72 



Air Pollution by Coal Smoke. 



[APRIL, 



this respect because they are also affected by the winter 

 smoke ; of the evergreens the most susceptible, owing to their 

 characteristic sunk stomata, are the conifers. 



Effect of Acid in Soot on Growth of Plants. 



The samples of soot were also nearly all found to be distinctly 

 acid in reaction to methyl orange, thus indicating the presence 

 of free mineral acids. The amounts of these acids thus brought 

 down by the rain were found in the more polluted parts of Leeds 

 to be as great as eighty pounds per acre. The deposition of 

 acid along with soot upon the leaves of plants is probably 

 one of the main causes of the early withering which is so 

 characteristic of many forms of town vegetation. Ash trees 

 in the purer parts of Leeds often retain their leaves six weeks 

 longer than those in the more contaminated districts. 



If, therefore, we regard the leaf as the factory of the plant, 

 we find that, owing to smoke pollution, the factory is actually 

 closed for six weeks out of the four or five months of its working 

 year, while during the remainder of the time, as our assimilation 

 experiments show, it will be working at less than half its normal 

 pressure. 



The presence of smoke contamination is usually made mani- 

 fest by an increased sulphur content in the leaves of trees. 

 From observations made at Leeds it appeared that in the more 

 smoke-infested areas there is not only an increased deposit 

 of sulphur compounds with the soot upon the surface of the 

 leaves, but that there is also an increased intake of sulphur 

 dioxide, which, owing to its germicidal action, tends very con- 

 siderably to lower the stamina and vitality of both plants and 

 animals. 



Sulphuric Acid in Rain Water. 



Sulphur dioxide, on coming in contact with air and 

 moisture, passes rapidly into sulphuric acid. The presence of 

 sulphuric acid in rain water has a harmful effect on the vege- 

 tation, and excessive acidity may check growth altogether. 

 Grasses watered with water, the acidity of which was 32 parts 

 per 100,000, were killed off in little more than three months, 

 and not a trace of vegetation of any kind was visible in the 

 iollowing spring ; while water, the acidity of which was 16 

 parts per ioo,oqo, proved fatal in less than a year. 



It was also shown that while the final effect of acidity was 

 to destroy vegetation altogether, smaller amounts had the effect 

 of reducing both the quantity and the quality of the herbage. 

 Thus, in every case a larger amount of acidity meant a decreased 



