INTERNAL FRICTION IN ROCKS 601 



rutile crystals are brownish in color and occasionally display the 

 geniculated twinning that is characteristic of this species. 



The foliation of the slate explains the lack of symmetry in the 

 expansion of columns of this rock during deformation. 



The Copper used in these experiments was taken from a rod 

 i inch in diameter, representing a good commercial grade of this 

 metal. Prior to being turned into columns for the experiments, the 

 pieces cut from the rod were annealed by being heated to bright 

 redness in the coal fire of a forge, being then allowed to cool down 

 gradually. 



The Lead employed in the experiments was "assay lead" which, 

 in order to free it from all air bubbles, was melted down and cast 

 in a heated iron mold, which was then allowed to cool slowly. 



METHODS EMPLOYED 



Several long round bars of nickel steel 2\ inches in diameter, all 

 of identical composition and from the same heat, and all having 

 been submitted to identical treatment in their manufacture, were 

 secured. For these the authors are indebted to the Bethlehem 

 Steel Company, which placed them at their disposal for the purpose 

 of the present investigation. 



This steel, which is very uniform in character, possesses a high 

 tensile strength, as well as a high elastic limit, and has the following 

 chemical composition: 



Carbon 30 per cent 



Manganese 74 per cent 



Silicon 162 per cent 



Phosphorus 035 per cent 



Sulphur 038 per cent 



Nickel 4 . 740 per cent 



The bars were sawed into lengths of about 3^ inches. These 

 were then bored and turned into tubes, the longitudinal sections 

 of which, with the final dimensions, are shown in the upper half 

 of Fig. 1. Two sets of these tubes were prepared, differing only in 

 the thickness of the wall of the central portion of the tube. In the 

 first set this has a thickness of 0.33 centimeter, while in the second 



