RELATION OF FORESTS TO WATER SUPPLY 



345 



FIG. 212. The decomposition of water. 



we notice is that there is always twice as much of one gas as 

 the other; that is, water is composed of two substances, one 

 of which is always pres- 

 ent in twice as large 

 quantities as the other. 



Hydrogen, a compo- 

 nent of water. On 

 testing the gases into 

 which water is broken 

 up by an electric cur- 

 rent, we find them to be 

 quite different. The gas 

 present in the smaller 

 quantity is oxygen, a 

 substance with which we are already familiar. The other gas, 

 hydrogen, is new to us and is interesting as being the lightest 

 known substance. An important fact about hydrogen is that 

 in burning it gives about as much heat as five times its weight 

 in coal. Its flame is blue and almost invisible by daylight, 

 but intensely hot. If platinum wire is placed in an ordinary 

 gas flame, it does not melt, but if placed in a flame of burning 

 hydrogen, it melts very quickly. 



The relation of forests to the water supply. When heavy 

 rains fall on a bare slope, or when snow melts on a barren hill- 

 side, a small amount of the water sinks into the ground, but 

 by far the greater part of it runs off quickly and swells brooks 

 and streams, thus causing floods and freshets. 



When, however, rain falls on a wooded slope, the action is 

 reversed ; a small portion runs off, while the greater portion 

 sinks into the soft earth. This is due partly to the fact that 

 the roots of trees by their constant growth keep the soil loose 

 and open, and form channels, as it were, along which the water 

 can easily run. It is due also to the presence on the ground of 



