156 THE POPULAR SCIENCE MONTHLY. 



be a vague reminiscence) Maxwell says ("Nature," vol. xvi, pp. 

 245, 246) : 



But, before we accept this somewhat promising hypothesis, let us try to con- 

 struct a rigid-elastic body. It will not do to increase the co-efficients of elasticity 

 without limit till the body becomes practically rigid. For such a body, though 

 apparently rigid, is in reality capable of internal vibrations, and these of an 

 infinite variety of types, so that the body has an infinite number of degrees of 

 freedom. 



The same objection applies to all atoms constructed of continuous, non-rigid 

 matter, such as the vortex-atoms of Thomson. Such atoms would soon convert 

 all their energy of agitation into internal energy, and the specific heat of a sub- 

 stance composed of them would be infinite. 



A truly rigid-elastic body is one whose encounters with similar bodies take 

 place as if both were elastic, but which is not capable of being set into a state of 

 internal vibration. "We must take a perfectly rigid body and endow it with the 

 power of repelling all other bodies, but only when they come within a very 

 short distance from its surface, but then so strongly that under no circumstances 

 whatever can any body come into actual contact with it. 



This appears to be the only constitution we can imagine for a rigid-elastic 

 body. And, now that we have got it, the best thing we can do is to get rid of 

 the rigid nucleus altogether, and substitute for it an atom of Boscovich a math- 

 ematical point endowed with mass and with powers of acting at a distance on 

 other atoms. 



But Boltzmann's molecules are not absolutely rigid. He admits that they 

 vibrate after collisions, and that their vibrations are of several different types, 

 as the spectroscope tells us. But still he tries to make us believe that these 

 vibrations are of small importance as regards the principal part of the motion of 

 the molecules. He compares them to billiard-balls, which, when they strike 

 each other, vibrate for a short time, but soon give up the energy of their 

 vibration to the air, which carries far and wide the sound of the click of 

 the balls. 



In like manner, the light emitted by the molecules shows that their internal 

 vibrations after each collision are quickly given up to the luminiferous ether; 

 If we were to suppose that at ordinary temperatures the collisions are not severe 

 enough to produce any internal vibrations, and that these occur only at temper- 

 atures like that of the electric spark, at which we can not make measurements 

 of specific heat, we might, perhaps, reconcile the spectroscopic results with what 

 we know about specific heat. 



But the fixed position of the bright lines of a gas shows that the vibrations are 

 isochronous, and therefore that the forces which they call into play vary directly 

 as the relative displacements, and, if this be the character of the forces, all im- 

 pacts, however slight, will produce vibrations. Besides this, even at ordinary 

 temperatures, in certain gases, such as iodine gas and nitrous acid, absorption 

 bands exist, which indicate that the molecules are set into internal vibration by 

 the incident light. The molecules, therefore, are capable, as Boltzmann points 

 out, of exchanging energy with the ether. But we can not force the ether into 

 the service of our theory so as to take from the molecules their energy of inter- 

 nal vibration, and give it back to them as energy of translation. It can not in 

 any way interfere with the ratio between these two kinds of energy which Boltz- 

 mann himself has established. All it can do is to take up its own due propor- 

 tion of energy according to the number of its degrees of freedom. We leave it 



