36 THE ATMOSPHERE. II. 



The chief causes which prevent or interfere with the growth 

 of plants in towns are : 



1. Diminished sunlight, often less than 50% of the intensity 



of the light in the country. 



2. The amount of sulphurous acid in the air. In heavy, calm 



weather the amount is often 10 to 20 times that present 

 in windy, clear weather. During fogs the amount is 

 even greater. 



3. The acidity of the rain. In Manchester often as much as 



70 parts of sulphuric acid per million were found. It 

 was noticed that the rime deposited on the leaves of 

 plants during frost sometimes contained as much as 

 400 parts sulphuric acid per million. 



Organic matter of a readily putrescihle nature is also present 

 in air where respiration of men or animals or decay of organic 

 matter (e.g., in marshy and malarious districts) takes place. To 

 this organic matter, perhaps, rather than to the increased car- 

 bon dioxide and diminished oxygen, the had effects of breathing 

 the atmosphere of close and crowded rooms are to be ascribed. 

 This organic matter is probably suspended, but is very finely 

 divided. 



Solid Matter. Air always contains large quantities of sus- 

 pended solids, some of which consist of micro-organisms and 

 their spores. The total number of micro-organisms in a given 

 volume of air can be estimated by aspirating a known volume 

 of air through a wide tube coated internally with sterilised 

 nutrient gelatine. The tube is afterwards kept in an incubator 

 for a few days and the number of colonies of micro-organisms 

 can be counted. By this and other processes the air of various 

 places has been examined bacteriologically.* The number 

 present varies enormously and is less at greater elevations. 

 Thus at Norwich 18 organisms in 10 litres of air were found 

 near the ground, 9 at a height of 180ft., and 7 at 300ft. At 

 S. Kensington it was found in 1886 that an average of 279 

 micro-organisms fell on each square foot of surface per minute, 

 and that an average of 35 were present in 10 litres of air. 



* P. F. Frankland, Pr. Royal Society, 40, 509. 



