372 Mr. S. T. Preston on some Dynamical Conditions 



dity between the gravific particles and the molecules of matter. 

 There may possibly be some who may be inclined to think that 

 this explanation was somewhat forced, or was warranted only as 

 explaining that special case, without being independently likely. 

 1 think that, on considering the subject, it will be found that the 

 explanation is in itself highly probable on independent grounds. 

 Where do we find substances in nature whose elastic rigidity 

 is the same. It would be in the highest degree unlikely that 

 portions of matter differing so vastly from each other in di- 

 mensions as a molecule and a gravific particle should have the 

 sa7ne elastic rigidity. If the elastic rigidity be not the same, 

 it is a strict dynamical fact, not a supposition, that the energy 

 of the particle after its rebound from a molecule, though the 

 same in amount, will not be the same in kind as before ; but 

 if the elastic rigidity of the large molecule be greater than that 

 of the minute particle, a part of the translatory motion of the 

 particle will be shivered into vibratory motion at the encounter ; 

 and therefore the particle will rebound with a less translatory 

 motion, the deficiency of translatory motion representing the 

 amount converted into vibratory motion at the encounter. It is 

 just as if a tuning-fork, a flexible ring, or any pliable elastic 

 object whatever, were thrown against a hard body (say the 

 hard surface of an anvil), when the body will rebound at a 

 less translatory motion than it struck, the def ciency of trans- 

 latory motion being compensated for by an accession of vibra- 

 tory motion. So with a gravific particle striking a molecule 

 of matter ; for mere size makes no difference in the principle. 

 It is therefore not an unnatural thing (but highly probable on 

 independent grounds) thai: the gravific particles should have 

 their velocities changed at impact against the molecules of 

 matter. The energy of the particle remains unaltered by the 

 impact ; only the distribution o^ the energy in the particle is 

 changed. 



15. The next question is. Do the particles which have thus 

 lost translatory motion and acquired an accession of vibratory 

 motion, recover their normal proportion of translatory motion 

 to vibratory motion again ? It has been pointed out by Sir 

 William Thomson that this must be the case. For it has been 

 demonstrated by Professor Clausius, in connexion with the 

 kinetic theory of gases, that, in the case of a system of particles 

 in free collision among themselves, the relation of the transla- 

 tory motion to the vibratory motion tends to assume a constant 

 value, so that when this relation is disturbed in any way it is 

 again restored. So, therefore, when the relation of the trans- 

 latory motion to the vibratory motion of the gravific particles 

 is disturbed by collison with the molecules of matter, this re- 



