1864.] CHEMIRTRY OF MANURES. 121 



food-supply of plants, — so finely divided, so delicately dropped^ 

 and so grandly replenished by the colossal water-service of the 

 world. As indeed of carbon, so of water, the atmosphere is, for 

 plants, the mighty reservoir' and ever-flowing fount. In point of 

 fact, every cubic foot of air upholds between two and three grains 

 of water invisibly dissolved ; and as fast as this condenses above 

 to floating clouds and falling rain, so, in annual quantity precisely 

 equal, is it fed below by the evaporation of " ihe seas and the 

 streams." This process however, like all the other great opera- 

 tions of Nature, is subject to perturbation, in the redress of which 

 human Art finds its appropriate sphere. In temperate climates^ 

 the formation, distribution, and condensation of rain-clouds take 

 place, on the whole, with sufficient regularity to insure, in ordinary 

 seasons, enough of this aliment to the crops. It is otherwise in 

 tropical regions. There, superfluous deluges of rain, and long- 

 protracted droughts, succeed each other ; so that artificial irrigation 

 becomes the prime condition of tropical husbandry. Irrigation 

 might, indeed, be fairly described as the high farming of the 

 tropics ; and water as their most precious manure. 



Water, indeed, is not merely the vehicle of all other aliments 

 for plants, it is also an aliment itself— in the sense that it assumes 

 the solid form in their tissues, entering into their chemical con- 

 sitution, and contributing largely to their weight. Wood, for ex- 

 ample, after having been thoroughly dried, still consists, for nearly 

 half its weight, of the elements of water. Water, moreover, is the 

 chief constituent of the sap of plants ; and its rapid evaporation 

 from their surfaces creates the internal vacuum to whi;h they owe 

 the astonishing suction-power of their roots ; as Halts first proved 

 by his capital experiments on this subject published in 1717. 



Supply of Nitrogen to Plants. — Last in order, because 

 least in quantity, yet by no means on that account lowest in 

 importance stands the nitrogen among the volatile constituents to 

 plants. It is of peculiar interest, as one of the costliest and most 

 eagerly-sought manurial elements, and as that concerning which 

 the principal agricultural controversy of the day is now raging. 

 Nitrogen like carbon, and the elements of water, has in the atmos- 

 phere its source and reservoir; flowing thence to living organisms, 

 and thither restored by their dec.iy and dissolut on after death. 

 It is thus difi'used, chiefly in combination with hydrogen, as am- 

 monia ; a gas in the highest degree diff'usible in air, soluble in 



