DUST, FOGS, AND CLOUDS. 345 
of wetting the dust in the air may be illustrated by passing air through ‘“‘ washing 
bottles,” after which it will still be found to be full of dust. Further, during 
wet weather, after rain has fallen for a long time, all the dust is not washed 
out of the air. It is still active as a fog-producer, though in a less degree than 
during dry weather. 
I believe that at present some attempts are being made to collect and 
estimate the dust in the air. These observations deal with the weight and 
composition of the dust. I would here suggest that other observations be 
made by this fog-producing power of the air, to get not the weight or compo- 
sition of the dust, but the relative multitude of the dust-specks in it at different 
times. There seems a possibility of there being some relation between dust 
and certain questions of climate, rainfall, &c. 
The composition of the dust will also be of great importance in determining 
its power as a cloud-producer, as it is evident some kinds of dust will have a 
greater attraction for water vapour than others. Fine sodic chloride dust, for 
instance, we would expect would condense vapour, before it was cooled to the 
saturated point, on account of the great attraction that salt has for water. The 
instrument for these observations might be made to depend, either on the density 
of the fog produced by steam, or on its density when produced by reduction of 
pressure, as in the air-pump experiment. 
Before making any experiment on the fog-producing powers of flames and 
combustion, it was necessary to test the effect of heat on the apparatus to be 
used, so as to be certain the effect was entirely due to the flame and nothing 
due to the heating of the apparatus used in collecting the hot gases. I accord- 
ingly experimented in the followimg manner :—The cotton-wool filter was 
detached from the experimental receiver, and there was placed between it and 
the receiver a short length of glass tube, so arranged that the air after passing 
through the filter should pass through the tube on its way to the receiver. The 
tube was so arranged that it could easily be taken out to be cleaned, and opened 
for introducing into it any substance the effect of which we might wish to test. 
The receiver was connected to an aspirator, by means of which filtered air was 
drawn into the apparatus. 
The glass tube was first carefully washed with soap and water, and then 
with sulphuric acid, the acid being carefully washed off before the tube was 
put in its place. Air was now drawn through the apparatus, the air being 
tested from time to time by the admission of steam into the receiver. At 
first the steam gave rise to cloudiness, but as the dust gradually got cleared 
out the clouding become less and less, till at last it disappeared, indicating 
a dustless state of the air in the receiver. After this condition was attained 
the glass tube, through which the filtered air was passing, was heated, 
to get the effect, if any, due to heating glass, and also to make sure that the 
VOL. XXX. PART I. 3G 
