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 p ly and 

 having a volume v Xi and at a temperature T°, can do 

 work by expanding till its pressure is reduced to 



p, and its volume 

 increased to v. If 

 it expands adia- 

 batically its tem- 

 perature will fall 

 to t°. Let us sup- 

 pose that t° is the 

 temperature of 

 the surrounding 

 medium : the gas 

 cannot therefore 

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

 If the gas is the substance which we 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 v u and we must 

 increase its pressure to p x . 



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

 of no° 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 



Fig. 30. 



