OZONE SULPHUR DIOXIDE. 35 



Determinations made by Thierry on Mont Blanc showed* 

 3*5 3'9 milligrams of ozone in 100 cb. metres of air at 

 Charaounix (1050 metres), while at the Grand Mulets (3020 

 metres) 9'4 milligrams per 100 cb. metres were present. 



At Montsouris (Paris) the amount found was 1*9 to 4*0 

 milligrams per 100 cubic metres of air. It thus seems highly 

 probable that the air from great altitudes contains more ozone 

 or hydrogen peroxide than that near the surface of the earth. 



A marked diminution in the average amount of ozone in the 

 air at Montsouris, Paris, and Marseilles was noted during the 

 outbreak of a cholera epidemic in France in 1884, the propor- 

 tion of ozone at Paris sinking from 2*0, the average of the 

 same period of the previous year, to -27, while at Marseilles 

 the diminution was from 2-17 to '86. This may have been 

 caused by the prevailing wind bringing air charged with 

 sulphur dioxide from the cities over the observatories.! 



In addition to the substances already mentioned, which 

 may be regarded as essential constituents, the atmosphere near 

 towns and in manufacturing districts contains other, accidental, 

 constituents. Some of these are very prejudicial to the life of 

 plants. Sulphur dioxide, which eventually becomes sulphuric 

 acid, is the commonest of these harmful impurities. It is 

 derived chiefly from the combustion of coal, though the decay 

 of animal and vegetable matter yields small quantities of sul- 

 phuretted compounds. 



The acid character of town rain is to be judged from the 

 analyses on page 31. It is mainly on account of this acidity 

 of the air and rain that there is difficulty and, in many cases, 

 impossibility of growing plants in our large towns, young 

 grasses being especially affected. 



Bailey: in 1892 described the results obtained by the exami- 

 nation of a large number of specimens of air collected in Man- 

 chester, Liverpool, and London, with the especial object of 

 determining the sulphur dioxide present. It was found that in 

 clear, breezy weather less than Imgm. of sulphurous acid 

 per 100 cubic feet was present in the air of Manchester ; but 

 during fogs the amount sometimes rose to as high as 34 to 50 

 milligrams. 



* J. C. Soc. 1897, abst. ii., 253. t Jour. Soc. Chem. Ind. 1885, 462. 



| British Association Report 1892, 679 and 781. 



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