II. THE ATMOSPHEKE 31 



industrial portion of the city, intercepted at least 40 per cent of the 

 daylight received at station 9 in the suburbs, and that there was a dis- 

 tinct correlation at stations 3, 4, 5 and 7 between the light intercepted 

 and the total suspended matter in the air. It was also shown that 

 leaves of laurel from the smokier parts of the town, possessed very 

 much lower assimilatory powers than similar leaves obtained from 

 near the suburban station 9, the figures ranging from 53 (station 5) to as 

 low as 11^- per cent (station 4) of the assimilating power, per unit area, 

 of the leaves from station 9. Further experiments showed that by 

 simply cleaning the laurel leaves with a cloth, their assimilatory powers 

 were considerably enhanced. 



The acid character of the rain was shown to have a distinct and 

 cumulative effect upon grasses, greatly diminishing the yield and. in- 

 creasing the proportion of crude fibre in the product, while greatly di- 

 minishing the proportion of nitrogenous matter. 



It was also noticed that acid rain had a considerable effect upon 

 the soil, the nitrogen as ammonia being increased with increasing 

 acidity while that as nitrates became very small and in the case of very 

 acid water being employed, disappeared altogether. The bacteria 

 content of the soil was also very seriously diminished by acid rain, the 

 nitrogen-fixing organisms and the nitrifying bacteria being especially 

 affected. 



Solid Matter. Air always contains large quantities of suspended 

 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 after- 

 wards 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. 1 The num- 

 ber 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 180 ft., and 7 at 300 ft. 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. 



In air are to be found dust particles of every description, blown up 

 from the surface of the ground and consisting of fragments of very 

 diverse character. 



In addition, air, even at considerable heights, contains solid particles 

 of a more definite and uniform character the minute particles left by 

 the evaporation of the tiny droplets of the spray from the sea. Each 

 drop of the spray loses its water by evaporation and leaves an ex- 

 cessively minute particle of dust, which remains suspended in the air 

 until washed out by rain. These saline particles are naturally most 

 abundant near the coast, as is evident from the analyses of rain-water ; 

 but at places far inland, chlorine is to be found in rain-water, and 

 sometimes in considerable quantities. 



1 P. F. Frankland, Pr. Koyal Society, 40, 509. 



