118 Adams and Cdker — Elastic Constants of Rocks. 



been exceeded, instead of split- 

 ting from top to bottom, broke 

 as if composed of a series of 

 rudely concentric shells. 



After a prolonged search for 

 isotropic glass in masses of suf- 

 ficient size to measure the elastic 

 constants, it was found that plate 

 glass answered the requirements. 

 A piece of one inch plate glass, 

 made in Great Britain, was 

 accordingly secured and was cut 

 into strips an inch wide, and 

 these again into three inch 

 lengths. The square prisms thus 

 produced were then properly 

 faced and polished. The glass 

 was found to be absolutely free 

 from all flaws and impurities, and 

 when examined between crossed 

 nicols, the prisms although an 

 inch thick, showed in one direc- 

 tion at right angles to vertical 

 axis absolute blackness through- 

 out a complete revolution, while 

 in the other direction at right 

 angles to this there was during 

 a revolution an alternation of 

 blackness with a pale grayish 

 illumination. This change was 

 so slight that considering the 

 thickness of the glass and the 

 sensitiveness of the test, the ma- 

 terial may be considered to be 

 practically free from internal 

 tension, and to be isotropic in 

 character. 



In order to get a good average 

 and to eliminate chance errors so 

 far as possible, seven of these 

 prisms were taken, and two com- 

 plete sets of determinations were 

 made on each of them, using in 

 every case different pairs of faces. 

 Fourteen determinations w ere 

 thus made of each of the elastic 





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