DUST, FOGS, AND CLOUDS. 343 
Let us now look more closely into the action of dust in producing cloudi- 
ness. It is very evident that the results are not always alike. In one case 
the condensed vapour takes the form of a fog, so fine that it easily floats in the 
air and never seems to settle. In another case the cloudiness is coarser 
grained and settles down slowly, and in another case it is a very coarse-grained 
mist which falls quickly (of course I am not here speaking of the coarse 
grainedness produced by a number of small particles combining to form one). 
From the experiments described, it would appear that, when the dust is 
present in great quantities, the condensed vapour forms a fog, because as there 
are a great number of dust nuclei each nucleus only gets a very little vapour, 
and is not made much larger or heavier, so it continues to float in the air. As 
the number of dust nuclei diminish, the amount of vapour condensed on each 
particle increases, their size and weight therefore also increase. So that as 
the density of the cloudiness decreases the size of the particles increases, and 
their tendency to settle down also increases. Fogs will, therefore, only be 
produced when there is abundance of dust nuclei and plenty of vapour. 
There is probably also something due to the composition of the dust particles ; 
some kinds of dust seem to form better nuclei than others. 
We now come to the question of what forms this dust. What is its 
composition? Whence its source? I have been unable to get any trustworthy 
information as to the chemical composition of the dust. The only analysis 
I have seen is of dust collected in rooms. Now it is evident that as this 
dust has settled down, it will be, so to speak, winnowed dust, and will there- 
fore contain too small a proportion of the finer particles. 
As to where this dust comes from, it is evident it will have many sources. 
Everything in nature which tends to break up matter into minute parts will 
contribute its share. In all probability the spray from the ocean, after it is 
dried and nothing but a fine salt-dust left, is perhaps one of the most important 
sources of cloud-producing dust. It is well known that this form of dust is 
ever present in our atmosphere, and is constantly settling on every object, as 
evidenced by the yellow sodium flame seen when bodies are heated. There is 
also meteoric dust, and volcanic dust and condensed gases. At present, 
however, I wish to confine our attention to the action of heat as a producer 
of atmospheric dust, and more especially in relation to its fog-producing 
power. 
Most of us on entering a darkened room, into which the sun is shining 
through a small opening in the shutters, have observed the very peculiar effect 
of the sun’s rays when seen under these conditions, the path of the beam 
of light being distinctly visible, shining like a luminous bar amidst the sur- 
rounding darkness. On closely looking at it, it is seen that this peculiar effect 
is produced by the dust motes floating in the air of the room reflecting the 
