than rain-water is used for growing crops, whether it is well, 

 or canal or tank water. 



209. Purity of waters. What quantity of solids is 

 contained in solution in a particular water intended to be 

 used for irrigation cannot be determined except by an ana- 

 lysis. This analysis for agricultural purposes need not be 

 an elaborate laboratory analysis at all. The readier method 

 of determining soluble salts in a soil is the electrical method, 

 which is applied in practice in field analysis of mineral sub- 

 stances. We will reserve this subject for consideration in 

 the part devoted to Agricultural Chemistry. Of all natural 

 waters rain-water is the purest and safest to use for irriga- 

 tion. Water of a river flowing through a granite country is 

 also very pure, containing only 2 or 3 grains of solid matter 

 in solution per gallon (i.e., 70,000 grains). The water of a 

 river flowing through a country containing more easily soluble 

 rocks (such as, limestones) often contains 20 to 30 grains 

 of solid matter in solution in every gallon. Spring or well 

 water contains a larger portion of solids in solution, as under 

 pressure at great depth such water absorbs larger volumes 

 of carbon-dioxide, sulphuretted hydrogen and other gases, 

 and it also dissolves saline matters of different kinds from 

 different rocks. Sea water contains as much as 2,400 grains 

 of solids in solution per gallon, of which about 2,000 grains 

 are common salt. Sea water is thus absolutely unfit for 

 purposes of irrigation. 



210. Depth of water, &*c. For all ordinary purposes 

 the questions of depth of water, volume of water that can be 

 raised per hour, and cost of the appliance used, are of first 

 consideration. This is the subject of water-lifts with which 

 we will deal in the next chapter. 



