198 THE MECHANISM OF LIFE 



Now we take the plain result of our experience that available 

 energy is continually decreasing in quantity (or that the entropy 

 of the universe is increasing). This is the bed-rock fact of our 

 experience: that everything that happens depletes the universe 

 of its store of available energy. " Every energy transformation 

 that occurs leaves an indelible imprint somewhere on the course 

 of events in the universe considered as a whole." There was an 

 original quantity of available energy in our universe, and this 

 is becoming exhausted as entropy increases. Now this quantity 

 being finite, and duration being infinite, it follows that the time 

 must come when there will be no available energy left (entropy 

 having attained its maximum), and so there must occur a com- 

 plete cessation of all energy transformations, or phenomena. 



But if past time is infinite, why has not this physical death of 

 the universe already occurred ? No matter how great a lapse 

 of duration is required, we can still imagine this to be possible. 

 Here, then, we have our physical impasse. The second law, 

 solidly based on our experience, says that entropy tends towards 

 a maximum, when universal happening must cease. Past time 

 is without limit, so that, no matter how slowly entropy is being 

 augmented, the maximum ought to be attained. But the uni- 

 verse is still the locus of energy transformations, so that entropy 

 has not attained its maximum value. 



Obviously the second law cannot be universally true, although 



.is true of our experience. Somewhere or other, or at some 

 time or other in the universe, it must be reversed or evaded. 

 Now since it is not logically necessary that entropy should always 

 tend towards a maximum value, we must next enquire under 

 what circumstances the second law can reverse itself; under 

 what conditions may water flow uphill of itself, or may heat flow 

 of itself from a region of lower to a region of higher temperature. 



The Reversal o! the Second Law. In order that such an 

 investigation may be possible we must choose some simple, 

 mathematically manageable case. Consider, then, the physical 

 condition of a small volume of some gas, say hydrogen, at 

 ordinary temperature and barometric pressure; it consists of an 

 enormous number of molecules which are moving very rapidly. 

 and so incessantly colliding with each other. It would be rather 

 like a swarm of midges in the air provided that the insects fl-\v 

 about in straight lines instead of avoiding each other. In a 

 volume of hydrogen equal to 1 decilitre (that is, a cube of 



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