352 JOHN AITKEN ON 
in a visible form in pure air, and that if it did condense in those circum- 
stances, the particles would be large. From the all-pervading presence of the 
infinitesimal atmospheric dust, the idea naturally suggests itself, that the blue 
sky may be caused by the light reflected by this dust. What seems to support this 
theory is that, as we ascend to high elevations, the sky becomes deeper blue, 
this being caused by fewer and only the finer of the dust particles being able 
to keep floating in the thin air at these elevations. Further, after rain the 
sky is darker blue, this deepening of the colour being caused by much of the 
dust being washed out by the falling rain. 
I wish now to apply the result of these experiments to the great fog 
question, which Dr ALFRED CARPENTER opened at the last Social Science 
meeting, and to which at present so much attention is being directed. The 
increased frequency and density of our town fogs are now becoming so great as 
to call for immediate action. But before doing anything, a much clearer 
knowledge of the conditions which produce a fog is necessary, or much time 
will be lost and expense uselessly incurred. I wish, therefore, to call attention 
to the teaching of the experiments described, so far as they bear on this 
important question. What I have to say on this point must, however, be 
received with reservation. The conditions of a laboratory experiment are so 
different and on so small a scale, that it is not safe to carry their teaching to 
the utmost limit, and apply them to the processes which go on in nature. 
We may, however, look to these experiments for facts from which to reason, 
and for processes which will enable us to understand the grander workings of 
nature. 
We have seen that fogs and clouds are produced by the condensation of 
vapour on the dust particles floating in the air. The condensation is 
produced by cold, the result of radiation or expansion of the air, either by 
reduction of barometric pressure or by the elevation of the air into higher regions. 
A fog, therefore, before it appears, is every particle of it represented by a 
particle of very fine invisible dust ; the thick visible fog was previously repre- 
sented by an invisible dust cloud. Now, it is very evident that if there is 
an enormous number of these dust particles in the air, so that they are very 
close to each other, then each particle will only get a very small amount of 
vapour condensed upon it. It will therefore become but little heavier, and 
will float easily in the air. To this light and dense form of condensation we 
give the name of fog. If there are fewer dust particles, then each particle 
gets more vapour, and each particle is heavier and settles sooner, It must 
not be supposed, from this, that rain only falls when these dust particles are 
few, and the vapour particles very large, because there seems to be always 
enough dust in the air to make the cloud particles small enough to keep 
suspended. Their union and fall as rain is determined by certain conditions 
