendency to condense 
the vapour ; each particle seems to have got its affinity satisfied, and 
true condensation begins, owing to the tendency of the saturated 
vapour to condense. The affinity of the particles has now little 
influence, but the size of the different particles has ; the larger ones 
getting the most moisture deposited on them. All the particles at 
this stage seem to cease to attract the vapour, and the vapour is no 
longer deposited on the whole number of particles ; but a compara- 
tively small proportion, and these the larger particles, receive the whole 
of the condensing molecules. This seems to be the reason why at 
about saturation a change occurs in the appearance of the condensa- 
tion, a sudden thickening of the air takes place along with an 
increased light reflecting power. 
It might be thought that if this incipient condensation, due to 
affinity, took place before the true condensation, then there ought 
to be some evidence of it in the every day phenomena of nature. 
Bor instance, does the upper moving front of a cumulus cloud show 
this incipient stage of condensation? At first sight we might 
expect it would. I do not think, however, that it has been noticed, 
possibly owing to the conditions necessary for seeing it not being 
easily obtained. We cannot expect to see it at the upper surface 
after the cloud is formed, the transition at that surface from 
saturated to dry air is far too sudden. If we are to see it, in the 
formation of a cumulus cloud, we must look for it just before the 
cloud begins to show, in the clear and rising air, where it is 
possible we may find a thickening taking place before, and at a 
lower level, than that at which the true cloud condensation begins. 
This as yet has not been observed, but in other conditions the 
VOL. xvil. 26/6/90 
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