J 11 THE RIVER. 



become desarts, there is always a crust of some of those 

 salts upon the surface ; and the beds of dried-up lakes 

 in warm climates contain quantities of the same, while 

 all their vicinity is sterile. On the surface of the neg- 

 lected lands, the coat is comparatively thin, but in the 

 basins that once were lakes (as in some of those in 

 Mexico,) it is several inches, or even feet, in thickness. 

 The greater thickness in the beds of the lakes, shows 

 that there must have been an accumulation there 

 while the bed was filled with water ; and hence it is 

 evident that the purification of the soil from saline 

 compounds, deleterious to vegetable and animal life, is 

 one of the most important functions of rivers ; and if 

 not so immediately necessary to the existing races of 

 beings, at least essential to their permanent continuation. 



Rivers also tend to purify the air, as well as to drain 

 the earth of deleterious matter. The current of water 

 that descends from the high ground, causes a gradual 

 motion in the air, by which that over different kinds 

 of surfaces is interchanged. This is all that is meant 

 by purifying the air. When it remains long over any 

 particular kind of surface, it ceases to take up the 

 effluvia, which, by stagnating, would be converted into 

 a poison. It is by changes of this kind, that winds, 

 hurricanes, and thunder-storms are said to clear the 

 air ; and what they do with violence, is silently done 

 by the ever-flowing current of a living stream. 



Nor, important though they be, are these all. Dead 

 animal and vegetable matters accumulate in water, and 

 then undergo decomposition, in the course of which 

 they give out gases which are pernicious. Disease is 

 always found about stagnant waters ; and " the reek o' 



