THE WORK OF LEA VES 175 



exhales in twenty -four hours carbon dioxide amount- 

 ing to about 1.2 per cent of his own weight, while 

 some bacteria give off in the same length of time twice 

 their own weight of carbon dioxide. It should be 

 remembered that foods contain other combustible sub- 

 stances besides carbon (e. g., hydrogen), and that the 

 above figures leave out of account the energy liberated 

 by burning these substances. They do not, therefore, 

 indicate the total amount of energy set free, but only 

 the principal part. 



By the method illustrated in Fig. 31, we can 

 measure approximately the volume of carbon dioxide 

 given off, and from this calculate the amount of energy 

 set free from the carbon of the food in a given time. 

 We may also find the weight of the carbon dioxide 

 (one liter of carbon dioxide weighs about two grams), 

 and so calculate how far the energy set free would 

 raise the organism if applied as mechanical energy. 

 Make these calculations in the experiment described 

 on page 34 (Fig. 31) . The experiment may be modi- 

 fied by setting the bottle containing the seeds upright 

 and bending the tube which passes through the 

 stopper so as to make its free end dip into a vessel 

 containing lye. 



The giving off of carbon dioxide is accompanied 

 by the absorption of oxygen : this process is called 

 respiration: it occurs in every living cell, since every 

 such cell has need of energy to perform its work. 





