36 
POPULAR SCIENCE REVIEW. 
ditions of a fog. He thought that the large area built on in 
the Metropolitan district has tended to keep the amount of fog 
constant, the extra houses supplying smoke, and also lessening 
the extent of evaporating surface. Sir Francis Knowles gave, 
also, the result of his calculations on the effects of coal combus- 
tion in London. Taking the daily consumption at about 22,000 
tons, there will be 166 lbs. of watery vapour per ton ejected by 
the chimneys. Nearly 4,000,000 lbs. of valuable ammoniacal 
liquor, with coal - tar and blacks, are, therefore, worse than 
wasted ; for there is about 13 lbs. of ammonia in the 166 lbs. of 
watery vapour. This waste may be almost entirely prevented 
by the often- suggested process of partially coking coal before 
using it. The liquors thus extracted from the coal would more 
than pay the cost of extracting them. The amount of ammonia 
annually wasted in London would be sufficient manure for six 
million quarters of corn. 
There are two elements in the physical history of fog which, 
in the recent discussion, seem to have been singularly neglected, 
namely, first, its very intense light- stopping power. This is 
obviously out of all proportion to the actual amount of carbon 
present. It has been shown in the pretty experiment given 
above that the quantity of pulverulent carbon necessary to 
produce the blackest smoke issuing from a river steamer’s 
chimney is small ; even this minimum, however, is signally 
absent from ordinary fog. 
The darkness of a dense fog is often, moreover, quite sepa- 
rable from opacity. We do not see moderately distant objects 
as if through smoke. Their outlines, where discernible, are 
distinct and unblurred. It is the light, and not the definition, 
which is deficient. Now it should be noted that exactly the 
same condition occurs with a snow-cloud. The blackness of an 
impending snow-storm is the only natural phenomenon which 
can in any way compare with that of fog. And yet we know 
that the impediment to light is situated in an infinite number 
of small masses of frozen water, in themselves transparent and 
lightly retractile. It is obvious that the same process is in 
action as that which renders a mass of transparent glass, when 
powdered, absolutely opaque : namely, the breaking up of the 
light by internal reflections and refractions of heterogeneous 
character. It seems far from improbable that this explanation 
may bear being extended to the case of fog, especially if Dr. 
Frankland’s hypothesis of its spheroidal cellular state be admit- 
ted. The yellowness may in such case be largely due to diffraction 
from bodies of very minute but similar size, bearing some simple 
ratio to the wave-length of yellow or orange-coloured light. 
Secondly, considerable corroboration of such a view may be de- 
rived from the fact mentioned by the Paris correspondent of the 
