WHY BO WE EAT OUR DINNER? 803 



tive steam-engine, with whom food stands in the place of fuel, while 

 the possible kinds of movement are infinitely more varied and special- 

 ized. I do not mean to advance any of those " automatic " theories 

 which have been so current of late years. Whether they are true or 

 false, they have nothing to do with our present subject. I only want 

 to put in a plain light an accepted scientific truth. Men differ enor- 

 mously from steam-engines in their possession of consciousness, wills, 

 desires, pleasures, pains, and moral feelings ; but they agree with them 

 in the purely physical mechanism of their motor organs. A man, like 

 a steam-engine, can not move without his appropriate fuel ; and if the 

 fuel is not supplied, the fire goes out and the man dies. The exact 

 manner in which the materials are utilized for keeping up this vital 

 flame is the question to which we must now address ourselves. 



Food-stuffs and coal agree essentially in the chief characteristics 

 of their chemical constitution. Both consist mainly of hydrogen and 

 carbon, and both possess energy in virtue of the fact that their affini- 

 ties for oxygen are not satisfied. Water contains hydrogen, and car- 

 bonic acid contains carbon ; but we can get no motion out of these, 

 because in them the oxygen has already united with the atoms for 

 which it had affinity, and the separation necessary for dormant energy 

 has ceased to exist. But in bread, meat, potatoes, or coal, the hydro- 

 gen and carbon remain in their free state, ready to unite with oxygen 

 whenever the chance is presented to them. All alike obtained their 

 energy in the same way. The rays of sunlight falling upon the leaves 

 of their original trees or plants separated the oxygen from the water 

 and carbonic acid in the air, and built up the free hydrocarbons in 

 their tissues. The energy which they thus drank in has remained dor- 

 mant within them ever since : in the case of the bread for a few short 

 months, in that of the coal for countless millions of geological cycles. 

 But, however long it may have rested in that latent form, whenever an 

 opportunity occurs the atoms will reunite with oxygen, and the energy 

 will once more assume the active shape. There is really only one se- 

 rious difference between coal and food, and that is that most foods con- 

 tain another element, nitrogen, as well as carbon and hydrogen ; and 

 this nitrogen is an absolute necessity for the animal if it is to continue 

 living. But there are good reasons for suspecting that nitrogen is not 

 itself a fuel, being rather analogous in its nature to a match, and hav- 

 ing for its business to set up the first beginnings of a fire, not to keep 

 the fire going when it has once been lighted. So that this apparent 

 difference of kind is really seen to be unimportant when we get to the 

 bottom of the question. 



The various matters which an animal eats consist of pure food-stuffs 

 and of useless concomitant bodies : just as coal consists of pure fuel 

 and of the useless mineral matter known as ash. When an animal eats 

 his dinner, the process of digestion and assimilation takes place, and 

 has the ultimate result of separating the pure food-stuffs from the 



