1920.] Air Pollution by Coal Smoke. 



77 



2. The thick deposit on the leaves of plants and trees, 

 still further blocks out the light. 



3. The choking of the stomata by the tarry, glutinous 

 matter tends to asphyxiate the plant, and effectively to 

 check its power of assimilation of carbon dioxide. 



4. The presence of free acids in the air tends generally 

 to lower the vitality of the plant. 



5. The free acids falling on the soil make it sour, and thus 

 limit the activity of the soil organisms, which must work 

 freely, if the soil is to maintain its fertility. 



While it is only possible to reduce the first three effects of 

 smoke pollution enumerated above by checking the output 

 of smoke, and using every effort to lessen the air pollution, 

 the remedy for the last lies in the hands of the farmers them- 

 selves. 



The acidity of the soil can be neutralised by a judicious 

 application of lime; in a smoke-infested area a dressing of 

 lime at least once in a rotation should be regarded as essential, 

 and the lack of such dressings has been responsible in many 

 cases for the failure of a crop, particularly of roots and 

 barley. 



