16 
Proceedings of the Royal Society 
of dust in the air tends to increase fogs, and that when the dust- 
particles are not so numerous the cloud-particles are larger, and settle 
down more quickly. 
It was shown that by simply heating any substance such as a 
piece of glass, iron, brass, &c., a cloud of dust was driven off, which, 
when carried along with pure air into the experimental receiver, 
gave rise to a dense fog when mixed with steam. So delicate is 
this test for dust, that if we heat the one-hundredth of a grain of 
iron wire, the dust driven off from it will give a distinct cloudiness 
in the experimental receiver, and if we take the wire out of the 
apparatus and so much as touch it with our fingers and again replace 
it, it will again be active as a cloud-producer. Many different sub- 
stances were tried, and all were found to be active fog-producers. 
Common salt is perhaps one of the most active. 
Heat, it is well known, destroys the motes in the air, and it might 
be thought that flame and other forms of combustion ought to give 
rise to a purer air. Such, however, is not the case. Gas was burned 
in a glass receiver, and supplied with filtered air for combustion, 
and it was found that the products of combustion of pure air and 
dustless gas gave rise to an intense^ fog-producing atmosphere. It 
may be mentioned here that the fog-producing air from the heated 
glass, metals, and burning gas, were each passed through the cotton- 
wool filter, and the air was in all cases made pure, and did not give 
rise to cloudiness when mixed with steam. 
It will be seen that it is not the dust motes which are revealed to 
us by a beam of sunlight when shining into a darkened room, 
that form the nuclei of fog and cloud-particles, as these may be 
entirely removed by heat, and yet the air remain active as a cloud- 
producer. The heat would seem to break up the larger motes which 
reflect the light into smaller and invisible ones. When speaking 
of dust, it is to these infinitesimally small and invisible particles we 
refer. The larger motes which reflect the light will no doubt be 
active nuclei, but their number is too small to have any important 
effect. 
It is suggested, and certain reasons are given for supposing that 
the blue colour of the sky is due to this fine dust. 
Other experiments were made to test the fog-producing power of 
the air and gases from different sources. The air to be tested was 
