TESTING BUILDING MATERIALS. 309 



presenting some of the facts I then intended to commuuicatej and 

 which I have since verified b}^ further experiments and observations. 



In a series of experiments made some ten years ago, I showed that 

 the attraction of the particles for each other of a substance in a liquid 

 form was as great as that of the same substance in a solid form. Con- 

 sequently_, the distinction between liquidity and solidity does not 

 consist in a difference in the attractive power occasioned directly by 

 the repulsion of heat ; but it depends upon the perfect mobility of the 

 atoms,, or a lateral cohesion. We may explain this by assuming an 

 incipient crystallization of atoms into molecules, and consider the first 

 efiect of heat as that of breaking down these crystals and permitting 

 each atom to move freely around every other. When this crystalline 

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

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

 mately an example of this in cast-steely in which no slipping takes 

 place of the parts on each other, or no material elongation of the 

 mass ; and when a rupture is produced by a tensile force, a rod of this 

 material is broken with a transverse fraction of the same size as that 

 of the original section of the bar. In this case every atom is sepa- 

 rated at once from the other, and the breaking weight may be consid- 

 ered as a measure of the attraction 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 thickness, 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 lower 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 into the other. 



According to the views I have presented, the difference in the 

 tenacity in steel and lead does not consist in the attractive 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 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 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, 

 turned down on a lathe in one part to about a half of 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 

 ijich 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 



