THE KINETIC THEORY OF GASES. 279 



a blow as a heavy molecule with a slower rate of motion. By Clausius's 

 hypothesis, the temperatures of two gases are believed to be equal 

 when the products of their masses into the square of their rates of 

 motion are equal. This is not quite the same thing as saying "when 

 the force of the blows they give is equal," but it may be taken as con- 

 nected with it. 



Supposing, then, that two gases are at the same temperature — that 

 when placed in contact neither gives up heat to the other — then the 

 product above mentioned must be equal for both. For it is obvious 

 that the specifically lighter gas must have the higher velocity: that is, 

 the molecules must be endowed with a higher rate of motion. 



What is that rate of motion ? Clausius was able to answer that ques- 

 tion : A molecule of hydrogen, the lightest gas known, if it moved in a 

 straight line, unimpeded in its motion by collision with any other mole- 

 cules or with any solid body, would pass through no less than a mile 

 and a quarter in a second. And a molecule of oxygen equally free to 

 move would travel through space with a velocity of rather less than 

 one- third of a mile per second. The relative rates of motion are there- 

 fore in inverse proportion to the square roots of the densities of the 

 gases. Thus, as oxygen is sixteen times as heavy as hydrogen, a 

 molecule of hydrogen would move through space in a straight line, 

 were it free to do so, at a rate four times as great as that at which a 

 molecule of oxygen moves. 



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 velocity. The question arises : Is there any possibility of so 

 lowering temperature that the motion of such moving molecules will 

 cease? Judging by 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 temperature;" 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 country. This temperature has 

 not been reached; it is unlikely that it will ever be reached; but an 

 approach has recently been made to it by liquefying hydrogen gas and 

 allowing it to boil at the atmospheric pressure. The temperature 

 reached in this manner is about —243° C. ; and Professor Dewar, who 

 has recently succeeded in liquefying hydrogen in quantity, will no doubt 

 be able to produce a still lower temperature by causing the liquid 

 hydrogen to boil iu a vessel connected with an air pump, so that the 

 pressure is reduced. For, just as raising the pressure raises the boil- 

 ing 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. Johnstone Stoney applied the kinetic 

 theory of gases, in a series of papers read before the Royal Dublin 

 Society, to the question of the existence of atmospheres on planets and 

 satellites. If a molecule happens to be moving on the surface of a 



