130 BULLETIN OF THE 



The Elasticity of Matter. — To select another illustration, the child 

 throwing his rounded marble downward on a stone pavement finds 

 te his surprise that it rebounds like his play-ball, and that he may, 

 without stooping, catch it in his hand. What explanation is to be 

 given of this direct and sudden reversal of movement ? To this 

 familiar quality of matter, we give the name of " elasticity." But 

 by what more simple formula of mechanics shall we represent 

 to ourselves this property elasticity t Kinematists abjuring alike 

 objective " qualities " and subjective " abstractions " have been 

 severely taxed in their attempts either to ignore the attribute or to 

 reduce the phenomenon to some phase of molecular vibration. 



Some few — consistent in their rejection of all quality from mate- 

 rial substance — have boldly denied the existence of elasticity ; or 

 rather have ventured to affirm that perfectly hard or inelastic atoms 

 or masses would on collision alike rebound, precisely as though 

 they were elastic* This startling conclusion — apparently necessi- 

 tated by their fundamental assumption " the conservation of 

 motion" — requires for the intelligent student of rational mechanics, 

 no discussion. 



Other kinematists have resolutely endeavored to explain the 

 resilience of colliding bodies as the special resultant of composite 

 motions. One of the most earnest of these has been the Italian 

 astronomer and physicist Angelo Secchi, who in an elaborate essay 

 on the ultimate identity of all the physical forces as simple modes 

 of motion, remarks: "It is evident that this 'elastic force' can be 

 admitted only as a secondary force derived from another antecedent 

 in an aggregate of atoms, that is in a compound molecule; and that 

 it cannot be admitted as pertaining to the elementary atoms. In- 

 deed, elasticity in its ordinary acceptation requires a void space 

 within the molecule to allow the form to be changed by compression 

 and afterward restored ; while on the contrary it is the necessary 

 condition of real atoms — by conception — to be impenetrable [in- 



* This thesis was maintained by John Herapath, in his work on Mathe- 

 matical Physics. 8vo. 2 vols. London, 1847 : (vol. i, pp. 106-137.) As 

 stated by Newton however, " Bodies which are either absolutely hard, or 

 so soft as to be void of elasticity will not rebound frorn^ one another. 

 Impenetrability makes them only stop. If two equal bodies meet directly 

 in vacuo, they will by the laws of motion stop where they meet, and lose 

 all their motion and remain in rest, unless they be elastic and receive new 

 motion from their spring." (Optics. 2d edition, 1717: book in, Qu. 31.) 



