282 THE POPULAR SCIENCE MONTHLY. 



and give a perfect account of it ; but if you attempt to trace tliat 

 back you will have a point where the equation will stop, and will be- 

 gin to talk nonsense. That is the point where you took away the 

 paper, and allowed the mixing to begin. If we apply that same con- 

 sideration to the case of the poker, and try to trace back its history, 

 you will find that the point where the equation begins to talk non- 

 sense is the pohit where you took it out of the fire. The mathemati- 

 cal theory sujjjjoses that the process of conduction of lieat has gone 

 on in a quiet manner, according to certain defined laws, and that if at 

 any time there was a catastrophe, one not included in the laws of the 

 conduction of heat, then the equation could give you no account of 

 it. There is another thing which is of the same kind. That is the 

 transmission of fluid friction. If you take your tea in your cup, and 

 stir it round with a spoon, it won't go on circulating round forever, 

 but comes to a stop ; and the reason is, that there is a certain friction 

 of the liquid against the sides of the cup; and of the different parts of 

 the liquid with one another. Now, the friction of the different parts 

 of a liquid or a gas is precisely a matter of mixing. The particles 

 which are going fast, and are in the middle, not having been stopped 

 by the side, get mixed, and the particles at the side going slow, get 

 mixed with the jjarticles in the middle. This process of mixing can 

 be calculated, and it leads to an equation of exactly the same sort as 

 that which applies to the conduction of heat. We have, therefore, in 

 these problems, a natural process which consists in mixing things 

 together, and this always has the property that you can go on mixing 

 them forever, without coming to any thing impossible ; but if you at- 

 tempt to trace the history of the thing backward, you must always 

 come to a state which could not have been produced by mixing, 

 namely, a state of complete separation. 



Now, upon this remark of Sir W. Thompson's, which you will find 

 further expi-essed in Mr. Balfour Stewart's book on the " Conservation 

 of Energy," a most singular doctrine has been founded. These writers 

 have been speaking of a particular problem, on which they were 

 employed at the moment. Sir W. Thompson was sj^eaking of the 

 deduction of heat, and he said this heat-problem leads you back to a 

 state which could not have been pi-oduced by the conduction of heat. 

 And so Prof. Clerk Maxwell, speaking of the same problem, and also 

 of the diff'usion of gases, said there was evidence of a limit in past 

 time to the existing order of things, when something else than mixing 

 took place. But a most eminent man, who has done a great deal of 

 service to mankind, Prof Stanley Jevons, in his very admirable book, 

 "The Principles of Science," which is simply marvellous for the num- 

 ber of examples illustrating logical principles which he has drawn 

 from all kinds of regions of science, and for the small number of 

 mistakes that occur in it, takes this remark of Sir W. Thompson's, 

 and takes out two very important words, and puts in two other very 



