4G JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 3 



by forcing the piston into the bomb by means of a powerful hydraulic 

 press. In order to determine the compressibility it is necessary to 

 measure the pressure and the decrease in volume. The pressure is 

 measured by means of a small coil of "therlo" wire the resistance of 

 which changes with pressure according to a known relation; and the 

 volume-decrease is determined by the movement of the piston, 

 which is measured with a dial micrometer. Pressures were read 

 to 1 megabar and piston displacements to 0.001 mm. As a means 

 of correcting for the compressibility of the kerosene, an exactly 

 similar series of measurements is carried out with some other sub- 

 stance of known compressibility. 



The diamonds were placed in a thin- walled steel capsule, and the 

 comparison body was a Bessemer steel cylinder the volume of which 

 was equal to the volume of the diamonds plus the volume of metal in 

 the capsule. In order to remove any air bubbles which might cling 

 to the diamond fragments, the material, having been placed in the 

 capsule, was covered with kerosene and the whole evacuated. 



With the apparatus used the most accurate measurements are 

 obtained in the range from 4,000 to 10,000 megabars.^ Accordingly, 

 it seemed best to make several series of readings within this range of 

 pressure. Readings were taken at 4,000, 6,000, 8,000 and 10,000 mega- 

 bars. Three separate runs were made with the diamond and two with 

 the steel, but the first series with the diamond was subject to slight 

 irregularities due to trouble with the electrical connections and was 

 therefore neglected in the final calculations. The method for calcu- 

 lating the results and for making the various corrections is described 

 in detail in the paper already referred to. 



Various ways of combining the data of the two series for steel 



and the two series for diamond led to substantially the same result. 



If the mean of the former be compared with the mean of the latter, 



Av 

 the relative decrease in volume, — , is found to be as shown in table 1. 



Vo 

 Table 1 

 Decrease of Volume of Dl^mond under Pressure 



^ 1 megabar = 0.987 atmosphere. 



