362 ; JOHN AITKEN ON 
the hydrochloric acid set free. In order to prevent this decomposition as much 
as possible, I have made some experiments at as low temperatures as possible, 
and the results are the same as when higher temperatures are used, allowance 
being made for the smaller amount of salt volatilised. 
The action of the products of combustion of sulphur would appear to be 
something like the following :—When the sulphur combines with the oxygen of 
the air, sulphurous acid is formed. I have shown in the first part of this paper 
that sulphurous acid has but little condensing power ; we must therefore look 
to the change which takes place in the sulphurous acid for the explanation of 
the wonderful condensing power of the sulphur products. The sulphurous acid 
becomes further oxidised in the air, and sulphuric acid is produced, and it is 
the great affinity which this sulphuric acid has for water which enables it to 
rob the air of its moisture and condense it in visible form. It does not seem 
to take long for the sulphurous to change to sulphuric acid in the air. A 
short time after the sulphur was burned in the cellar all smell of sulphurous 
acid was gone, and I am informed by Dr Wallace that he has found that all 
traces of sulphurous acid cease at a short distance from calcining ironstone 
bings in which much sulphur is being burned. The gradual thickening of the 
sulphur fog will probably be in part due to this gradual change of sulphurous to 
sulphuric acid. The gradual thickening of these fogs is also in part due to 
the slow evaporation of the water from the sides of the receiver, and subsequent 
condensation on the absorbing nuclei. 
I find that the fumes from highly concentrated sulphuric acid have a fog- 
producing power similar to the products of combustion of sulphur. If we 
highly heat a glass rod wetted with sulphuric acid, or heat the acid in a 
platinum cup, and admit a little of the fumes into the receiver, they are found 
to have a very strong fog-producing power. 
The above represents something like what the action of sulphuric acid is in 
moist air, in which there are no other vapours or gases with which this acid tends 
to combine. Before considering these more complicated effects I shall describe 
some experiments made to test the action of acid vapours on moist filtered air. 
The apparatus consisted of the air-pump arrangement, with test receiver or 
flask, one pipe as before being connected with the air-pump, and the other with 
the filter. Between the receiver and the filter was placed a test tube, in which 
was placed the acid to be experimented upon. The filtered air was caused to 
bubble through the acid on its way to the moist air in the receiver, the acid 
being generally kept at the temperature of the room. 
When nitric acid is put in the test tube and filtered air passed through it, it 
is found that its vapour always gives rise to fumes when mixed with the moist 
air in the receiver. These fumes—as cloudy condensation in unsaturated air 
may be called—may therefore be produced without nuclei when nitric acid is 
