1911.] The Mechanism of Carbon Assimilation. 



109 



as no reversal of the films was made, and an important control experiment 

 was omitted; nevertheless they bring out several points of interest not 

 shown in the final series. 



One of the films (A) was in C0 2 -free air, the other (B) was in a tube 

 connected with a supply of carbon dioxide. With air in both tubes, B was 

 o- 56 hotter than A when both were exposed to light ; in the following 

 table of readings the observed temperature difference is corrected for this 

 want of symmetry : — ■ 



C0 2 slowly entering B. 



2.34 

 2.36 

 (At this point it 



s noticed that film A v 



almost destroyed. Two new films were 

 therefore prepared.) 



i shaded for 4 minutei 



These figures show remarkable variations of the temperature difference 

 with time. It will be noticed that several minutes are required for the 

 maximum temperature difference to be established, and that this difference 

 does not persist for more than two minutes, but gradually falls off until, 

 if the exposure to light is continued uninterruptedly, the temperatures of 

 the two films become equal, and ultimately the one in carbon dioxide 

 becomes hotter than the control film in air. It was always noticed that 

 the film in C0 2 -f ree air was " scorched " and destroyed sooner than the 

 other, and, regarding each film merely as an absorber of heat, it is obvious 

 that the one in which the chlorophyll is more rapidly destroyed must also 

 be the one in which the amount of heat absorbed in unit time falls off more 

 rapidly. This probably explains the ultimate reversal of the temperature 

 difference, for both films were being gradually destroyed, but by the time 

 the film in carbon dioxide had lost its photolytic efficiency the one in air 



