PQOD-DIGEST10N AND RESPIRATIOX 53 



in the direction in which it was driven at first. If the second blow 

 arrives just at the time in which this last movement starts it will 

 increase it, and a repetition of the blows at the correct moments 

 will finally result in a vigorous vibration of the pendulum. We 

 have converted the energy of the blows into a movement of a large 

 mass. Unless the blows are timed to the natural rate of the 

 pendulum, some of them will push it in the wrong direction and 

 undo the work of the rest. The energy of blows so delivered, 

 instead of setting the system into its natural rate of vibration, will 

 be wasted as heat. Similarly, the light energy taken up by 

 resonance is converted into molecular movement of the natural 

 rate, and may so increase this movement that chemical change 

 occurs, and is thus used in chemical work without becoming heat. 



What, then, is the reason why chlorophyll has this especially 

 great absorption in that part of the spectrum which we see as red ? 

 It has been found by measurement that the energy of the rays in 

 the solar spectrum is greatest about the yellow. This is due to the 

 fact that these measurements were made on a high sun in a clear 

 atmosphere. Since the atmospere absorbs rays of short wave- 

 length more than it does those of the longer wave-length, it is 

 possible that the position of the chlorophyll band may be in that 

 of maximum energy for the greater part of the day and the greater 

 part of the weather, especially in the higher latitudes. 



So far we have seen what provision is made for absorbing 

 radiant energy, and we next inquire as to the chemical changes 

 which it causes to take place. We saw that the final result is that 

 carbon dioxide is decomposed and that oxygen is given off. Let 

 us take the carbon dioxide part of the problem first. It is easy to 

 show that starch is the final product (E., p. 186). Now starch is an 

 insoluble carbohydrate formed by the union of a large number of 

 glucose molecules by removal of the elements of water from two 

 neighbouring ones, in a similar way to the union of two amino- 

 acids described above. The advantage of its being insoluble will 

 be seen later, when we come to learn about enzymes. The forma- 

 tion of starch from glucose is not a result of the light, so that what 

 we have to account for is the production of glucose or a similar 

 hexose. From the composition of glucose we see that hydrogen 

 has to be introduced into the molecule of carbon dioxide to start 

 with. This, of course, comes from the water present. The final 

 net result is : 



6CO., + 6H 2 O = CoH^O,. + 6Oo 



and we see that there is a large increase in chemical potential 

 energy, which has come from the sun. But this process must 

 consist of several stages. What is known about these? We 



