DUST, FOGS, AND CLOUDS. 353 
on which the present inquiry throws no light. But of clouds there are vast 
degrees of texture, the fog being the finest grained, most dense and pie 
almost never settling down. 
From this view it will be seen that the vapour condenses on the solid matter 
floating in the air, whether that matter be fine dust or condensed smoke. 
This view I am aware is different from the one generally received, namely, 
that cloud particles are hollow vesicles, hollow to enable them to float, and 
that smoke, &c., attaches itself to the outside of these vesicles. 
Since, then, fogs are produced by an over-abundance of fine atmospheric 
dust in a moist atmosphere, and as we have but little control over the moisture 
in the air, our attention must be directed principally to the diminution of the 
atmospheric dust, if we wish to reduce the density of fogs. We have seen 
that all forms of combustion, however perfect, are great producers of this less 
than microscopic dust. The brilliant flame, the transparent flame, and the 
smoky flame are all alike fog-producers. Perhaps there may be some form of 
combustion which is not a dust-producer, or some form of combustion which may 
give a coarse-grained dust. If there is, it ought to be more generally known. 
As a correction of the present form of combustion, perhaps something could 
be done to arrest the dust before it escapes into the atmosphere. But any 
plan which at present suggests itself is too troublesome and expensive ever to 
be put into general use. To prevent mistakes I may here remark, that when 
speaking of the dust produced by combustion, I do not mean the dust usually 
spoken of in connection with fires, as it is comparatively heavy, and soon 
settles to the ground, nor do I refer to smoke or soot. The dust I refer to is 
the invisible dust, so fine that it scarcely settles out of the air. If we put air 
into the experimental receiver and leave it for days without any communica- 
tion with the outer air, we will still find it fog-producing, though in a very 
marked degree less than at first. 
All our present forms of combustion not only increase the number and 
density of our town fogs, but add to them evils unknown in the fogs which 
veil our hills and overhang our rivers. In the country the fogs are white and 
pure, while in towns they are loaded with smoke and other products of 
imperfect combustion, making the air unwholesome to breathe and filthy to live 
in. But why should these two miseries always come together? Either the fog 
or the smoke is bad enough alone ; why should the smoke which usually rises 
and is carried away by the winds fall to the ground when we have fogs? I 
think that the conditions which account for the fog also account for the smoke 
falling. When we have fogs, the atmosphere is nearly saturated with vapour, 
and the smoke particles, being good radiators, are soon cooled, and form 
nuclei on which the vapour condenses. The smoke particles thus become 
loaded with moisture, which prevents them rising, and by sinking into our 
VOL. XXX. PART I. 3H 
