110 



SCIENCE. 



[N. S. Vol. VIII. No. 205. 



' These rates of motion are calculated for 

 the temperature of melting ice. But as the 

 effect of rise of temperature is to quicken 

 the rate of motion of molecules of gases, so 

 fall of temperature will cause a decreased 

 velocit3^ Tlie question arises : Is there 

 any possibility of so lowering temperature 

 that the motion of such moving molecules 

 will cease ? Judging bj' the rate at which 

 the pressure of a gas decreases with fall of 

 temperature, there is : That temperature 

 has been called the 'absolute zero of tem- 

 perature ;' it lies 273° below the melting 

 point of ice on the Centigrade scale, or at 

 —460° on the Fahrenheit scale, the one 

 commonly in use in this countrj\ This 

 temperature has not been reached ; it is un- 

 likely that it will ever be reached ; but an 

 approach has recently been made to it by 

 liquefying hj'drogen gas and allowing it to 

 boil at the atmosphei-ic pressure. The tem- 

 perature reached in this manner is about 

 — 243° Cent.; and Professor Dewar, who 

 has recently succeeded iu liquefying hydro- 

 gen in quantity, will no doubt be able to 

 produce a still lower temperature, by caus- 

 ing the liquid hydrogen to boil in a vessel 

 connected with an air pump, so that the 

 pressure is reduced. For just as raising 

 the pressure raises the boiling point of a 

 liquid, as exemplified in the boiler of a 

 steam engine, so lowering the pressure 

 lowers the boiling point. 



It is now many years since Dr. John- 

 stone Stoney applied the Kinetic Theory 

 of Gases, in a series of papers read before 

 the Eoyal Dublin Societies, to the question 

 of the existence of atmospheres on planets 

 and satellites. If a molecule happens to 

 be moving on the surface of a planet at a 

 rate which would carry it away from the 

 planet more rapidly than the planet can 

 draw it back, that molecule will escape into 

 space. It is not theoretically impossible, 

 although practically unrealizable, to con- 

 struct a gun which would fire a bullet 



vertically into the air at such a rate that 

 the bullet might never return to the earth. 

 What, then, would occur to it? Well, it 

 would wander on through space as a little 

 planet, performing an ellipse round the sun, 

 as indeed many aerolites, or ' shooting stars,' 

 are known to do. It might, indeed, chance 

 to come within the range of attraction of 

 some planet — e.g., Jupiter — massive enough 

 to hold it ; or it might actually fall on the 

 surface of a planet ; in the former case it 

 would act like a little satellite and revolve 

 round that planet, as the numerous stones 

 of which Saturn's rings are composed re- 

 volve round Saturn ; in the latter case it 

 would simply become part of that planet, as 

 the falling stars which reach the earth 

 form, after their fall, a portion of the earth. 



The molecule of gas, which we have been 

 considering, differs in no particular from a 

 bullet, in its wanderings or in its fate. If 

 it chance to come within the sphere of at- 

 traction of a planet of sufficient mass to re- 

 tain it, it will, according to Dr. Stoney, 

 form part of that planet's atmosphere. If 

 not, it will wander on until it may, by 

 chance, come near enough to the sun to fall 

 a victim to its enormous attractive force, 

 and it will then become part of the sun's 

 atmosphere. 



Dr. Stoney has summed up the results of 

 various inquiries of this kind in a memoir, 

 entitled ' Of Atmospheres upon Planets and 

 Satellites.'* 



One important point has been omitted in 

 the sketch given of the Kinetic Theory. It 

 is this : When it was said that a molecule 

 of oxygen moves at the rate of about one- 

 third of a mile per second it was not im- 

 plied that all molecules are moving at that 

 rate. Some, urged on by collisions from 

 behind, acquire a much more rapid rate ; 

 others, hindered in their motion by colli- 

 sions with other molecules moving more 



* 'Royal Dublin Society,' Vol. VI., Nov., 1897, pp. 

 305-328. 



