350 JOHN AITKEN ON 
flame, taking care not to heat the glass. Products of combustion from a clear 
part and from a smoky part of a fire were tested, and found to be about equally 
foggy, and both much worse than the air of the room. 
From these experiments it would appear that combustion under all condi- 
tions is bad as a fog-producer ; bad, whether the combustion be perfect, as in a 
Bunsen flame and a clear fire, or imperfect as in a smoky flame and smoky fire. 
It is therefore hopeless to expect that by adopting fires having a perfect com- 
bustion, such as the gas ones now so much advocated, we would thereby 
diminish the fogs which at present, under certain conditions, envelop our 
towns, and give rise to so much that is both disagreeable and detrimental. All 
fires, however perfect the combustion, are fog-producers when accompanied by 
certain conditions of moisture and temperature. From this it will be observed 
that it is not the visible dust motes seen in the air that form the nuclei of fog 
and cloud particles, as these may be all destroyed by combustion, and yet the air 
remain fog-producing. No doubt these motes also play their part in the condensa- 
tion, but their number is too small to be of importance. The fog and cloud nuclei 
are a much finer form of dust, are quite invisible, and though ever present in 
enormous quantities in our atmosphere, their effects are almost unobserved. 
A number of experiments have been made by burning and highly heating 
different substances to test their fog-producing powers, and I have found that 
highly heated sodic chloride, as, for instance, when burned in an alcohol flame, 
or salt water spray heated in a BunsEN flame, gives rise to an extremely dense 
fog when tested with steam. But perhaps the most active of all substances I 
have yet tried is burning sulphur. The fog produced when steam has been 
blown into air in which a very little sulphur has been burned is so dense that if 
ever fog was “cut” it might or should be. So dense is it that it is impossible 
to see through a depth of more than 5 centimetres of it. The sulphides when 
burned also give similar results. 
These experiments evidently introduce a new element into the investigation. 
We have here not only to do with the attraction of the different molecules of 
the same kind, but the gaseous molecules in this case have also chemical 
affinities for each other. It is very difficult to understand this marvellous 
fog-producing power of burned sulphur. Sulphur in burning gives rise to 
sulphurous acid. Now from experiment I have made with sulphurous acid 
prepared from sulphite of soda and sulphuric acid, and also from copper and 
sulphuric acid, the sulphurous acid being carefully dried with sulphuric acid, I 
do not find it active as a fog-producer. It gives riseto no fumes, it does not 
increase the fogging of dusty supersaturated air, and produces no fog in filtered 
supersaturated air. 
Sulphuric acid vapour, it is well known, gives rise to dense fumes by com- 
bining with the moisture of the air, and I find, under certain conditions, it also 
