362 THE PHILOSOPHY OF BIOLOGY 



of the envelope ; and, being assumed perfectly elastic, 

 they rebound from each other, and from the walls of 

 the vessel, with the same velocity which they had when 

 they collided. The pressure of the gas (say that of 

 steam at a temperature of 110 C., and a pressure of 

 120 Ibs. to the square inch in a steam boiler) is the sum 

 of the impacts of the molecules on the walls of the 

 containing vessel. When the temperature is high 

 the molecules are moving at a higher mean velocity 

 than when the temperature is lower, and their mean 



free path tends to become 

 greater. The volume of a 

 certain mass of gas, that is, 

 the volume occupied by a 

 certain very great number 

 of molecules, is greater the 

 higher is the temperature, 

 provided the envelope is 

 one capable of yielding. If 

 _p we reduce the capacity of 

 the envelope in which the 

 FlG 2Q gas is contained, the pressure 



will rise, for the intrinsic 



energy of the gas is still the same ; but we have done 

 work on it, and by the law of conservation this work, 

 or at least the energy represented by it, must still 

 exist. It is represented by the decreased length of 

 free path of the molecules, and this means that the 

 impacts on the walls of the vessel will be greater than 

 they were. There is, therefore, a certain relation 

 between the volume of a gas and its pressure, and this 

 relation can be represented by an equation involving 

 the temperature, the pressure, and the volume. 



The diagram represents the pressure and the volume 

 of a gas when these things change. There are two 



