THE ACTIVITIES OF THE ORGANISM 117 



are so sharpened that he can follow every molecule in 

 its course ; such a being, whose attributes are still as 

 essentially finite as our own, would be able to do what 

 is at present impossible to us." ^ For the temperature 

 of the gas depends on the velocities of the molecules, 

 and in any part of the gas these velocities are very 

 different. Suppose that the demon saw a molecule 

 approach which was moving at a much greater velocity 

 than the mean : he would then open the door in the 

 aperture and let it pass through from - to +. On 

 the other hand, should a molecule moving at a velocity 

 much less than the mean approach he would let it 

 pass from + to - . In this way he would sort out 

 molecules of high from those of low velocity. But 

 the collisions between the molecules in either division 

 of the vessel would continually produce diversity of 

 individual velocity, and in this way the difference of 

 temperature between + and - would continually be 

 increased. Heat would thus flow from a region of 

 low to a region of high temperature without an equi- 

 valent amount of work being expended. 



Now we must not introduce demonology into 

 science, so, lest this fiction of Maxwell's should savour 

 of mysticism, or something equally repugnant, we 

 shall state the idea involved in it in quite unexception- 

 able terms. The conclusions of physics are founded 

 on the assumption that we cannot control the motions 

 of individual molecules. In a mass of gas, or liquid, 

 or in a solid, the molecules are free to move and do 

 move. Their individual velocities and free paths vary 

 considerably from each other. These motions and 

 paths are un-co-ordinated — " helter-skelter " — if we 



^ Impossible, in the sense that while we are unable to " abrogate " a 

 physical law. Maxwell's finite demon could, although his faculties were similar 

 in nature to ours. 



