I. THE ATMOSPHEKE 29 



stant during the day in August, but that from 6 to 9 p.m. no ozone 

 could be found in the atmosphere. 



Determinations made by Thierry 1 on Mont Blanc showed 3-5-3-9 

 milligrams of ozone in 100 cubic metres of air at Chamounix (1050 

 metres), while at the Grand Mulcts (3020 metres) 9'4 milligrams per 

 100 cubic 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 

 cholera epidemic in France in 1884, the proportion 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 0*86. This 

 may have been caused by the prevailing wind bringing air, charged 

 with sulphur dioxide from the cities, over the observatories.' 2 



ACCIDENTAL GASEOUS CONSTITUENTS. 



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 sulphuretted compounds. 



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

 on page 25. 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 3 in 1892 described the results obtained by the examination 

 of a large number of specimens of air collected in Manchester, Liver- 

 pool and London, with the especial object of determining the sulphur 

 dioxide present. It was found that in clear, breezy weather less than 

 1 milligram 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. 



The chief causes which prevent or interfere with the growth of 

 plants in towns are 



1. Diminished sunlight, often less than 50 per cent 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 ten to twenty 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 seventy 



1 Jour. Chem. Soc., 1897, Abstracts, ii. 253. 



2 Jour. Soc. Chem. Ind., 1885, 462. 



3 British Association Eeport, 1892, 679 and 781. 



