I Q20.] 



A ik Pollution by Coal Smoke 



7 1 



a characteristic difference between the contamination arising 

 from the factory shaft and that attributable to the very im- 

 perfect combustion of the domestic range. 



Effect on the Growth of Plants. 



The solid impurities, present in the air of towns and due to 

 pollution by coal smoke, play a very important part in detri- 

 mentally affecting the growth of plants. Their effect in 

 diminishing the amount of sunlight in industrial towns may be 

 gathered from the fact that, in 1907, the number of hours 

 of bright sunshine registered in the centre of Leeds was 1,167, 

 as compared with 1,402 at Adel, some four miles to the north. 

 In other words, the smoke-cloud hanging over the centre of the 

 town curtailed the duration of bright sunshine by about 17 per 

 cent. If, however, there is measured, not the number of 

 hours of bright sunshine, but the actual intensity of the light, 

 it is found that not only is there a greater curtailment, but 

 that there is a sharply* defined correlation between that 

 intensity and the known solid impurities in the air. 



In Hunslet, in the centre of the industrial area, fully 40 per 

 cent, of the light was shown to be shut off. The energy of 

 sunlight is required by the green leaf for the conversion of 

 carbon dioxide into carbohydrates, and when 40 per cent, of 

 that energy is cut off by the smoke cloud, the effective growth 

 of the plant must be very seriously checked. 



Further, the greater part of the material of which the leaves 

 of plants is composed is taken from the atmosphere. The 

 leaves of plants possess minute pores, or stomata, by means of 

 which they absorb carbon dioxide from the air, this carbon 

 dioxide being converted in the plants into starches, sugars, 

 or other carbohydrates. Soot, as has already been pointed 

 out, is not pure carbon, but contains varying amounts, occa- 

 sionally as much as 40 per cent, of a thick oil or tar, which 

 causes the soot to adhere tenaciously to vegetation, so that it 

 cannot easily be removed by the rain. It thus hinders the 

 intake and assimilation of carbon dioxide necessary to the 

 growth of the plant. The black adhesive film which thus 

 settles on vegetation affects the leaves of trees and ever- 

 greens in particular. 



In addition to blackening the vegetation the black deposit 

 covers the whole leaf with a kind of varnish and fills up the 

 pores or stomata, thus effectively checking the natural process 

 of transpiration and assimilation. Evergreens suffer most in 



