OF COAL AND RATE OF EVAPORATION, 41 
it issues from the chimney-top to discolour the atmosphere 
is prodigious, but its weight is inconsiderable. 
This discolouring power is rapidly abated and soon de- 
stroyed by the aggregation of the particles through electri- 
cal or other causes, and quick precipitation ensues, each 
particle being assisted in its descent by an associated 
burden of dust and other matters. The dense heavy cloud 
we see floating off to leeward of such large cities as Man- 
chester, is not composed of black smoke; indeed, it may 
be questioned if, at the distance of half-a-mile from its 
source, and unless drifted like sand before the gale, there 
is then a single particle of black smoke in the atmosphere. 
Those who have “ crofted pieces” under the current of their 
chimney can estimate the correctness of this remark as to 
the rapid precipitation of black smoke. The dust which 
descends, associated with the black smoke, has been invari- 
ably found to contain iron. This may be shown on the 
most minute scale by placing a particle of the soot be- 
tween the wet folds of a pink piece dyed with madder at 
that point of the process when the piece has been washed 
out of the “tin.” The chemical law of displacement in 
this experiment is beautifully shown by the iron taking 
the place of the alumina that formed the basis of the pink 
colour, and itself producing a black with the liberated 
alizarine. What we see floating in the atmosphere is dust 
associated with the volatile constituents of the coal which 
have been distilled off through imperfect combustion. It 
is scarcely in place in this paper to go further into this 
important subject; but I may remark that this rapid dis- 
appearance of black smoke by precipitation, more particu- 
larly in dry weather, is one of the marked tests of the 
proper action of the fires below, while if the smoke hangs 
lazily, keeps together, and is visibly black for a greater 
distance, we may be certain that something is going up 
VOL. XV. G 
