THE WATER OF SOILS. 189 



other liquid. Second, that as its density decreases in cooling 

 after a certain point is reached, it freezes at the surface instead 

 of at the bottom, as other liquids do; and as solid water (ice) 

 is lighter than fluid water, ice stays at the surface and is readily 

 melted when spring comes. Third, since its temperature 

 changes more slowly than that of any other liquid, it serves to 

 prevent injuriously rapid changes of temperature in plants and 

 animals as well as in soils. Its high " heat of fusion " also 

 serves to prevent quick freezing of plant and animal tissues, so 

 that the hrief prevalence of a low temperature may be more 

 readily borne. Finally, the large amount of heat absorbed in 

 evaporation of water serves to keep both plants and animals 

 cool under excessive external temperatures which would other- 

 wise quickly destroy life. 



Capillarity or Surface Tension. In this table it will be 

 noted, first, that water rises higher in fine ("capillary") or 

 hair tubes than the other fluids mentioned, which fairly rep- 

 resent all others. Xo other fluid approaches water in the 

 height to which it will rise l in either soils or plant tissues. 

 Were its capillary factor no higher than, c. g., that of oil or 

 alcohol, trees could not grow as tall as we find them, and the 

 water supply from the substrata, and all the movements of 

 water in the soil, and hence plant growth, would be similarly 

 retarded. It is easy to verify these differences by immersing 

 a cylinder of clay soil (or a cotton wick) in water on the 

 one hand, and in oil or alcohol on the other. Notwithstanding 

 the greater fluidity of alcohol as compared with water, the 

 latter will be found to fill the porous mass much more 

 quickly. 



The smaller the diameter of the tube, the higher will the water rise 

 in it, and the greater will be the curvature of its upper surface, to which 

 the rise is sensibly proportional. Hut in the case of liquids which do 

 not " wet " the walls of the tube (as in that of mercury and glass), the 

 curve (meniscus) is convex, instead of concave, and the liquid is 

 depressed instead of rising. 



It is in its relations to heat, however, that water is specially 

 distinguished from other substances; and these differences are 



1 Excepting only the water-solutions of certain salts, among which common 

 salt, kainit and nitrate of soda are of agricultural interest. Common salt may in- 

 crease the capillary rise to the extent of more than live percent. 



