134 Mr. G. J. Stoney on the Internal Motions of Gases 



be allowed to expand without a change of temperature, d and / 

 become larger, while the velocities are but little changed from 

 what they were before*. If the gas be changed for another, for 

 instance if hydrogen be substituted for oxygen at the same tem- 

 perature and pressure, the number of molecules remains the 

 same, d also remains the same, / is probably a little altered. 

 But the mass of each molecule is now but one-sixteenth of the 

 mass of each molecule of oxygen ; and to compensate for this, 

 the velocities with which the molecules move are 16^ or upwards , /& 

 of 250 times as swift. 



3. The molecular motions consist of two very distinct parts — 

 the motions of the molecules among one another, and the mo- 

 tions in the interior of each molecule. In an eminently gaseous 

 body the former of these classes of motion engrosses a large part 

 of the vis viva f; but the motions are irregular, and on this 

 account, as well as probably because they are too coarse, they 

 are unfitted to absorb or develope vibrations in the luminous 

 aether, except possibly in a very small degree, just as the wa- 

 ving of the hand may produce a slight sound. So far, there- 

 fore, as the rectilinear motions of the molecules are concerned 

 the gas will be eminently transparent. But each molecule is also 

 a little separate system, the constituent parts of which are in 

 energetic motion among themselves. These minuter motions 

 are interfered with by the neighbouring molecules only during 

 the instants when the path of a molecule is being deflected upon 

 its close approach to another molecule ; and as the intervening 

 times when the path is rectilinear are long in comparison with 

 these brief instants, the internal motions of each little system 

 will be for the most part undisturbed and therefore regular. 

 And accordingly we must presume that it is they which influence 

 the eether, producing those bright lines which constitute nearly 

 the whole spectrum of an incandescent gas, and are in a corre- 

 sponding degree influenced by the aether, absorbing these same 

 rays. How wonderfully regular the internal motions of the 

 molecules are, and at the same time how complex, appear to be 



* That the velocities will be somewhat altered seems evident when we 

 bear in mind the great change which, when a gas expands, has been effected 

 in the ratio of the periods of perturbation to the whole time (see next sec- 

 tion). But it seems equally plain that the alteration of the velocities will 

 be but small except at high temperatures, and in such gases as have lines 

 which are much dilated by the perturbations at high temperatures. This 

 is in conformity with the comparative fixity of the two specific heats of a 

 gas, within the limits to which experiments have been pushed. 



t Nearly two-thirds in those simple gases in which the two specific heats 

 bear to one another their usual ratio. Considerably less than two-thirds 

 in most compound gases, in all which suffer condensation in the act of 

 combining. See a memoir by Clausius in the Philosophical Magazine for 

 1857, vol. xiv. p. 125. 



