J. Henry on testing Building Materials. 



37 



of breaking down these crystals, and permitting each atom to 

 move freely around every other. When this crystalline arrange- 

 ment is perfect, and no lateral motion is allowed in the atoms, 

 the body may be denominated perfectly rigid. We have ap- 

 proximately an example of this in cast-steel, in which no slip- 

 ping takes place of the parts on each other, or no material elon- 

 gation of the mass ; and when a rupture is produced by a tensile 

 force, a rod of this material is broken with a transverse fracture 

 of the same size as that of the original section of the bar. In 

 this case every atom is separated at once from the other, and the 

 breaking weight may be considered as a measure of the attrac- 

 tion of cohesion of the atoms of the metal. 



The effect, however, is quite different when we attempt to pull 

 apart a rod of lead. The atoms or molecules slip upon each 

 other. The rod is increased in length, and diminished in thick- 

 ness, until a separation is produced. Instead of lead, we may 

 use still softer materials, such as wax, putty, &c, until at length 

 we arrive at a substance in a liquid form. This will stand at 

 the extremity of the scale, and between extreme rigidity on the 

 one hand, and extreme liquidity 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 attractive cohe- 

 sion of the atoms, but in the 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 had 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 un- 

 equally ; 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 diam- 

 eter 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 di- 

 ameter. 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 separated, while the outer sur- 

 face presents no appearance of change. 



From this it would appear that metals should never be elon- 

 gated by mere stretching, but in all cases by the process of wire- 

 drawing, or rolling. A wire or bar must always be weakened 



