24 Introduction. 



bon dioxide is broken down and the chemical changes are 

 produced which result in building the sugars, starches and 

 cellulose of plants. Others of these waves are much longer 

 than the light waves and these have a wonderful power in 

 producing heating effects when they fall upon certain sub- 

 stances, one of which is water. 



When bright sunshine is allowed to pass through a 

 large lens the glass is but little warmed by the passage, 

 but if paper is held at the light focus it is quickly set on 

 fire by the dark or invisible rays. That it is the dark rays 

 may be proved by allowing the light to pass first through a 

 solution of iodine in bisulphide of carbon which permits 

 the dark waves to easily pass while it cuts down or stops 

 the light waves. When these dark waves are brought to 

 a focus in water it is made to boil quickly under their in- 

 fluence. 



On the other hand if sunlight is first passed through a 

 solution of alum in water, which stops the dark waves but 

 allows the light waves to pass, then when they are focused 

 upon water but little heating effect is noted. 



29. How Water is Evaporated. It is the fact that water 

 does not allow the long dark waves from the sun to pass 

 readily through it which causes it to evaporate so rapidly 

 from ocean, lakes and streams, and from the soil and the 

 leaves of vegetation. When these waves fall upon water 

 they set its molecules in such rapid vibration that the sur- 

 face tension, or force of cohesion, is overcome and many of 

 the water molecules are thrown out into the air in the form 

 of invisible vapor. Were the water not so opaque to the 

 dark waves, neither snow nor ice would be as rapidly 

 melted in the spring nor would there be so much evapora- 

 tion from the ocean as we now have, hence rains would be 

 less frequent and the land less productive. 



30. How Chemical Changes Are Produced by Ether 

 Waves. When the light waves and especially the shorter 

 rlark waves fall upon many substances they appear to set 



