DISCOURSE OF W. B. TAYLOR. 327 



posed lead, the stone first gives way along the outer lines or those 

 of least resistance, and the remaining pressure must be sustained by 

 the central portions around the vertical axis of the cube. After 

 this important fact was clearly determined, lead and all other inter- 

 posed substances were discarded, and a method devised by which the 

 upper and lower surfaces of the cube could be ground into perfect 

 parallelism. - - - All the specimens tested were subjected to 

 this process, and on their exposure to pressure were found to give 

 concordant results. The crushing force sustained was therefore 

 much greater than that heretofore given for the same material." * 



In the same communication, interesting remarks are made on the 

 tensile strength of materials, particularly the metals. "According 

 to the views 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:" that is on the differ- 

 ence of lateral adhesion of the molecules, as exemplified in ice and 

 water. A bar of soft metal as lead subjected to tensile strain, 

 by reason of the greater freedom of the exterior layers of mole- 

 cules, exhibits a stretching and thinning; while the interior mole- 

 cules being more confined by the surrounding pressure, are less 

 mobile, permit less elongation of the mass, and are therefore the 

 first to commence breaking apart. Accordingly on ultimate sepa- 

 ration, each fragment exhibits a hollow or cup-like surface of 

 fracture, where the interior portion of the material has first parted : 

 the depth of the concavity being somewhat proportioned to the 

 malleability or ductility of the substance. "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 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 a 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." f 



* Proceed. Am. Assoc. Providence, Aug. 1855, pp. 102-112. 



fThis conclusion is not at all in opposition to the ascertained fact of the 

 increased strength imparted to an iron rod by " thermo-tension," discovered by 

 Professor WALTER R. JOHNSON, in 1838. (Journal of Franklin Institute, Oct. 1839, vol. 

 xxiv. n. s. pp. 232-236.) 



