70 THE PHILOSOPHY OF BIOLOGY 



of heat taken from the reservoir and the amount given 

 to the refrigerator. 



This series of operations is called a direct Carnot 

 cycle. But the mechanism can be worked backwards. 

 In this case heat passes from the refrigerator into the 

 working substance, which was at a lower temperature. 

 The working substance, or gas, is then compressed, 

 as the result of which operation it is heated to just 

 above the temperature of the reservoir. The heat it 

 thus acquires is then given up to the reservoir. 



In the direct Carnot cycle, therefore, energy passes 

 from a state of high potential to a state of low potential 

 and work is done by the mechanism. In the reversed 

 Carnot cycle energy passes from a state of low potential 

 to a state of high potential and work is done on the 

 mechanism. The Carnot engine is thus perfectly 

 reversible. No energy is dissipated in its working. 

 It is, of course, a purely imaginary mechanism. 



In the metabolism of the green plant carbon dioxide 

 and water are taken into the tissues of the leaf and 

 are transformed into starch. But the energy of the 

 compounds, carbon dioxide and water, is much less 

 than that of the same compounds when built up into 

 starch. Energy must therefore be derived from 

 some source, and this source is said to be the ether. 

 Solar radiation is absorbed by the green leaf, and this 

 energy is employed to produce the chemical trans- 

 formation. Just how this is effected we do not 

 positively know, in spite of much investigation. It 

 is possible that formaldehyde is formed from carbon 

 dioxide and water, polymerized, and then converted 

 into starch. It is possible that the absorbed electro- 

 magnetic vibrations are converted into electricity in 

 the chlorophyll bodies of the leaf, though when 

 radiation is absorbed in physical experiments it is 



