94 WRITINGS OP JOSEPH HENRY. [1855- 



nomena, while in solid bodies both the attractive and repul- 

 sive are evident. Thus, if we place a heavy weight on the top 

 of a vertical iron bar its length will be infinitesimally dimin- 

 ished. If the weight be removed, the atoms, by repulsion^ 

 will spring back to their original distances, and this ma}^ be 

 repeated any number of times with the same result, provided 

 the weight is not so great as to cause any permanent change 

 which consists in a new arrangement of the atoms. If we 

 now suspend the bar from one end, and apply a weight to the 

 other, the bar will be minutely elongated; and if the weight 

 be removed, the atoms, by their attraction, will return to their 

 normal position. In this state the atoms are at the distance 

 which constitutes a neutral condition. If pushed together, 

 they fly apart whenever the compressing force is removed; 

 and if drawn in the direction of the length of the body, they 

 are brought into the region of attraction, and tend to bring 

 the bar back to its original length when the elongating force 

 is remitted. 



This constitution of matter may be represented by a series 

 of balls separated from each other by helical springs. If we 

 attempt to elongate this bar the springs will be drawn out. 

 When we attempt to compress the mass the several spires of 

 the springs will be compressed closer together, and an action 

 similar to repulsion will be produced. 



This repulsion of the atoms is further demonstrated by the 

 elasticity of a body, or the force with which it tends to restore 

 itself to its former condition when disturbed by any extra- 

 neous force. The elasticity for instance of a rod of tempered 

 steel is exhibited when we bend it. It tends to return to its 

 first form in obedience to two forces. The atoms on the 

 convex side, after the rod has been bent, are slightly sepa- 

 rated, and are therefore in the region of attraction, while those 

 on the concave side are brought nearer, and thus tend to 

 repel each other. If this be the case, there should be a line 

 somewhere near the middle of the bent rod, in which the 

 atoms are neither compressed nor distended ; and that such 

 a neutral line does really exist can be shown by polarized 

 light, which enables us, when the experiment is made on a 



