FEB. 4, 1921 ADAMS: COMPREvSSIBlUTY OF DIAMOND 47 



In table 1 the second column shows the "observed" relative de- 

 crease in volume corresponding to the pressures given in the first 

 column. The arbitrary zero with reference to the volume change was 

 at 4000 megabars. 



The decrease in volume was assumed to be a linear function of the 

 pressure, that is, 



— X 103 = a + 6 (^ - p^) X 10-\ (1) 



and by the method of least squares the values of the constants a and 

 b were calculated from the data in table 1 . The value found for b 

 "was 0.442, which, multiplied by 10 ~^ is the difference between /3, 

 the compressibility of diamond, and /3', that of steel. That is, /3' — ^ 

 = 0.44 X 10 "^ If the compressibility of steeP be taken as 0.60 X 

 10 ~^ the compressibility of diamond is 



/3 = o.i6 X io~* per megabar. 



This value is estimated to be correct within =t 0.02 X 10~^ 



From this result it is evident that the compressibility of diamond is 

 remarkably low; indeed, of all substances whose elastic behavior is 

 known, diamond is by far the most incompressible. Its nearest com- 

 petitor, tungsten, is nearly twice as compressible (/3 = 0.27 X 10~®), 

 and the majority of solids decrease their volume more than ten times 

 as much for a given increment of pressure. If a diamond were buried 

 100 miles below the surface of the earth the pressure due to the super- 

 incumbent rock — a higher pressure than has ever been attained in the 

 laboratory — would decrease the volume only about three-fourths of 

 one per cent. 



It is noteworthy that the other modification of carbon, graphite, 

 is much more compressible, the value of (3, according to Richards,'' 

 being 3.0 X 10"^ 



Relation of compressibility to other properties.- — ^Einstein^ has 

 derived the following equation, involving the compressibiUty 3 



« Compare P. W. Bridgman, Proc. Amer. Acad. 47: 366. 1911. E. GrCneisen, 

 Ann. Phys. (4) 33: 1262. 1910. 



' T. W. Richards, Joum. Amer. Chem. Soc. 37: 1646. 1915. 



8 A. Einstein, Ann. Phys. (4) 34: 170. 1911. The specific heat per gram-atom, Ct of 

 any monatomic solid, according to Einstein, is a universal function of p/T, thus: 



R, h and k being universal constants. 



