310 THE HUMAN BODY. 



And finally we may oxidize the acetic acid so as to get carbon 

 dioxide and water 



We get, in both cases, from one molecule of alcohol, two 

 of carbon dioxide and three of water; and six atoms of oxy- 

 gen are taken up. In each stage of the gradual oxidation a 

 certain amount of heat is evolved; and the sum of these is 

 exactly the amount which would have been evolved by burn- 

 ing the alcohol completely at once. 



The food taken into the Body is for the most part oxi- 

 dized in this gradual manner; the products of imperfect 

 combustion in one set of cells being carried off and more 

 completely oxidized in another set, until the final products, 

 no longer capable of further oxidation in the Body, are car- 

 ried to the lungs, or kidneys, or skin, and got rid of. A great 

 object of physiology is to trace all intermediate compounds 

 between the food which enters and the waste products which 

 leave; to find out just how far chemical degradation is carried 

 in each organ, and what substances are thus formed in vari- 

 ous parts : but at present this part of the science is very im- 

 perfect. 



The Utilization of Energy in the Human Body. In 

 the steam-engine energy is liberated as heat; some of the heat 

 is used to evaporate water and expand the resulting steam ; 

 and then the steam to drive a piston. But in the living Body 

 it is very probable (indeed almost certain) that a great part 

 of the energy liberated by chemical transformations does not 

 first take the form of heat; though some of it does. This, 

 again, does not affect the general principle: the source of 

 energy is essentially the same in both cases; it is merely the 

 form which it takes that is different. In a galvanic cell 

 energy is liberated during the union of zinc and sulphuric 

 acid, and we may so arrange matters as to get this energy as 

 heat; but on the other hand we may lead much of it off, as 

 a galvanic current, and use it to drive a magneto-electric 

 machine before it has taken the form of heat at all. In fact, 

 that heat may be used to do mechanical work we must reduce 

 some of it to a lower temperature: an engine needs a con- 

 denser of some kind as well as a furnace; and, other things 

 being equal, the cooler the condenser the greater the proper- 



