ITS AFFINITY FOR SOLID SUBSTANCES. 39 



Thus when water falls in rain or trickles along the surface of the 

 land, it absorbs these gaseous substances, carries them with it wherever 

 it goes, conveys them to the roots, and into the circulation of plants, and 

 thus, as we shall hereafter see, makes them all minister to the growth 

 and nourishment of living vegetables. 



Again, water possesses the power of dissolving many solid substances. 

 If sugar or salt be mixed with water in certain quantities, they 

 speedily disappear. In like manner, many other bodies, both simple 

 and compound, are taken up by this liquid in greater or less quan- 

 tity, and can only be recovered by driving ofTthe water, through the aid 

 of heat. 



Thus it happens that the water of our springs and rivers is never 

 pure, but holds in solution more or less of certain solid substances. 

 Even rain water, washing and purifying the atmosphere as it descends, 

 brings down portions of solid matter which had previously risen into the 

 air in the form of vapour, and as it afterwards flows along or sinks into 

 the surface of the soil, it meets with and dissolves other solid substances, 

 the greater portion of which it carries with it wherever it enters. In 

 this way solid substances are conveyed to the roots of plants in a fluid 

 form, which enables them to ascend with the sap ; and the supply of 

 these naturally solid substances is constantly renewed, by the succes- 

 sive passage of new portions of flowing water. We shall hereafter be 

 able to see more clearly and to appreciate more justly this beautiful ar- 

 rangement of nature, as well as to understand how indispensable it is to 

 the continued fertility of the soil. 



Nor is it merely earthy and saline substances which the water dis- 

 solves, as it thus percolates through the soil. It takes up also sub- 

 stances of organic origin, especially portions of decayed animal and ve- 

 getable matter, — such as are supposed to be capable of ministering to 

 the growth of plants, — and brings them within reach of the roots. 



This solvent power of water over solid substances is increased by an 

 elevation of temperature. Warm water, for example, will dissolve 

 Epsom salts or oxalic acid in much larger quantity than cold water 

 will, and the same is true of nearly all solid substances which this fluid 

 is capable of holding in solution. To this increased solvent power of 

 the water they absorb, is ascribed, among other causes, the peculiar 

 character of the vegetable productions, as well as their extraordinary 

 luxuriance, in many tropical countries. 



2°. But the affinity which water exhibits for many solid substances is 

 little less important and remarkable. 



When newly burned lime is thrown into a limited quantity of water 

 the latter is absorbed, while the lime heats, cracks, swells, and finally 

 falls to a white powder. When thus perfectly slaked, it is found to be 

 one-third heavier than before — every three tons having absorbed one 

 ton of water. This water is retained in a solid form, more solid than 

 water is when in the state of ice, and it cannot be entirely separated 

 from the lime without the application of a red heat. When you lay 

 upon your land, therefore, four tons of slaked lime, you mix with your 

 soil one ton of water, which the lime afterwards gradually gives up, 

 either in whole or in part, as it combines with other substances. To 

 this fact we shall return when we hereafter consider the various ways 



