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Proceedings of the Pioyal Society 
with chloride of sodium vapour, by highly heating the salt in a 
platinum tube, and afterwards cooling and drawing the vapour 
along with air into the experimental receiver. 
Many other salts — such as chloride of calcium, bromide of potas- 
sium, &c. — when highly heated, have also the power of determining 
cloudy condensation in unsaturated air. Potassium and sodium, 
when burned, give also similar results, and the fogging is, in all 
cases, proportional to the moisture in the air. 
Experiments have also been made on a larger scale in a cellar, the 
air in which was damp but not saturated. The temperature was 
about 45° Fahr., and the wet and dry bulb thermometers showed a 
difference of from |° to 1° during the experiments. A short time 
after a little sulphur bad been burned the whole cellar was filled 
with a dense white dry fog, which remained for some hours. A 
similar result was also got by sprinkling some salt over an alcohol 
flame. The salt dust so produced determined a decidedly foggy 
condensation in the unsaturated atmosphere. 
Experiments were also made by burning sulphur in the open air. 
When the air is dry, the fumes can only be traced a short distance ; 
but when there is more moisture the condensation is more evident, 
and in certain conditions of the atmosphere the products of com- 
bustion can be seen floating away in the passing air, leaving the 
sulphur in a pale thin stream of vapour, which gradually increases 
in size and rolls away in a horizontal cloudy column ten or fifteen 
feet in diameter, clearly marked out from the surrounding air. 
This fog-producing power was shown to be probably due to the 
affinity which the sulphuric acid, resulting from the combustion of 
the sulphur, has for water vapour. 
It was shown that cloudy condensation may take place without 
dust particles being present. It will probably take place in a 
highly supersaturated atmosphere. It will also happen when some 
substance is vaporised in dustless air and cooled to a temperature 
far below that corresponding to its tension. When the substance 
condenses and forms nuclei of condensation for the water vapour, 
cloudy condensation will also take place in dustless air. When 
there are gases present which combine and form new compounds, 
and the temperature is much too low for them to remain as vapours, 
the molecular strain seems under these conditions to be sufficient to 
