AND THE NATURE OF FORCE. 497 



again. It may be remarked that the atoms are assumed to be rigid. 

 Herschel's reasoning is based upon the assumption that relative motion 

 is necessarily destroyed in the collision of rigid bodies. 



A rigid body is such that the distances between its parts are 

 invariable. It is possible that rigidity is a necessary condition of 

 continuous matter. However that may be, we shall assume our 

 continuous atoms to be rigid, the truth of the assumption to rest on 

 the coincidence of results with observation. 



We have now to discuss the question, Whether is it true, or probable, 

 that the collision of rigid bodies involves the diminution of relative 

 motion ? 



It may be premised that the laws of collision have been deduced 

 from experiment ; the question now being whether the experimental 

 results have been attributed to their proper causes ? 



The following are the results of experiment, as far as the change 

 in relative motion by collision is concerned. When bodies collide, 

 they change their form and acquire a common motion, in which 

 case they are called inelastic ; or else, after undergoing a certain 

 amount of compression, they partially or wholly regain their form, to 

 all appearance ; in which case, they separate with a relative velocitv 

 which is always less than their relative velocity before impact ; in 

 this case, they are termed imperfectly elastic. 



Besides the above changes, motions always continue in the interior 

 of the bodies after they separate, or after they have acquired a com- 

 mon motion as wholes. These motions sometimes take the form of 

 heat, sometimes of visible or audible vibrations, or both combined. 



As before noticed, where the bodies are inelastic the change of form 

 is always greatest, other things being equal. 



Also, the time occupied by the impact is greater, as the change of 

 form is greater for the same bodies. Where the change produced in 

 the relative velocity by the impact is less, it is always found that 

 there is less change in the condition of the bodies themselves, less 

 heat is developed, the vibrations are of smaller extent, the change of 

 form is less, and the time of contact is less. 



From all these phenomena, it is fair to draw, as a very probable 

 conclusion, that the principle of conservation of energy holds with 

 regard to these bodies — that where the energy of the bodies con- 

 sidered aa wholes is diminished, we always find a cei-tain amount of 

 energy in the parts; and we may be satisfied that if we could measure 

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