THE LICHENS OF SOUTH LANCASHIRE. 89 
towns expanded enormously, and entirely new towns and urban districts 
sprang into being. This process still continues, and in all our urban 
districts there is a steady and rapid encroachment of the town on the open 
country. 
Atmospheric Pollution.—]It need hardly be said that the flora of a densely- 
populated district like South Lancashire has been deleteriously affected by 
human activities in many ways. With most of these we need not here 
concern ourselves, and we shall confine ourselves mainly to a consideration 
of the effects of atmospheric pollution on the flora, and more especially on 
the lichen-flora, since the pollution of the air by smoke and chemical fumes 
has more deleteriously affected cryptogamic plant-life in South Lancashire 
than any other agency. 
The degree to which our air is polluted by smoke, and the effect of this 
atmospheric pollution on the vegetation, are not fully recognized. The 
average town-dweller thinks that when the suburbs are passed, and the open 
country reached, the air is pure and the vegetation quite unaffected by town- 
smoke. То the trained eye, however, the cryptogamic plants, and especially 
the mosses and lichens, tell their own tale. The condition of the lichens in 
any district is a sure indication of the relative purity of the air, since lichens, 
especially the corticole and rupestral species, are exceedingly intolerant 
of atmospheric impurity, and rapidly begin to deteriorate as soon as the 
atmosphere is smoke-polluted to such an extent that there is the slightest 
deposition of combustion-products upon them or upon the surfaces upon 
which they grow. 
In some parts of our district fumes from chemical manufacture and ore- 
smelting play a not unimportant part in the pollution of the atmosphere; but 
generally in South Lancashire, as in other populous manufacturing areas, 
the chief cause of atmospheric impurity is the continual discharge into the 
air of the noxious products resulting from the consumption of bituminous 
coal, which is burnt more or less imperfectly in such enormous quantities in 
our domestic fires and in the furnaces of our factories. According to 
Prof. Lewes (10 0), coal contains from 0°3 to 3°5 per cent. of sulphur, chiefly 
in the form of iron pyrites ; and a result inseparable from the burning of coal 
is the liberation of sulphur dioxide (SO), which is ultimately oxidized in the 
atmosphere into sulphuric acid (H,SO,), and brought down in the rain. 
The sulphur compounds, such as sulphur dioxide and sulphuretted hydrogen, 
are the most harmful in their effect on vegetation of all the products of coal- 
combustion. Among the other noxious products emitted from our chimneys 
are unconsumed carbon and certain tarry and mineral matters, which 
together constitute what we know as soot. For details indicating the extent 
to which large industrial towns in the North of England are responsible for 
polluting the atmosphere, we may refer to the results obtained by the 
Manchester Air Analysis Committee (13) and to the investigations of 
LINN. JOURN,—BOTANY, VOL. XLIII, H 
