480 Tire WATER SHOULD NOT BE STAGNANT. 



diJfTusion over the soil may be ascribed to the layer of visible manure 

 which it leaves everywhere behind it. Thus the Nile and the Ganges 

 fertilize the lands over which their annual floods extend, and partly 

 m this way do some of our smaller streams improve the fields over 

 which they either naturally flow or are artificially led. 



2°. Or if the water hold in solution, as the hquid manures of the 

 farm-yard do, substances on which plants are known to feed, then to 

 diffuse them over ilie surface is a simple act of liquid manuring, from 

 which the usual benefits foflow. Such is the irrigation which is prac- 

 tised in the neighborhood of our large towns, where the contents ol" 

 the common sewers are discharged into the waters which subsequent- 

 ly spread themselves over the fields. (For an interesting account of 

 the effects of such irrigation in the neighborhood of Edinburgh, see 

 Stephens, On IiTigaUon and Draining, p. 75.) In so far also as any 

 streams can be supposed to hold in solution the washings of towns or 

 of higher lands — and there are few which are not more or less im- 

 pregnated in this manner — so far may their beneficial action, when 

 employed for purposes of irrigation, be ascribed to the same cause. 



3°. But spring waters which have run only a short way from their 

 source are occasionally found to be valuable irrigators. In such cases, 

 also, the good effect may be due in vihoie or in part to substances held 

 in solution by the water. Thus, in lime-stone districts, and especially 

 those of the mountain lime-stone formation (Lee. XL, § 8,) — in which 

 copious springs are not unfrequently met with — the water is generally 

 impregnated with much carbonate of lime, which it slowly deposites as 

 it flows away from its source. To irrigate with such water is, in a re- 

 fined sense, to lime the land, and at the same time to place within the 

 reach of the growing plants an abundant supply of this substance, in a 

 form in which it can readily enter into their roots. (Some of the water 

 used in the well-known scientific irrigations at Closeburn Hall, in 

 Dumfries-shire, appears to have been impregnated with lime. See 

 Stephens, p. 43.) 



In other districts, again, the springs contain gypsum and common 

 salt, and sulphate of soda and sulphate of magnesia, and thus are ca- 

 pable of imparting to plants many of those inorganic forms of matter, 

 without which, as we have seen, they cannot exhibit a healthy growth. 



4°. Again, it is observed that the good effects of irrigation are pro- 

 duced only by running watei — coarse grasses and marsh plants spring- 

 ing up when the water is allowed to stagnate (_Low's Elements of 

 Agriculture^ 3d edition, p. 472.) This is explamed in part by the 

 fact that a given quantity of water will soon be deprived of that por- 

 tion of matter held in solution, of which the plants can readily avail 

 themselves, and that when this is the case it can no longer contribute 

 to their growth in an equal degree. 



But there is another virtue in running water, which makes it more 

 wholesome in the living plant. It comes upon the field charged with 

 gaseous matter, with oxygen and nitrogen and carbonic acid, in propor- 

 tions very different from those in which these gases are mixed together 

 in the air (Lee. II., § 6.) To the root, and to the leaf also, it carries 

 these gaseous substances. The oxygen is worked up in aiding the 

 decomposition of decaying vegetable matter. The carbonic acid is 



