342 JOHN AITKEN ON 
they might change their state. Arrived at this point, the presumption was 
very strong that water vapour could be cooled below the boiling-point for the 
pressure without condensing. It was on looking for some experimental illus- 
tration of the cooling of vapour in air below the temperature corresponding to 
the pressure that I thought that the dust in the air formed “free surfaces” on 
which the vapour condensed and prevented it getting supersaturated. Arrange- 
ments were at once made for passing the air experimented on through a cotton- 
wool filter, and it was then that I first found that air which was free from dust 
gave no cloudy condensation when mixed with steam, and that the super- 
saturated air remained perfectly clear. 
Shortly after this, the investigation had to be abandoned, and all that 
remained of it was a sketch of the apparatus in my notebook, together with a 
description of the experiments made with it, till about the middle of November 
last, when the investigation was continued. The apparatus with which the 
experiments were made before the Society is the same as when used in the first 
experiments. 
The conclusions which may be drawn from these experiments are—lIst, 
that when water vapour condenses in the atmosphere, it always does so on 
some solid nucleus; 2d, that the dust particles in the air form the nuclei 
on which it condenses; 3d, if there was no dust in the air there would be no 
fogs, no clouds, no mists, and probably no rain. As we do not at present know 
anything about the temperature of condensation of vapour where there are no 
free surfaces, we cannot tel] whether the vapour in a perfectly pure atmosphere 
would ever condense to form rain; but if it did, the rain would fall from a 
nearly cloudless sky. 
I have said that if there was no dust there would be no fogs, clouds, nor 
mists; but that is not all the change which would be wrought on the face of 
nature by the absence of dust. When the air got into the condition in which 
rain falls—that is, burdened with supersaturated vapour—it would convert 
everything on the surface of the earth into a condenser, on which it would 
deposit itself. Every blade of grass and every branch of tree would drip with 
moisture deposited by the passing air; our dresses would become wet and 
dripping, and umbrellas useless; but our miseries would not end here. The 
insides of our houses would become wet; the walls and every object in the 
room would run with moisture. 
We have in this fine dust a most beautiful illustration of how the little 
things in this world work great effects in virtue of their numbers. The im- 
portance of the office, and the magnitude of the effects wrought by these less 
than microscopic dust particles, strike one with as great wonder, as the great 
depths and vast areas of rock which, the palzontologist tells us, is composed 
of the remains of microscopic animals. 
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