290 W. A. Norton — Experiments on Wood, Iron, and Steel bars. 



propagation inward of the greater disturbed condition of the 

 molecules of the upper and lower fibers. 



14. The general correspondence in tbe phenomena of set and 

 altered deflection, that obtain with different materials altogether 

 precludes the idea that they may result, either wholly or in a 

 considerable degree, from irregular strains subsisting in certain 

 parts of the bar before the stress is applied, and which are more 

 or less modified by the stress ; as some persons have conjec- 

 tured. The change that supervenes must be a general one, or 

 one in which all the molecules participate, though in diverse 

 degrees according to the amount of molecular displacement. 

 The especial character of the change, for each individual mole- 

 cule, must depend upon the kind of strain to which the mole- 

 cule is exposed, whether tensile, compressive, or shearing; and 

 not on the nature of the material subjected to strain. 



15. If, as experiment has established, when the distance be- 

 tween two contiguous molecules has been forcibly altered, the 

 molecules, when again left to their mutual actions, no longer 

 exert, at the same distance, effective actions of the same 

 intensity as before, it is apparent that the molecules in the act of 

 displacement have experienced some change, either m their dimen- 

 sions, or in their internal mechanical condition. This change 

 must result from the change that took place in the mutual 

 action of the molecules when they were urged nearer to each 

 other, or separated to greater distances. It must be experienced 

 by the ultimate molecule, whether this be indentical with the 

 integrant molecule or not — that is whether we regard the in- 

 tegrant molecule as a single ultimate molecule, or as a group of 

 ultimate molecules. For it is plain that a group of ultimate 

 molecules could not undergo an internal change, that abides 

 after all external actions have ceased, unless its constituent 

 molecules have suffered a change, by reason of which they no 

 longer act upon one another with the same intensities of force 



It is well known that with Physicists the " chemical atom " 

 has come to be replaced by the "ultimate molecule." Of the 

 probable physical constitution of the ultimate molecule differ- 

 ent conceptions have been formed. To those Physicists who 

 regard it as made up of a limited number of precisely similar 

 atoms, endued with unvarying forces — of attraction at certain 

 distances, and repulsion at other distances — I leave it to recon- 

 cile this conception with the legitimate inference to be drawn 

 from experimental results, that the ultimate molecule is liable 

 to a change of mechanical or physical condition, with every 

 slight displacement it may experience — a change which sub- 

 sists after the constraining cause of the displacement has ceased 

 to act; and may, under different conditions, either be per- 

 manent, or gradually subside with fluctuations. 



