IRRIGATION. 231 



280. Pure water acts only by its air. All water exposed 

 to air, absorbs different proportions of its oxygen and nitro- 

 gen. This is a very slow process. It is found that most, 

 natural waters give out, by boiling, from every hundred 

 cubic inches of water, 3^ cubic inches of air. This air con- 

 tains 8 or 9 per cent, more oxygen, than an equal bulk of 

 common air. Water is generally filled or saturated with 

 air ; it will take up no more by a month's exposure. If 

 this water is boiled, and again exposed to the air, it will 

 absorb, in 24 hours, as follows : — let there be taken any 

 number of measures of air, which are composed of 20 of 

 oxygen and 80 of nitrogen. If 100 measures are absorbed 

 by water, it is in this proportion : 



Of nitrogen, 46.43 



Of oxygen, 53.57 



so that oxygen is three times more absorbable than nitrogen. 



281. If now, there is expelled by boiling, the air from 

 pond or river water, it is found to contain, 



Nitrogen, . . . . . . . 45.29 



Oxygen, 18.63 



so that two-thirds of the oxygen have disappeared ; this is 

 the only fact which concerns the farmer. The oxygen has 

 been absorbed by natural waters and two-thirds retained. 

 What has become of it 1 It has gone, it is not said all of 

 it, but in irrigation a large portion to convert insoluble into 

 soluble geine. Irrigation is chiefly employed on grass-lands. 

 The green sward here may not be broken up. What if it 

 was 1 What if by ploughing, it was exposed to the action 

 of the air? Remember the properties of geine. Air con- 

 verts the insoluble to soluble, by forming carbonic acid, that 

 is, the air combines with the carbon of the geine, and forms 



