354 WRITINGS OF JOSEPH HENRY. [1855 



ity on the other, we may find a series of substances gradually 

 shading from one extremity to the other. 



According to the views I have presented the difference in 

 the tenacity in steel and lead does not consist in the attract- 

 ive cohesion of the atoms, but in their capability of slipping 

 upon each other. From this view it follows that the form of 

 the material ought to have some effect upon its tenacity, and 

 also that the strength of the article should depend in some 

 degree upon the process to which it has been subjected. 



For example, I have found that softer substances in which 

 the outer atoms have freedom of motion, while the inner ones 

 by the pressure of those exterior are more confined, break 

 unequally; the inner fibres (if I may so call the rows of atoms) 

 give way first and entirely separate, while the exterior fibres 

 show but little indications of a change of this kind. 



If a cylindrical rod of lead three quarters of an inch in 

 diameter be turned down on a lathe in one part to about half 

 an inch, and then be gradually broken by a force exerted in 

 the direction of its length, it will exhibit a cylindrical hollow 

 along its axis of half an inch in length, and at least a tenth 

 of an inch in diameter. With substances of greater rigidity 

 this effect is less apparent, but it exists even in iron, and the 

 interior fibres of a rod of this metal may be entirely sepa- 

 rated, while the outer surface presents no appearance of 

 change. 



From this it would appear, that metals should never be 

 elongated by mere stretching, but in all cases by the process 

 of wire-drawing, or rolling. A wire or bar must always be 

 weakened by a force which permanently increases its length 

 — without at the same time compressing it. 



Another effect of the lateral motion of the atoms of a soft 

 heavy body, when acted upon by a percussive force with a 

 hammer of small dimensions in comparison with the mass 

 of metal, (for example, if a large shaft of iron be hammered 

 with an ordinary sledge,) is a tendency to expand the surface 

 so as to make it separate from the middle portions. The 

 interior of the mass by its own inertia becomes as it were an 

 anvil, between which and the hammer, the exterior portions 



