An Introduction to a Biology 



diet the exact state of the weather a fortnight in advance, 

 we should not procure the warmer raiment until we knew 

 that it would be needed. 



Another phenomenon which may be looked at from these 

 two points of view is that of the causation of heat ; it is 

 believed that the heat of a substance is occasioned by the 

 mean speed at which the molecules of which it is composed 

 are travelling. To deal with the three differences between 

 our two aspects of things in turn ; it is evident first that, 

 while our ignorance of the speed of an individual molecule 

 is so great that we try to conceal it by saying that it is deter- 

 mined by chance, our knowledge of the average speed of 

 myriads of them is so accurate that certain laws of thermo- 

 dynamics have been formulated. Secondly, it has been cal- 

 culated that a curve representing the frequency of the various 

 speeds spread over the molecules is an ordinary curve of 

 error ; so that although that which is true of the mass is 

 also true of some of the molecules, it is by no means true of 

 all of them. Thirdly, the only point of view from which 

 we can regard this phenomenon at present is that from which 

 we can only discern the mass-result ; but that it is by no 

 means inconceivable that there may be another point of 

 view is evident to all who are familiar with Clerk Maxwell's 

 demon, a being who is so essential to my argument that I 

 shall make no apology for quoting his creator's description 

 of him in full : " One of the best-established facts in thermo- 

 dynamics is that it is impossible in a system enclosed in an 

 envelope which permits neither change of volume nor passage 

 of heat, and in which both the temperature and pressure 

 are everywhere the same, to produce any inequality of 

 temperature or pressure without the expenditure of work. 

 This is the second law of thermo-dynamics, and it is un- 

 doubtedly true so long as we can deal with bodies only in 

 mass, and have no power of perceiving or handling the separate 1 

 molecules of which they are made up. But if we conceive 



1 My italics. 

 155 



