DUST, FOGS, AND CLOUDS. 351 
gives rise to a dense fog with steam, but I also find that these fumes and fog 
owe their formation to dust. This is illustrated by the followimg experiment. 
In a retort was placed a quantity of sulphuric acid. The stopper of the 
retort was removed, and in its place was put a tube connecting the retort with 
a cotton-wool filter. The neck of the retort was connected to a wash-bottle by 
means of a glass tube. An aspirator drew the air out of the wash-bottle, and 
thus kept up a current of air from the filter through the retort to the wash- 
bottle, the air bringing the sulphuric acid vapour along with it. At first, when 
unfiltered air passed, dense fumes filled the retort and wash-bottle, but when 
the filter was introduced the cloudiness gradually disappeared. The absence 
of dust entirely prevented any foggy condensation, even though there were 
chemical affinities. After the experiment had been continued for some time, 
slight fumes began to appear, even when filtered air was passing, but this only 
happened when the acid became very concentrated, and much acid evaporated, 
and the fumes with filtered air were very slight, while unfiltered air gave very 
dense fumes. 
It is not necessary to suppose the want of dust prevented the chemical 
affinities from acting, it only prevented the new compound from condensing 
in cloud form. When the acid was weak its vapour would combine with the 
moisture in the air, but would remain as vapour when there was no dust for it 
to condense upon. But when the acid became highly concentrated, the mole- 
cular strain would be greatly increased on account of the vapour tension being 
greatly in excess of that due to the temperature, and it would then seem to be 
able to condense without the presence of a “free surface.” There is, of course, 
the possibility that the filtering of the air was not perfect. I may remark here 
that the fumes of highly concentrated sulphuric acid are found to be an excellent 
fog-producer. If we dip a glass rod in the acid, and heat it highly, and allow 
a little of the fumes to pass into the experimental receiver, steam will now give 
a very dense fog indeed. 
The effect of dust in producing the cloudy form of condensation of other 
vapours than water was tried. With all the vapours experimented on, which 
included alcohol, benzol, and paraffin oil, it was found that pure air gave no 
clouding whatever, while unfiltered air gave more or less cloudiness with all 
of them. 
The cause of the blue colour of the sky has long afforded interesting 
matter for speculation. The theory which seems most satisfactorily to explain 
its blue colour depends upon the property which very small particles of matter 
have of scattering only the rays of the blue end of the spectrum, and the 
question is, What are these very small particles composed of? It has been 
suggested that they are very small particles of condensed water vapour. Now, 
we have shown the high improbability of water vapour ever condensing out 
