28 ELEMENTARY AGRICULTURAL CHEMISTRY 



apparently no power of utilising nitrogen, except when it is in 

 a state of combination, best in the form of nitrates. 



Oxygen is the most active and perhaps most variable con- 

 stituent. Its average proportion in dry air is nearly 21 per 

 cent, by volume, or 23*2 per cent, by weight. In consequence 

 of the large number of processes of oxidation taking place, the 

 proportion of oxygen is liable to local variations, but these are 

 not so great as might be expected, because of t^e influences of 

 diffusion, convection, and wind, and the compensating action of 

 vegetation. In towns and over marshy places the amount of 

 oxygen is generally found to be slightly less than in the open 

 country or over the sea. The limits of variation in the outside 

 air are not wide, perhaps from 20*5 to 21*03 per cent. 



Argon, a recently discovered constituent, is of little im- 

 portance from an agricultural or, indeed, from any practical 

 standpoint. The gas, which is about 19*9 times as heavy as 

 hydrogen, is remarkable for its inert character ; so far as is 

 known, it takes no part in any chemical processes, and appears 

 to be incapable of uniting with any other element, or even with 

 itself. Its molecule, unlike that of most gases, consists of one 

 atom. Its amount in the atmosphere is about 0'94 per cent, 

 by volume, or 1*3 per cent, by weight. Helium, neon, krypton 

 and xenon are other new elements which have been found in 

 air in excessively small quantities. So far as is known, they 

 play no part in any chemical change. • 



Carbon Dioxide is a small but important constituent. Its 

 percentage amount is very variable, being increased by the 

 combustion and decay of all organic bodies and by respira- 

 tion. The average amount in the air as a whole is estimated 

 at about 0-03 per cent, by volume, but the tendency of recent 

 .investigations has been to give somewhat lower numbers. On 

 the land its proportion is greater during the night than in 

 the daytime* but over the sea this daily variation cannot be 



