176 FIRESIDE SCIENCE. 



the eartli is due to the condensation of aqueous 

 vapor previously existing in the atmosphere, and 

 which is supplied in great measure from the surface 

 of the sea, the area of the latter compared with 

 that of the land being very great, necessarily so 

 perhaps to furnish the requisite extent of evaporat- 

 ing surface. This water is, as is well known, 

 perfectly fresh and pure, the saline constituents of 

 the ocean having no sensible degree of volatility at 

 the temperature at which the vapor has been raised. 

 No sooner, however, does it reach the earth than 

 its solvent powers are brought into requisition, and 

 it becomes contaminated with or takes up a large 

 number of substances, which it holds in solution. 

 The waters of rivers and springs invariably contain 

 a greater or less amount of alkaline and earthy 

 salts, which have been washed out- of the earth by 

 percolating rains. In the water we daily use for 

 household purposes, and that which we furnish to 

 our animal's, are found considerable quantities of 

 these salts, together with numerous other sub- 

 stances. We are accustomed to regard these as 

 impurities, and they are such, strictly considered ; 

 but these very impurities are of vital consequence 

 to the living system. These matters exercise an 

 important influence upon the body in health and 

 disease, and if they were entirely absent, physical 

 weakness, and probably death, would ensue. It 



