~354 JOHN AITKEN ON 
streets add their murky thickness to the foggy air. This seems to explain the 
well-known sign of falling smoke being an indication of coming rain. That the 
colour or blackness of what is called a pea-soup fog is due to smoke, is, I 
think, evident from the fact that a town fog enters our houses and carries its 
murky thickness into our rooms, and will not be induced to make itself 
invisible however warmly we treat it. It will on no account dissolve into 
thin air, however warm our rooms, for the simple reason that heat only 
dissolves the moisture and leaves the smoke, which constitutes a room fog, 
to settle slowly, and soil and destroy the furniture. If the fog was pure, that 
is to say, was a true fog and nothing but a fog, such as one sees in the 
country, it would dissolve when heated, as every well-conditioned country 
fog does—at least I never remember meeting a fog in a country house. 
But while admitting the bad effects of a fog aggravated by smoke, yet we 
must not forget the probable good effects of the smoke. It has been else- 
where pointed out that the suspended smoke or soot may exercise the well- 
known disinfectant properties possessed by the different forms of carbon. 
Before utterly condemning smoke it will be necessary fully to consider its 
value as a deodoriser. And further, we must remind those who are crying 
for more perfect combustion in our furnaces and grates, that combustion, 
however perfect, will not remove or diminish fogs. It will, however, make 
them cleaner, take away their pea-soupy character, but will not make them 
less frequent, less sulphurous, less persistent, or less dense. 
We have shown that sulphur in its different forms when burned is most 
active as a fog-producer. Now, almost all our coals contain sulphur, which is 
burned along with the coal, and it is certainly worth considering whether some 
restriction ought not to be put on the amount of sulphur in the coal used in 
towns. The quantity of burned sulphur that escapes from our chimneys is 
very great. Suppose we put the amount of coal annually consumed in the 
London district at a little over 7,400,000 tons. Now, the average amount of 
sulphur in English coal is more than 1:2 per cent. Suppose that it is 1 per 
cent., so as to be within the mark, that would give 74,000 tons of sulphur 
burned every year in London fires, or at the rate of about 200 tons in an 
average day, and the amount will be greater in a winter day—a quantity some- 
what alarming, and quite sufficient to account for the density of our fogs. Its 
presence and effects during our fogs is very evident in the discoloured metal 
on our street door and in our houses. 
But, like smoky fires, burnt sulphur is not an unmitigated evil. During 
fogs the air is still and stagnant ; there is no current to clear away the foul 
smells and deadly germs that float in the air, and which might possibly be more 
deadly than they are if it were not for the powerful antiseptic properties of the 
sulphurous acid formed by the burning sulphur. Before condemning the 
