12 INTRODUCTION TO GENERAL PHYSIOLOGY 



Now, one of the most significant and important results of 

 modern physiological investigation is that this first law has been 

 shown to apply to the human body itself. The amount of chemical 

 energy taken in the form of food can be measured, and that given 

 out in different forms can be converted to heat. When this is done, 

 the balance is found to be so close as to be practically perfect. 



Although, as we have said, there is no loss in the conversion of 

 any one kind of energy to any other kind, so far as the final total 

 sum is concerned, there is a certain limitation in the case of heat. 

 With this circumstance the second law of energetics deals. This 

 law may be looked at in two ways. In the first place, it expresses 

 the fact that, while any other form of energy can be completely 

 converted into heat, heat itself, under the conditions in which we 

 live, can only be partially converted into other forms of energy. 

 The proportion is given by the well-known formula relating the 

 fall of temperature along which the work is done to the actual 

 height of the temperature above a particular point, at which heat 

 energy is absent, called the absolute zero of temperature. How do 

 we find out where this zero is ? Take a volume of a gas at o C. 

 Owing to the heat energy present in it, the molecules are in a state 

 of movement, and sufficiently far apart that the volume taken up 

 by them is so small as to be a negligible fraction of the total 

 volume. Lower its temperature by one degree. The kinetic 

 energy of the molecules is reduced, so that the volume taken up 

 by the gas is diminished, if we keep the pressure from altering. 

 This diminution in volume is found to be 1/273 of its initial volume. 

 Hence, if the temperature is lowered by 273, the volume will be 

 reduced to nothing, provided, of course, that nothing happens to 

 change the nature of the gas, and that we disregard the volume of 

 the molecules themselves. This temperature is the absolute zero 

 at which heat energy is absent altogether ; the kinetic energy of 

 the molecules has disappeared. In actual fact, of course, the 

 volume cannot decrease beyond that point at which the mole- 

 cules touch one another. The reason why heat has the peculiar 

 position as regards conversion to other forms of energy is, therefore, 

 because the temperature at which we work is so far above that at 

 which heat energy is absent. We can never completely get rid of 

 it ; whereas we can have a total absence of mechanical, electrical, 

 or chemical energy. 



The other aspect of the second law is that otfree energy. W T e 

 have seen that the various forms of energy, with the exception of 

 heat, can be entirely converted into other forms ; and, so long as 

 they are not changed into heat, we may be said to be free to use 

 the whole of them. But, if ever we allow any of this free energy 

 to be " degraded," as is often said, to heat, we can only use a part 



