114 NOT ALTOGETHER ABOUT PLANTS 



motion which he uses in his factories are expensive to pro- 

 duce, but the Hght which she uses falls everywhere and 

 has no cost. Yet man has never been able to imitate the 

 invention of nature ; he has no such device by means of 

 which he can use sunlight to do his work. But he does 

 make use of the hght-using device of nature. It furnishes 

 him fuel for his engines as well as food for his body. 



Already you know something of these factories of nature 

 which use Hght for their power. You know that their 

 product is more important than the product of any fac- 

 tories of man. You know that these factories of nature 

 are the green leaves and that their product is food. You 

 know that man uses the products of plants in his factories 

 as well as at his meals, and you know that there would be 

 no products of plants if it were not for the green leaves. 



For fuel man first used wood. Now he depends more 

 upon coal. Coal as well as wood owes its existence to the 

 work of green leaves. Coal is the transformed bodies of 

 plants which Uved thousands of years ago. Those plants 

 fell in the shallow water in which they lived and their 

 bodies were covered. Other deposits covered them, and 

 movements of the earth's crust, as well as the weight 

 of the deposits above, caused much pressure upon thedi. 

 At last they became transformed into layers of coal. It 

 is common to find 'in coal outlines of the stems and leaves 

 from which it came. When we burn it, we are releasing 

 the energy of sunlight which shone on the green leaves of 

 long-gone ages. 



38. Evaporation. — Evaporation is the name of a proc- 

 ess. It is the changing of a substance from a liquid or 

 a solid into a gas. Usually we think only of liquids as 



