DUST, FOGS, AND CLOUDS. 361 
after fifteen or twenty minutes the receiver was full of a dense white fog, 
which remained for a long time. 
Similar results were got by vapourising chloride of sodium. The salt was 
in some cases vapourised by a Bunsen flame. It was also vapourised by 
placing it on a piece of hot iron, and the receiver held over it to collect the 
vapour, which condensed and formed nuclei, which determined the condensa- 
tion of the water in unsaturated air. In some experiments the salt was 
vaporised in a heated platinum tube and drawn along with air through a coil 
of pipe to cool it, before admitting it into the receiver. In these experiments 
the density of the fogging was in proportion to the vapour present, and if the 
experiment was made in a wetted receiver, the fog took some time to attain its 
maximum density. 
The condensing power of sulphur products and salt can be illustrated in 
another way. The air with either of these substances in suspension, is drawn 
through a coil of pipe to cool it. If now this stream of air is made to strike 
any wetted surface, the wetted surface looks as if it had suddenly become 
heated—a stream of condensed vapour flows away from it. This vapour is, 
of course, invisible if ordinary air is used, and without the powerfully con- 
densing nuclei. 
Experiments on a larger scale were also made with these two substances. 
A little sulphur was burned in a cellar, the air of which was damp, but not 
saturated. The temperature was about 43° Fahr., and the wet and dry bulb 
thermometers showed a difference of from 3° to 1° during the experiments. 
After the sulphur was burned a fogginess was evident, but, on returning half an 
hour afterwards, the fogging was found to have increased very greatly in density, 
the air was very thick, and not the slightest smell of sulphurous acid perceptible. 
This fog hung about the cellar for many hours. The experiment was repeated 
with chloride of sodium, the salt being sprinkled over an alcohol flame. The 
result was similar to the sulphur products, a fogging which gradually increased 
in density, and very slowly cleared away. 
Experiments have also been made by burning sulphur in the open air. 
When the air is dry the fumes can only be traced a short distance, but as the 
amount of moisture increases the cloudiness becomes more and more evident, 
and in certain conditions of the atmosphere the cloudiness can be distinctly 
seen flowing away in the passing air, leaving the sulphur in a pale thin stream 
of vapour, which gradually increases in size and density, and rolls away in a 
horizontal cloudy column, ten or fifteen feet in diameter, clearly marked out 
from the surrounding air. 
There may be a certain amount of doubt as to the action of the heated salt 
in these experiments. When heated in the BunsEN flame it is probable decom- 
position of some of the salt takes place, and part of the result may be due to 
VOL, XXX. PART I. 31 
