310 TESTING BUILDING MATERIALS. 



entirely separated, 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 compress- 

 ing 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, the ten- 

 dency would be 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 ex- 

 terior portions are stretched longitudinally and transversely. I here 

 exhibit to the Association a piece of iron originally from a square bar 

 four feet long, which has been so hammered as to produce a perfora- 

 tion of the whole length entirely through the axis. The bar can be 

 seen through as if it were the tube of a telescope. 



This fact appears to me to be of great importance in a practical 

 point of view, and may be connected with many of the lamentable 

 accidents which have occurred in the breaking of the axles of loco- 

 motive engines. These, in all cases, ought to be formed by rolling, 

 and not with the hammer. 



The whole subject of the molecular constitution ot matter offers a 

 rich field for investigation^ and isolated facts, which are familiar to 

 almost every one, when attentively studied, will yield results alike 

 interesting to abstract science and practical art. 



