ON THE NATURE OF LIFE 195 



burns. These things seem to us to be so inevitable that we find 

 it difficult to imagine them happening in the opposite way. It 

 appears to be ridiculous to think of water running uphill of itself; 

 of a table rising up of itself from the floor ; of a cold poker becom- 

 ing red-hot by exposure to the air; or of the ash, the water 

 vapour, and the carbonic acid combining together (with absorp- 

 tion of heat) to form a cigarette. But why should not these 

 things happen ? Puzzle over this, and we find we can give 

 no reason other than that no one has seen them happen. 



Nevertheless, there is no logical reason why they should not 

 happen, and we can easily imagine them to do so. We can even 

 picture them or imagine ourselves so situated that they appear 

 to happen. Take the case of the red-hot poker: it is red-hot 

 because its molecules are moving so rapidly that their atoms 

 radiate and so give off visible vibrations. While they glow they 

 are vibrating much more quickly than are the molecules of air 

 which come into contact with them, and so they accelerate the 

 velocity of the latter just as a quickly moving billiard ball would 

 accelerate the motion of one that is moving less rapidly when the 

 two collide. There is, therefore, an excess of kinetic energy in 

 the motions of the molecules of the poker compared with that 

 in the molecules of the air in the room, and so the excess becomes 

 levelled down, so to speak. When the poker has cooled to the 

 room temperature all the energy which it contained is still in 

 existence, but it is now uniformly distributed between the metal 

 and the air which came in contact with the latter instead of 

 being concentrated in the poker. 



Imagine that we could actually see the molecules and their 

 motions, and that we could photograph them so as to make a 

 kinematographic record. Working the latter, we should then 

 have a picture of the transfer of molecular motions from the hot 

 metal to the cold air, but if we were to work the record backward 

 just the same motions and. molecules would be thrown upon the 

 screen, but in the reverse order. W^e should first see molecules 

 of air and metal moving at certain rates in equilibrium with 

 each other, and we should then see the air molecules begin to 

 move more slowly and to give up some of their kinetic energy 

 to the molecules of the poker, which would move more and more 

 rapidly, until they become red-hot and give off visible radiation. 

 Or imagine a record of a cigarette smoker to be worked back- 

 wards ; we should then seem to see the smoke and ash solidify to 



