COMPRESSIBILITY OF METALS. 199 



Richards gives the initial compressibility of thallium which con- 

 tained a slight amount of lead as 23 X 10 -7 . The initial compressi- 

 bility may be found graphically from the data of the table just given, 

 or it may be computed. I have passed a three constant power series 

 curve through the points at 3000, 6000, and 9000 kg., and from this 

 find the initial compressibility at 30° to be 34.8, and at 75°, 36.7 X 

 10 -7 . This is so much higher than the value of Richards that it seems 

 difficult to ascribe to experimental error or slight impurity. The 

 crystal structure of thallium does not seem to have been determined 

 as yet; the difference in these values of compressibility strongly 

 suggests that the crystal system cannot be cubic. 



Cerium. This material I owe to the kindness of Dr. A. W. Hull of 

 the General Electric Company. It was provided in the form of a 

 square bar. I extruded it at about 400° to a round section 0.3 cm. in 

 diameter, thereby reducing its section about four fold, and obtaining a 

 piece 4.5 cm. long. This was mounted as a compression specimen in 

 the lever apparatus for long specimens. The purity of this specimen 

 was presumably not high, as its mean temperature coefficient of re- 

 sistance between 0° and 100° was only 0.001, but since the compressi- 

 bility seems never to have been measured, and impurity usually has 

 only an additive effect on compressibility, I thought it of sufficient 

 interest to run through the measurements. 



The two regular runs were made, at 30° and 75°. Because of 

 trouble with the pressure apparatus the maximum pressure of these 

 runs was only 10000 instead of the usual 12000 kg., but it did not 

 seem worth while to go to the labor of repeating the measurements 

 over the greater range. The average departure from a smooth curve 

 of the 23 observed points (no discards) was 0.47%, and the maximum 

 departure from linearity was 2.2% of the maximum pressure effect. 

 The final results are given by the formulas : 



„ AT r 

 At 30° — = - 10- 7 (35.74 - 19.0 X 10~ 5 p) p 



I 



AV 

 At 75° — = - 10- 7 (35.80 - 19.7 X 10- 5 p) p. 



I 



o 



This compressibility is not inconsistent with that to be expected 

 for cerium from its position in the periodic table. 



Calcium. Material from two sources was used. The first was 

 obtained from the General Electric Company and was cut from the 

 same block 1 inch in diameter as I used for previous determinations 



