262 FROM NEBULA TO NEBULA 



constant; the entropy of the universe tends to a maxi- 

 mum", he proceeds : 



The famous Scotch physicist. Clerk Maxwell, has conceived 

 of this case. Imagine a vessel which is divided by a partition into 

 two halves, both charged with a gas of perfectly uniform tem- 

 perature. Let the partition be provided with a number of small 

 holes which would not allow more than one gas molecule to pass 

 at a time. In each hole Maxwell places a small, intelligent being 

 (one of his "demons"), which directs all the molecules which 

 enter into the hole, and which have a greater velocity than the 

 mean velocity of all the molecules, to the one side, and which 

 sends to the other side all the molecules of a smaller velocity than 

 the average. All the undesirable molecules the demon bars by 

 means of a little flap. In this way all the molecules of a velocity 

 greater than the average may be collected in the one compart- 

 ment, and all the molecules of a lesser velocity in the other com- 

 partment. In other words, heat for heat consists of the move- 

 ments of molecules will pass from the one constantly cooling 

 side to the other, which is constantly raising its temperature, and 

 which must therefore become warmer than the former. 



In this instance heat would therefore pass from a colder to 

 a warmer body, and the entropy would diminish. 



Nature, of course, does not know any such intelligent beings. 

 Nevertheless, similar conditions may occur in celestial bodies in 

 the gaseous state. When the molecules of gas in the atmosphere 

 of a celestial body have a sufficient velocity which in the case 

 of the earth would be nkm. (7 miles) per second and when 

 they travel outward into the most extreme strata, they may pass 

 from the range of attraction out into infinite space, after the man- 

 ner of a comet, which, if endowed with sufficient velocity when 

 near the sun, must escape from the solar system. According to 

 Dr. Johnstone Stoney, it is in this way that the moon has lost its 

 original atmosphere. This loss of gas is certainly imperceptible 

 in the case of our sun and of large planets like the earth. But 

 it may play an important part in the household of the nebulae, 

 where all the radiation from the hot celestial bodies is stored up, 

 and where, owing to the enormous distances, the restraining force 

 of gravity is exceedingly feeble. Thus the nebulae will lose their 

 most rapid molecules from their outer portions, and they will 

 therefore be cooling in these outer strata. This loss of heat is 

 compensated by the radiation from the stars. If, now, there were 

 only nebulae of one kind in the whole universe, those escaped 

 molecules would finally land on some other nebula, heat equilib- 

 rium would thus be established between the different nebulae, and 

 the "heat-death" be realized. But we have already remarked 

 that the nebulae enclose many immigrated celestial bodies, which 



