AIR AND SUNLIGHT 17 



our energy we owe either directly or indirectly to the sun. With- 

 out it there could be no life on the earth, our oceans would become 

 vast bodies of ice, and all our lands would be frost-bound the year 

 round. The movement of the winds and waters, and changes of 

 temperature all depend on the sun's action. This solar energy or 

 sunshine is a sort of motion which comes to us at the rate of one 

 hundred and eighty-six thousand miles in a second of time and 

 it is this energy which does almost the entire work of the world. 

 If we let bright sunshine pass through a lens and hold a piece 

 of paper at the proper distance, the light rays come together 

 at a point or focus, and the paper is quickly set on fire by the 

 heating powers of the dark or invisible rays from the sun. We 

 can easily prove that this is true by placing a solution of iodine in 

 bisulphide of carbon between the sun and the lens. This shuts 

 out the light rays but allows the dark rays to pass through. When 

 this is done, we find that the same heating effect is produced as 

 before. If a solution of alum water is substituted for the carbon 

 bisulphide solution of iodine, the heat rays will be sifted out, and 

 the light rays when focused on the paper produce no apparent 

 heating effects. 



The fact that water absorbs these heat rays instead of transmit- 

 ting them is of vast importance to us. Were it not for this fact, 

 neither snow nor ice would melt rapidly in the spring. There 

 would be but little evaporation, rains would be of rare occurrence, 

 and lands would be much less productive. Besides these invisible 

 heat rays that come from the sun there are other waves that pro- 

 duce colors and still others that are capable of producing certain 

 chemical changes of use to photographers. Summing up, we may 

 say, then, there are three principal kinds of rays that reach us from 

 the sun: 1. The invisible heat rays which furnish us warmth. 

 2. The light rays which produce our colors. 3. The actinic rays 

 which produce chemical changes. 



As to how these rays from the sun reach us is a question of physics 

 rather than of agriculture, but it is sufficient to say that the sun- 

 light reaches us through a medium known as the ether. This 

 medium, according to Mendelejeff, the great Russian chemist, is a 

 gas one million times lighter than hydrogen, the lightest gas now 

 known to most of us, and with a power of diffusion so great that a 



PRAC. AGRICUL. 2 



