210 BRIDGMAN. 



Richards gives for the average compressibility to 500 kg. (no dis- 

 tinction possible between the initial and the average compressibility) 

 the value 29 X 10 -7 , pressure being expressed in megabars. The cor- 

 rection for converting to kilograms would reduce this figure by about 

 2%. The formulas above give for the average compressibility to 

 500 kg. at 20° the value 29.4 X 10~ 7 . Considering the number of signi- 

 ficant figures given by Richards, the agreement is within the possible 

 experimental error, and thus we again have presumptive evidence of 

 the equal compressibility of magnesium in all directions. 



Magnesium is the only one of the non-cubic metals that may be 

 expected to have equal compressibility in all directions. The other 

 metals of this group measured were Bi, Sb, Cd, Sn, Zn, and Te. A 

 complete description of their behavior would demand a determination 

 of the compressibility in different directions specified with respect to 

 the crystalline axes. This is much beyond the scope of the present 

 work. For some of these metals the most that can be obtained from 

 these measurements is some idea of the magnitude of the variation 

 with direction that may be expected. This is given roughly by the 

 variation in the numbers obtained for the compressibility with differ- 

 ent methods of preparing the sample. The first strong evidence of 

 this effect was found with bismuth. 



Bismuth. The material used for these measurements was com- 

 mercial electrolytic bismuth, obtained through the courtesy of the 

 U. S. Lead Refinery, Inc. The properties of this bismuth have already 

 been discussed in considerable detail 18 ; the only measurable impurity 

 is 0.03% of silver. Measurements were made on four samples. Two 

 of these were castings, one cast in an iron mold, and the other in a thin 

 walled graphite mold, chilled by slowly lowering into water. The 

 other two specimens were extruded from the casting, the reduction 

 in diameter being 2 to 1. Measurements on two of these samples, the 

 casting in the iron mold, and one of the extruded pieces, were made 

 with the preliminary apparatus, before the greatest accuracy was 

 obtained. The initial compressibility at 30° of the extruded piece 

 was 31. S X 10- 7 , and that of the casting 25.6 X 10~ 7 . The measure- 

 ments of the casting were repeated to be sure that there was no error. 

 Although these measurements were not as accurate as those finally 

 made, still the accuracy was certainly one or two per cent, and the 

 difference between these two samples was far beyond any possibility 

 of experimental error. 



Measurements on the two other samples, the casting made in 

 graphite (density at 20° 9.803) and one of the extruded pieces (density 



