PHILOSOPHICAL SOCIETY OF WASHINGTON. 331 



crushing dynamometer. " This was in accordance with a plan 

 adopted by Rennie, and that which appears to have been used 

 by most if not all of the subsequent experimenters in researches 

 of this kind. Some doubt however was expressed as to the 

 action of interposed lead, which induced a series of experiments 

 to settle this question ; when the remarkable fact was discovered 

 that the yielding and approximately equable pressure of the 

 lead caused the stone to give way at about half the pressure it 

 would sustain without such an interposition. For example, one 

 of the cubes precisely similar to another which withstood a pres- 

 sure of upwards of 60,000 pounds when placed in immediate 

 contact with the steel plates, gave way at about 30,000 pounds 

 with lead interposed. This interesting fact was verified in a 

 series of experiments embracing samples of nearly all the mar- 

 bles under trial, and in no case did a single exception occur to 

 vary the result. 



" The explanation of this remarkable phenomenon (now that 

 the fact is known) is not difficult. The stone tends to give way 

 by bulging out in the centre of each of its four perpendicular 

 faces, and to form two pyramidal figures with their apices op- 

 posed to each other at the center of the cube, and their bases 

 against the steel plates. In the case where rigid equable pres- 

 sure is employed, as in that of the thick steel plate, all parts 

 must give way together. But in that of a yielding equable 

 pressure as in the case of interposed 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 interposed substances 

 were discarded, and a method devised by which the upper and 

 lower surfaces of the cube could be ground into perfect paral- 

 lelism. . . . 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. "Ac- 

 cording 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 difference 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 molecules, exhibits a stretching and 

 thinning ; while the interior molecules being more confined by 

 the surrounding pressure, are less mobile, permit less elongation 



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

 105 



