Adams and CoTcer — Elastic Constants of Bocks. 123 



It is, however, in any case much more compressible than 

 steel, which has a value for D of from 26,098,000 to 27,547,000 

 (18 tol9xlO n , C.G.S.)\ 



The compression to which the rocks were subjected in this 

 investigation ranged from 6,000 to 17,310 pounds to the square 

 inch. Most of the rocks, however, were subjected to a load 

 of from 9,000 to 15,000 pounds per square inch, and their 

 bulk compression was determined for these loads as maxima. 

 Higher pressures could not be employed without incurring the 

 risk of breaking the specimen, and at the same time of de- 

 stroying the measuring apparatus. One apparatus was in fact 

 so destroyed. 



The question arises as to whether under still higher pressures, 

 if rupture could be avoided, the ratio of load to compression 

 would be maintained. Judging from the deportment of much 

 stronger substances, such as steel, when similarly tested, it is 

 inferred that this ratio of bulk compression will remain con- 

 stant for very much higher pressures, or until deformation 

 sets in and the rock begins to flow. 



With regard to the accuracy of the results obtained by this 

 method as compared with those obtainable by any method in 

 which cubic compression is actually produced and measured, 

 it may be observed that by far the best method of this kind 

 hitherto suggested seems to be that proposed by Richards and 

 Stall. f We have endeavored to make use of this method in 

 order to obtain results for purposes of comparison with those 

 given in the present paper, but have not hitherto succeeded in 

 overcoming certain experimental difficulties. The experi- 

 mental errors in this method, though apparently small, still 

 exist, and in applying it to rocks, which are much, less com- 

 pressible than the substances examined by Richards and Stull, 

 these errors become proportionately more serious. Moreover, 

 higher pressures than those used in the method employed in 

 the present paper could scarcely be employed in this direct 

 method, while difficulties dependent on the possible lack of 

 absolute continuity in the substance of the rock and the danger 

 of minute air-filled spaces, would probably present themselves 

 in the case of most rocks. It seems that all things being con- 

 sidered, the indirect method here employed is probably as 

 accurate as any direct method which can be used. The attempt 

 to apply Richards and Stull's method to the same rocks is still 

 being continued, however, and it is hoped that satisfactory 

 results may be eventually obtained by its use. 



McGill University, Montreal. 



* Illustrations of the C.G.S. System of Units with, tables of Physical 

 Constants. Macmillan & Co., 1902, p. 60. 



f New method of Determining Compressibility. Published by the Carnegie 

 Institution, Washington, D. C, Dec., 1903 (No. 7). 



