AT OUR HOMES 217 



remains of plant growths. The coal age in the earth's his- 

 tory as given in Geology must have been a time when climatic 

 conditions were unlike those of the present day. Vegetation 

 grew rank in an atmosphere warm, and laden with moisture 

 and carbon dioxide. During long periods of time accumula- 

 tions of woody material of great thickness must have been 

 kept from decay by submergence in the waters of marshy 

 regions. Under the pressure of layers of earthy material 

 washed in upon it, this vegetable matter largely shut away 

 from the air became changed in course of time into the varied 

 forms with which we are familiar as coal. The several layers 

 (" veins") of coal, separated by layers of slate or other rock 

 material, represent successive periods of accumulation and 

 of submergence. Through later periods, and probably when 

 vegetation did not flourish so luxuriantly, these thicknesses 

 of vegetable matter became buried in the depths where as 

 coal they are now found. 



It should be mentioned here that all this carbon represents 

 " stored solar energy'' used by plants in the separation of the 

 carbon from the carbon dioxide of the air. Enormously 

 long periods of time must have been involved in this forma- 

 tion of the coal layers so nearly indispensable to the welfare 

 of mankind. It is calculated that at the present rate of 

 increase in the production and use of coal the world's supply 

 will become exhausted in somewhat more than a hundred 

 years. Long before then, however, unless other forms of 

 energy are employed for doing the world's work, the rise in 

 prices due to a limited supply is likely to make them prohibi- 

 tive for most people. 



All means for illumination in common use in homes depend 

 upon the fact that when the temperature of a substance has 

 been raised sufficiently high it becomes incandescent, i.e., 

 it gives out light. In candle, kerosene lamp, and illuminating 

 gas flames the carbon particles present in the gas or vapor 



