196 THE MECHANISM OF LIFE 



form the cigarette, and the latter to grow to its original, unlit, 

 unsmoked size. If the molecules were visible, we should see those 

 of the ash, the water, the carbonic acid gas, and the particles of 

 smoke combine together to make up the molecules of the cellulose 

 of the tobacco and paper. We should see energy taken up from 

 the air and become concentrated, so to speak, in the glowing 

 cigarette end, and then pass into the potential, chemical state in 

 the tobacco and paper. 



Or, again, take the famous illustrations of Einstein's theory of 

 relativity. A man looking down from a stationary balloon 

 might have seen the explosion of a mine on the Menin Ridge, but 

 suppose that, during the few seconds of that occurrence, the; 

 balloon had been moving away from the earth with the velocity 

 of light; then our observer would have seen nothing happen, 

 although someone in a stationary balloon would have seen the 

 earth, stones, smoke, etc., thrown up into the air. Suppose that 

 the observer had witnessed the explosion from his balloon, and 

 that the latter had then immediately started to move away from 

 the earth with a velocity greater than that of light; then he 

 would (assuming he had a telescope powerful enough) have seen 

 the whole train of events proceed backwards. Smoke, stones, 

 dust, and earth would appear to come together from nowhere 

 and coalesce to form the solid ground. 



Apparently, then, there is no logical reason why any physical 

 change should not go in either direction. Water runs downhill, 

 but it might run uphill. (Note that we can make it go uphill 

 by forcing it through a closed pipe, but what we are to imagine is 

 it running uphill itself. We can spend energy in forcing it up, 

 and then we find that we have spent more power than can be 

 recovered by allowing the water to run downhill again and work 

 a pump.) We can think of, or imagine, or picture, the water 

 going uphill of itself, and we cannot find any reason why it 

 should not except that it does not. 



In the abstract, therefore, all physical changes are reversible, 

 but it happens that we are living in an universe in which they 

 are irreversible. This is our experience, and we can generalise it 

 in several ways. The most comprehensive way of doing so is to 

 say that in all things that happen a certain mathematical func- 

 v tion, called entropy, increases in quantity. What is meant by 

 entropy we shall explain presently. Note, in the meantime, that 

 the necessary condition that any phenomenon or physical change 



