364 THE PHILOSOPHY OF BIOLOGY 



but the conception of its operation is of the 

 greatest value in the consideration of energy-trans- 

 formations, and it is for this reason that we discuss 

 it here. 



Consider a gas, or some other substance capable 

 of expanding or contracting. It contains intrinsic 

 energy, and it is capable of doing work. Thus, since 

 a gas can expand indefinitely it can be made to do 

 mechanical work. A mass of gas at a pressure pu and 

 having a volume Vi, and at a temperature T°, can do 

 work by expanding till its pressure is reduced to 

 „o ^o p, and its volume 



\ \ increased to v. If 



\ \N it expands adia- 



""" V^ batically its tem- 



i\^^ perature will fall 



: ^^^^?^=^^/o^ pose that t° is the 



; .JT^""*=^z"' temperature of 



1 1 i> the surrounding 



pjj, 30 ' medium : the gas 



cannot therefore 

 cool further, and we can obtain no more work from it. 

 If the gas is the substance which v/e wish to employ 

 as the working substance in the Carnot engine, we 

 must therefore bring it back to the condition repre- 

 sented by A . That is, we must raise its temperature 

 to T°, we must reduce its volume to Vi, and we must 

 increase its pressure to pi. 



Thus the steam of an engine is (say) at a temperature 

 of 110° C, and a pressure of 120 lbs. to the square inch. 

 When it has passed through the cylinder and condenser 

 it is water at a temperature of, say, 15° C, and it is at 

 atmospheric pressure. We must, therefore, bring it 

 back to its former condition by heating this water in 



