THE LEAF 145 



that heat is motion, which by loosening the particles of 

 a body causes its decomposition. But light is also motion, 

 a regular, undulatory form of motion. The following 

 rough comparison will help us to explain the decom- 

 posing effect of light. Suppose two light bodies, say 

 two wooden balls, float side by side on a smooth surface 

 of water. We throw a stone into the water near them. 

 Circles will radiate from the stone, and every time a 

 new wave passes under the floating balls it will separate 

 them, will break the connection between them, driving 

 one of them up to its crest, and plunging the other 

 into its hollow. To this stone, which produces the 

 circles in the water, we may compare the sun, with 

 the waves of light continually running from it and 

 diverging to infinity ; with the sole difference that 

 these waves travel about 190,000 miles a second, 

 and are so fine, and follow upon one another so rapidly, 

 that on the average 50,000 of them are included within 

 a single inch. 



These waves, following upon one another at an 

 almost inconceivable rate, come into contact in the 

 leaf with still smaller atoms of carbon and oxygen, 

 combined in carbonic acid, loosen them and break up 

 their combination. The oxygen is set free while the 

 carbon immediately enters into combinations of another 

 kind. The first of these new compounds that we 

 recognise under the microscope is starch. 



We have just seen how the heat and light of the 

 burning magnesium can be stored. The same is true of 

 the rays of the sun. We cannot simply seize and shut 

 up a ray of sunlight ; but we grow plants for the 

 purpose, and these not only extract carbon from the 

 air by means of their leaves, but together with the 

 carbon absorb and store, concealed in it, the sun's rays. 

 It is the rays of the summer sun which warm us in our 

 wood fires ; it is again the same rays that give us light 

 in our candles for our long winter evenings. 



K 



