370 THE PHILOSOPHY OF BIOLOGY 



undergo the adiabatic expansion i->2 until its tem- 

 perature falls to that of the refrigerator, T^"". We now 

 compress the gas while keeping it at this temperature, 

 that is, we cause it to undergo the isothermal con- 

 traction 2->3, during which operation it is giving up 

 heat to the refrigerator, so that there is again a flow 

 of heat-energy. We then compress it still further 

 without allowing heat to escape from it, that is, we 

 cause it to undergo the adiabatic contraction 3^4. 

 During this operation the gas rises in temperature to 

 T2°- It is now in the condition that it was when the 

 cycle commenced. 



In this cycle of operations heat first entered, and 

 then left the gas, and with this entrance or rejection 

 of heat, the condition of the gas with respect to its 

 power of doing work changed. We investigate this 

 flow of heat, and the concomitant change of properties 

 of the substance, with regard to which the flow took 

 place, by forming the concept called entropy. We 

 make the convention that when heat enters a substance 

 the entropy of the latter increases, and when heat 

 leaves it its entropy decreases. We call the quantity 

 of heat entering or leaving a substance Q, and the 



temperature of the substance T. Then ^ is pro- 

 portional to the change of entropy of the substance 

 when the quantity of heat, Q, enters or leaves it. 



Now it is a fact of our experience that heat can 

 only flow, of itself, from a hotter to a colder body. 

 Consider two such bodies forming an isolated system, 

 the temperature of the hotter one being T^ , and that 

 of the colder one Ti'. Let Q units of heat flow from 

 the body at T/ to that at Ty no work being done. 



Then the loss of entropy of the hotter body is ^o, and 



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