THE COMPOSITION OF WATER 



79 



ward and after a longer or shorter journey, meets with a non- 

 porous layer of rock through which it cannot pass; and which 

 effectually hinders its downward passage. In such regions, 

 there is an accumulation of water, and a well dug there would 

 have an abundant supply of water. The non-porous layer is 

 rarely level, and hence the water whose vertical path is ob- 

 structed does not " back up " on the soil, but flows down hill 

 parallel with the obstructing non-porous layer, and in some 

 distant region makes an outlet for itself, forming a spring 

 (Fig. 38). The streams originating in the springs flow through 

 the land and eventually join larger streams or rivers ; from 

 the surface of streams and rivers evaporation occurs, the 

 water once more becomes vapor and passes into the atmos- 

 phere, where it is condensed and again falls to the earth. 



Water which has filtered through many feet of earth is far 

 purer and safer than that which fell directly into the rivers, or 

 which ran off from the land and joined the surface streams 

 without passing through the soil. 



72. The Composition of Water. Water was long thought 

 to be a simple sub- 

 stance, but toward the 

 end of the eighteenth 

 century it was found 

 to consist of two quite 

 different substances, 

 oxygen (O) and hy- 

 drogen (H.) 



If we send an elec- 

 tric current through 

 water (acidulated to 

 make it a good con- 

 ductor), as shown in Figure 39, we see bubbles of gas rising 

 from the end of the wire by which the current enters the water, 



FIG. 39. The decomposition of water. 



