ON FOG. 
29 
yellow variety came under his notice. Indeed, the surprised 
Parisians were energetically discussing the new invader as 
though it were Phylloxera, Aphis vastator, or the Colorado 
bug. 
The part which suspended carbon actually plays in coal- 
smoke seems to have been first correctly pointed out by the 
late Mr. C. Wye Williams in his Treatise on the Combustion of 
Coaly and the Prevention of Smoke. A pretty experiment as to 
its great light- extinguishing power is quoted by the writer of 
an excellent article on this subject in a weekly contemporary, 
the Engineer , to which once for all the writer may acknowledge 
his indebtedness : — 
‘ The weight of carbon in a cubic foot of black smoke is not 
equal to that of a single grain. Of the extraordinary light- 
absorbing property and colouring effect produced by the in- 
appreciable myriads of atoms of this finely - divided carbon 
forming part of the cloud, some idea may be formed by arti- 
ficially mixing some of it, when in the deposited form of soot, 
with water. For this purpose collect it on a metallic plate held 
over a candle or gas-jet, and touching the flame. Let a single 
grain weight of this soot be gradually and intimately mixed on 
a palette, as a painter would with a palette-knife — first with a 
few drops of gum- water, enlarging the quantity till it becomes 
a spoonful. On this mixture being poured into a glass globe 
containing a gallon of water, the whole mass, on being stirred, 
will become opaque, and of the colour of ink. Whatever, then, 
may be the quantity or number of its atoms, we see from the 
cloud of incombustible matter with which this carbon is so inti- 
mately associated as smoke, that even attempting its separation 
and collection independently of its combustion borders on 
absurdity.’ 
4 But even if it were possible/ says the same authority, ‘ to 
totally prevent the evolution of carbon in a finely- divided state, 
we should have advanced but a short way on the path to be 
trodden. The carbon renders, it is true, a fog dirty and dark, 
but it is not the carbon which is the principal cause of the sore 
throats, bronchitis, and “ colds in the head,” which are beyond 
question brought on by the persistent inhalation of London fog. 
These are more than probably due to the presence in the fog of 
sulphurous acid gas, carbonic acid, and carbonic oxide, the two 
last fruitful causes of violent headache. No matter how perfect 
the combustion of the coal might be, these gases would be as 
plentiful as ever. To illustrate this truth, we may cite the 
Metropolitan Railway. The engines on it burn Welsh smoke- 
less coal, and the conditions of combustion are made as perfect 
as science can make them ; but the tunnels are filled with an 
atmosphere charged with carbonic acid, carbonic oxide, and 
