90 MESSRS. J. А. WHELDON AND W. G. TRAVIS ON 
Cohen (2) and Ruston (17) at Leeds. A few figures may, however, be given 
here. It has been calculated that 35,000 tons of soot are poured into the 
atmosphere every year from the domestic chimneys alone of Manchester. 
Prof, Cohen has estimated the amount of soot emitted per annum from the 
factories of Leeds at 5000 tons, and from house-fires, 30,000 tons. Near 
to one of the chief industrial areas in Leeds, Mr. A. G. Ruston (17) found 
that fully 40 per cent. of the sunlight during the year of his investigation 
was shut off by the smoke in the atmosphere, and that the solid impurities 
which reached the ground as the result of coal-combustion amounted to the 
high figure of 1565 pounds per acre. Аз illustrating the relative amounts 
of sulphur acids in the air in various districts, we take from Macfie’s (12) 
book some of the figures which he quotes from Angus Smith showing 
the amounts of sulphuric acid (sulphates) obtained by analyris of 
rainfall :— Valentia (Ireland) taken as 100, England (inland country 
places) 202-2, Waterloo (near Liverpool) 418:7, St. Helens 1215'8, 
Liverpool 14502, Manchester (average 1869-70) 1641:9, near an alkali- 
works (isolated) 2685. 
Although the air is constantly renewed by movements of the atmosphere, 
and eleansed by rain and snow-fall, yet the discharge of impurities into the 
atmosphere is continuous, and the impurities are wafted about and deposited 
on every surface with which the air comes in contact. The solid impurities 
in coal-smoke are usually mostly deposited near the place of origin, and the 
amount of deposition falls off considerably outside the towns. In some parts 
of South Lancashire, however, the towns are so close together that in those 
areas a continuous smoke-zone is created. The solid impurities may be 
rapidly deposited near their place of origin, but the noxious gases are 
diffused over much wider areas; and there is not a square mile in the 
whole of South Lancashire, even in the parts most remote from towns, 
where the vegetation is not visibly affected by smoke. Mr. Albert 
Wilson (25) has shown that the smoke of South Laneashire and the 
West Riding of Yorkshire affects practically the whole of the North of 
England. 
Under the conditions of atmospheric pollution which obtain in South 
Lancashire, we cannot be surprised that lichens, which of all plants are the 
most sensitive to atmospheric impurity, should be most seriously affected. 
A large nuinber of lichens have their habitat on bare rocks and tree-trunks. 
Tn our district such surfaces are smoke-begrimed to such an extent that the 
growth of lichens and mosses on them is inhibited or greatly hindered. As 
pointed out by Wheldon and Wilson (26 & 27), the rain as it trickles 
down such tree-trunks or rock-surfaces becomes more and more charged with 
acid impurities as it descends, and these prove fatal to young and tender 
cryptogamic plant-growths. A film of soot on rock, bark, or other surfaces, 
apart from any other injurious character, also serves to mask the surface ; 
