50 JOURNAL OF the; WASHINGTON ACADEMY OF SCIE^NC^S VOI^. 11, NO. 3 



but in accordance with the well-known fact that, in general, a for hard 

 and highly incompressible substances is approximately 0.25, it has 

 been assumed to have this value. The agreement between (3 calculated 

 and 13 obser\^ed is satisfactory except for lead. Poisson's ratio, how- 

 ever, is a difficult property to measure accurately, and the large dis- 

 crepancy in the case of lead, as well as the smaller discrepancies for 

 the other metals may, not unreasonably, be ascribed to an error in 

 the values of a. For example, if o- for lead were 0.40 instead of 0.45 

 as given by Griineisen, equation (8) would yield a value of (3 almost 

 exactly equal to the observed value. 



Since a small change in a leads to a large change in (3, or, conversely, 

 since a given change in /j corresponds to a much smaller variation in 

 a, this equation may be used to calculate with high precision the value 

 of the important elastic constant, a, for all those elements for which 

 the compressibility and specific heat are known. Thus, granting the 

 validity of the reasoning by which Debye arrived at his formula — 

 the remarkable success of Debye's specific heat formula in accurately 

 representing the variation of specific heat with temperature would 

 seem to substantiate the soundness of his theory — we may state with 

 considerable confidence that Poisson's ratio for diamond is not far 

 from 0.25. 



In the course of the measurement of the compressibility of diamond 

 the density of the fragments was determined by the pyknometer 

 method and found to be 3.513 at 25°. This is in good agreement with 

 the value 3. 514 at 18° obtained by Cohen and Olie.^^ 



1 dv 



Summary. — The cubic compressibility — . ■ — of clear, colorless 



Vq dp 



diamond of density 3.513 was measured by comparison with soft 

 steel. Assuming the compressibility of steel to be 0.60 X 10"*' per 

 megabar, the compressibility of diamond is 



O.I 6 X io~^ per megabar. 

 This is the lowest compressibility of any known substance. 



From a consideration of various formulas expressing the relations con- 

 necting compressibility with other physical properties, it appears that 

 the low compressibility of diamond is intimately related to its high melt- 

 ing point, its low expansion coefficient, and its high atomic frequency. 



Very accurate values of Poisson's ratio for the various elements 

 might be obtained by Debye's equation if the compressibility and 

 specific heat were previously determined. 



'5 E. Cohen and J. OuE, Jr., Zeitschr. Phys. Chem. 71: 391. 1910. 



