308 JOHN AITKEN ON 
of the amount of air present, that is, independent of the pressure of the air, 
and depends only on the temperature. 
After air has become what is called “ saturated ” with vapour, that is, when 
the vapour tension is that due to the temperature, a momentary condition of 
stability is attained. Suppose the temperature to fall, a change must now take 
place. All the water cannot remain as invisible vapour ; some of it must con- 
dense out into its visible form. It is this condensed water held in mechanical 
suspension in the air to which we give the names of fog, cloud, mist, and rain, 
phenomena having some resemblance to each other, yet possessing marked 
differences. The particles composing a fog, for instance, are so fine they 
scarcely fall through the air, a cloud is a little coarser in the grain, while a 
mist is coarser still in texture, and rain is any of these while falling, whether it 
be a wetting mist or a drenching rain. And the question now comes, Why 
this difference? Why should the water vapour condense out of the air in one 
case in particles so minute they seem to have no weight, and remain 
suspended in the air, while in another case they are large grained and fall 
rapidly 2 
As the key to the answer to this question is given by a very simple experi- 
ment, it will be well for us here to have a clear conception of the conditions of 
that experiment. Here are two large glass receivers, both connected to this 
boiler by means of pipes. If we now allow steam to pass into this receiver, 
which we shall call A, you will see the steam whenever it begins to enter. 
There it comes, rising in a dense cloud, and soon you see the receiver gets filled 
with the condensed vapour, forming a beautiful white foggy cloud, so dense 
that you cannot see through it. Let us now pass some steam into the other 
receiver, which we shall call B. Observe—nay, you may strain your eyes as 
much as you please, you cannot see when the steam begins to enter, and now 
it has been rushing in for some time, and yet you cannot see it. There is not 
the slightest appearance of cloudiness in the receiver, yet it is as full of water 
vapour as the receiver A, which still remains densely packed with fog. 
Now, why this difference in the two cases? Simply this. The receiver A, 
which is so full of fog, was at the beginning of the experiment full of ordinary 
air—the air of this room—while the other receiver B was also full of the air 
of this room, but before entering the receiver it was passed through a filter of 
cotton-wool, and all dust removed from it. The great difference, then, between 
the appearance of these two receivers is due to the dust in the air. Dusty air 
—that is, ordinary air, gives a dense white cloud of condensed vapour. Dust- 
less air gives no fogging whatever. 
But why should there be this difference in the two cases? Why should 
dust have this peculiar action? or rather, Why does not the water vapour 
condense into its visible form in air free from dust? The air is “ super- 
