2i4 PETER GUTHRIE TAIT 



things are in contact the hotter cannot take heat from the colder without external 

 agency. 



" Now let A and B be two vessels divided by a diaphragm and let them contain 

 elastic molecules in a state of agitation which strike each other and the sides. 



" Let the number of particles be equal in A and B but let those in A have the 

 greatest energy of motion. Then even if all the molecules in A have equal velocities, 

 if oblique collisions occur between them their velocities will become unequal, and I 

 have shown that there will be velocities of all magnitudes in A and the same in 

 B, only the sum of the squares of the velocities is greater in A than in B. 



"When a molecule is reflected from the fixed diaphragm CD no work is lost or 

 gained. 



" If the molecule instead of being reflected were allowed to go through a hole 

 in CD no work would be lost or gained, only its energy would be transferred from 

 the one vessel to the other. 



" Now conceive a finite being who knows the paths and velocities of all the 

 molecules by simple inspection but who can do no work except open and close a 

 hole in the diaphragm by means of a slide without mass. 



" Let him first observe the molecules in A and when he sees one coming the 

 square of whose velocity is less than the mean sq. vel. of the molecules in B let 

 him open the hole and let it go into B. Next let him watch for a molecule of B, 

 the square of whose velocity is greater than the mean sq. vel. in A, and when it 

 comes to the hole let him draw the slide and let it go into A, keeping the slide 

 shut for all other molecules. 



"Then the number of molecules in A and B are the same as at first, but the 

 energy in A is increased and that in B diminished, that is, the hot system has got 

 hotter and the cold colder and yet no work has been done, only the intelligence of 

 a very observant and neat-fingered being has been employed. 



" Or in short if heat is the motion of finite portions of matter and if we can 

 apply tools to such portions of matter so as to deal with them separately, then we 

 can take advantage of the different motion of different proportions to restore a 

 uniformly hot system to unequal temperatures or to motions of large masses. 



" Only we can't, not being clever enough." 



To this is appended a pencilled annotation by Thomson : 



"Very good. Another way is to reverse the motion of every particle of the 

 Universe and to preside over the unstable motion thus produced." 



In an undated letter, which must have been written about this time, 

 Maxwell constructed the following Catechism : 



" Concerning Demons. 



"i. Who gave them this name? Thomson. 



"2. What were they by nature? Very small BUT lively beings incapable of 

 doing work but able to open and shut valves which move without friction or 

 inertia. 



