RECENT COSMOGONIES 225 



and that they have the same direction of rotation in common with 

 their moons and with the central body, the sun. It is only the 

 outermost planets, like Uranus and Neptune, in whose cases the 

 tidal effects were not of much consequence, that form exceptions 

 to this rule. 



Our author's second aim is to point out what he re- 

 gards as Nature 's method of self -restoration by revers- 

 ing,^ as it were, the thermal hour glass and starting the 

 heat current on the downward course once more. Quot- 

 ing Clausius ' old maxim, * ' The energy of the universe is 

 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, 



