344 JOHN AITKEN ON 
light, and becoming visible as they pass through the path of the beam. We 
are struck by the marvellous amount of dust thus revealed ever floating in our 
atmosphere, and which under ordinary conditions of light are not observed. It 
is known that when air containing this dust is highly heated or passed through 
a flame, all these motes are destroyed, and the path of the sun’s rays becomes 
invisible. 
Returning uow to the question of fogs, one might naturally conclude from 
what we have said that air which had passed over or through a flame or 
through a fire, where the combustion was perfect, ought to be nearly dustless, 
and, therefore, ought not to be a good medium for fogs. Before, however, 
coming to any conclusion on this point, it was deemed necessary to make more 
direct experiments, and we shall presently see that, however natural our con- 
clusion is, it is very far wrong. Heating the air may cause the dust motes to 
become invisible; but so far as my experiments go, they prove that the 
heating of the air by the flame does not remove the dust, but rather acts in 
the opposite way, and increases the number of the particles. The heat would 
seem to destroy the light-reflecting power of the dust, by breaking up the 
larger motes into smaller ones, and by carbonising or in some way changing 
their colour, and thus make them less light-reflecting. 
Powerful as the sun’s rays are as a dust revealer, I feel confident we have 
in the fog-producing power of the air a test far simpler, more powerful and 
delicate, than the most brilliant beam at our disposal. When steam escapes 
into the air it condenses on the dust particles, and thus by simply magnifying 
their size, makes their number evident to the eye. Every fog particle in the air 
was represented by a dust particle before the steam was added, but these were 
invisible to the eye till increased in size by the vapour. This would seem to 
indicate a condition of the atmosphere too impure to be true, yet I think we 
are justified in our conclusion, as it has been shown that when there is no dust 
there is no fogging. In the future, therefore, we will be compelled to look upon 
our “ breath” as seen on a cold morning, as evidence of the dusty state of the 
air. And every puff of steam as it escapes into the atmosphere will remind 
us still more powerfully of the same disagreeable fact. If it was not for 
dust we would never see our “breath,” nor would wreathes of steam be 
seen floating in the air, nor would our railway stations and tunnels be 
thick with its cloudiness. The only consolation we have is, this fine dust is 
not easily wetted. The air we breathe is not deprived of all its dust in its 
passage through the lungs. The air which we exhale is still active as a fog- 
producer. If, for instance, we inhale the air by the nostrils, and pass it by the 
mouth to the experimental receiver, we find it still full of dust and fog- 
producing. We might have expected, that after passing over so much wetted 
surface, the dust would have been all taken out of the air. This difficulty 
i 
