II. THE ATMOSPHEKE . 19 



would raise the temperature of the same weight (or about ^^ of the 

 volume) of water through the same interval of temperature. It is thus 

 evident that by cooling a given volume of warm water through one degree 

 (or any fixed interval of temperature) enough heat is extracted to raise 

 the temperature of about 3200 volumes of air by the same amount. 



Hence the potency of currents of warm water, e.g., the Gulf 

 Stream, in affecting climate and the slowness with which water is 

 cooled and converted into ice by cold winds, etc. 



The mean temperature of the lower layers of the atmosphere varies 

 greatly, one important factor being the latitude, which mainly deter- 

 mines the amount of heat received from the sun. The temperature 

 is also found to vary greatly along the same parallel of latitude, with 

 the nature of the soil and particularly with the proximity or otherwise 

 of large areas of water, places near the sea-coast always enjoying a 

 more uniform climate than those far inland. Then, too, the height 

 above the sea-level greatly affects the temperature of a place, there be- 

 ing on the average a fall of 1 C. for about every 350 ft. above the sea- 

 level, but the rate is very variable. 



The most important discovery in recent meteorological research is 

 that at a height of about 9 miles, the temperature of the atmosphere 

 remains practically constant at about - 56 C. The air above this height 

 is probably almost motionless and takes no part in the circulation 

 which is so constantly in play in the lower layers. Above this 

 isothermal layer, the composition of the atmosphere probably changes 

 rapidly with increasing elevation, the carbon dioxide, water vapour, 

 argon, oxygen and nitrogen in turn diminishing until, in the highest 

 regions, probably little but hydrogen remains. 



COMPOSITION OF THE ATMOSPHERE. 



Air is a mechanical mixture of various gases and vapours, but it 

 invariably contains suspended solid matter, some of which consists of 

 micro-organisms. 



The main gaseous constituents are oxygen, nitrogen, argon, 

 carbon dioxide, water vapour, ammonia, oxides of nitrogen or nitric 

 acid vapour, and ozone. All these are subject to variation, but to 

 'very different extents. 



Nitrogen, the largest constituent, is the least variable in amount. 

 It usually constitutes about 78 per cent by volume or 75 '5 per cent by 

 weight of dry air. Its function in the atmosphere has usually been 

 regarded as mainly that of a diluent. Apparently very few reactions, 

 in which nitrogen takes part, occur in the atmosphere. Eecently, 

 however, many obscure changes in which the free nitrogen of the air 

 enters into combination with oxygen and hydrogen have been observed, 

 some of which are of the greatest importance in agriculture. These 

 changes occur by the agency of micro-organisms in the soil or plant 

 (vide Chap. IV). 



Very high temperatures, such as obtain in the blast furnace, will 

 cause nitrogen to combine with carbon in the presence of alkalies, with 

 the production of cyanides. Nitrogen may be made to combine with 



