IRRIGATION. 201 



water. First, before it can be understood how irri- 

 gation acts, let it be considered, how pure water 

 acts ; it is not. said rain water, for that acts in a 

 double way, both by its purity and impurity. The 

 more impure, the better manure is water. The pur- 

 er water is, the less is it fit for irrigation. 



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

 exposed to air, absorbs different proportions of its oxy- 

 gen and nitrogen. This is a very slow process. It 

 is found that most natural waters give out, by boil- 

 ing, from every hundred cubic inches of water, 3 1-2 

 cubic inches of air. This air contains 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 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 meas- 

 ures are absorbed by water, it is in this proportion, 



46*43 of nitrogen, 



53*57 of oxygen ; 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, 



45*29 of nitrogen, 



18*63 of oxygen ; so that 2-3 of the oxygen 



