BRIDGMAN. — MERCURY UNDER PRESSURE. 363 



be made that the material obeys Hooke's law within the stress range 

 employed, and that it is perfectly homogeneous and isotropic. With 

 these assumptions the identical relations are rigorously exact The 

 first of these assumptions seems open to but little question. It is the 

 second which is likely to give the most trouble, particularly when the 

 substance experimented on is in the form of hollow tubes, as when 

 the elastic constants have been determined for glass. This method 

 was employed by Amagat 12 and by de Metz,!^ among others. 



Another method is to measure the linear compressibilty of the sub- 

 stance in question when subjected to hydrostatic pressure all over and 

 to calculate the cubic compressibility simply by multiplying the linear 

 compressibility by three. This seems preferable to the method of the 

 last paragraph, because the assumptions are reduced to a minimum. 

 The stress, a hydrostatic pressure, is in this method the same during 

 the indirect determination of the cubic compressibility as it is during 

 the actual use of the material in the piezometer. There seems much 

 less chance for complications to be introduced by nonhomogeneity of 

 the material than there is when the compressibility is calculated from 

 two elastic constants, such as Young's modulus and the torsion coeffi- 

 cient. The only assumption made here is that the material is equally 

 compressible in every direction, an assumption which is open to verifi- 

 cation by direct experiment. This verification has been made by pre- 

 vious experiments in the paper cited. This method has also been 

 used by several previous experimenters, for example Buchanan,^* 

 Amagat, 15 and Richards. 1^ 



In the paper already cited, the linear compressibility of steel was 

 determined at room temperatures up to 6500 kgm./ cm.^. These data 

 were inadequate for the present purpose, however, because they did not 

 run to high enough pressures, and the effect of temperature on com- 

 pressibility was not determined. But the method there used is entirely 

 satisfactory, and new results have since been obtained by it for the 

 purposes of this paper. Briefly, the method consists in enclosing the 

 metal to be experimented on, which is in the form of a rod, in a long 

 steel cylinder, within which it is exposed to the action of hydrostatic 

 pressure all over. The change produced by the pressure in the length 

 of the rod with respect to that of the cylinder is measured by a ring 



" Amagat, C. K., 107, 618-G20 (1P88). Ann. de Chim. et Phys. (G), 22, 

 95-141 (1891). 



" De Mctz, loc. cit. 



" Buchanan, Proc. Roy. Soc. Lon., 73, 296 (1901). 



" Amagat, C. R., 108, 727-730, 1889. 



*^ Richards, loc. cit. 



