EDITOR'S TABLE. 



411 



make any effort to enter into the state of 

 mind of the parties addressed, and their 

 expositions, therefore, often fail from 

 lack of adaptation. Sometimes a sub- 

 ject familiar to teachers of great capa- 

 city is still too abstruse to be grasped 

 by common minds. Sometimes the ex- 

 pounder does not understand the sub- 

 ject himself ; and not unfrequently hy- 

 potheses are invented to explain unex- 

 plainable things, and which serve only 

 to increase existing difficulties. A 

 marked illustration of this is afforded 

 by a lecture delivered not long ago be- 

 fore the Koyal Institution, by the emi- 

 nent physicist and mathematician, Sir 

 William Thomson, who announced as 

 his topic of discourse the curious sub- 

 ject, " Maxwell's Sorting Demons." 



The lecture was mainly devoted to 

 an explication of the phenomena of the 

 diffusion of liquids and the principles 

 it involves. Professor Thomson bad 

 many tubes prepared, each containing 

 two liquids of different colors, to rep- 

 resent the progress of diffusion, while 

 some ingenious experiments were made 

 by throwing the spectra of various 

 solutions upon the screen with an elec- 

 tric light. The diffusibility of solids 

 and gases was also referred to, and a 

 just tribute paid to the memory of Gra- 

 ham, whose name stands most promi- 

 nently associated with this branch of 

 research. 



Sir William Thomson's reasons, how- 

 ever, for bringing forward these phe- 

 nomena of diffusion were that they 

 stand very closely related to the pres- 

 ent theories and speculations concern- 

 ing the molecules of matter, and which 

 aim to account for their motions. In 

 diffusion, the molecules gradually in- 

 termingle, according to definite laws, 

 which are variable in different cases. 

 The molecules do not move capricious- 

 ly or irregularly, as all chemical action 

 and all crystallization prove. But why 

 do they move this way or that, and 

 why always go the same way in the 

 same conditions? This "why" is the 



perplexing word of science, and when 

 we get down among objects the very 

 existence of which is hypothetical it 

 carries us far beyond our depth. But 

 Professor Maxwell thinks he gives us 

 aid here by inventing a host of little 

 demons living creatures with wills and 

 infallible intelligence which sort the 

 molecules and regulate their extraor- 

 dinary motions. In a very brief ab- 

 stract of his lecture which Sir William 

 Thomson has published, he thus ex- 

 plains the attributes and offices of these 

 remarkable agents : 



Clerk Maxwell's " demon " is a creature 

 of imagination having certain perfectly well- 

 defined powers of action, purely mechanical 

 in their character, inveuted to help us to un- 

 derstand the " dissipation of energy " in na- 

 ture. He is a being with no preternatural 

 qualities, and differs from real living animals 

 only in extreme smallness and agility. He 

 can at pleasure stop, or strike, or push, or 

 pull any single atom of matter, and so mod- 

 erate its natural course of motion. En- 

 dowed ideally with arms and hands and fin- 

 gers two hands and ten fingers suffice he 

 can do as much for atoms as a piano-forte 

 player can do for the keys of the piano just 

 a little more, he can push or pull each atom 

 in any direction. 



He can not create or annul energy ; but, 

 just as a living animal does, he can store up 

 limited quantities of energy, and reproduce 

 them at will. By operating selectively on 

 individual atoms he can reverse the natural 

 dissipation of energy, can cause one half of 

 a closed jar of air, or of a har of iron, to be- 

 come glowingly hot and the other ice cold; 

 can direct the energy of the moving mole- 

 cules of a basin of water to throw the water 

 up to a height and leave it there proportion- 

 ately cooled (1 Fahr. for seven hundred and 

 seventy -two feet of ascent) : can " sort" the 

 molecules in a solution of salt or in a mixture 

 of two gases, so as to reverse the natural pro- 

 cess of diffusion, and produce concentration 

 of the solution in one portion of the water, 

 leaving pure water in the remainder of the 

 space occupied ; or. in the other case, sepa- 

 rate the gases into different parts of the con- 

 taining vessel. 



The classification, according to which the 

 ideal demon is to sort them, may he accord- 

 ing to the essential character of the atom : 

 for instance, all atoms of hydrogen to be 

 let go to the left, or stopped from crossing to 



