COMPRESSIBILITY OF METALS. 209 



gives at once without question the cubic compressibility of magnesium, 

 and the data are accordingly given here. 



Magnesium. This material I owe to the kindness of Professor C. C. 

 Bidwell, who stated that it was of unusually high purity, but the exact 

 chemical analysis was not known. Professor F. A. Saunders was kind 

 enough to make a spectroscopic analysis for me with the following 

 results: Fe, none, Zn, perhaps none, Ca small, Cu small, Al faintest 

 trace, Sr none, Cd faintest trace. 



The material was originally provided in the form of a square bar, 

 about 1.3 cm. on a side and 1 cm. long. I extruded this in two stages 

 at 500° to a piece of round section 0.3 cm. in diameter. It was straight- 

 ened after extruding by rolling between iron plates heated to a dull 

 red, and then annealed by heating for several hours to 300°, sealed in 

 a glass tube to protect from oxidation. The final specimen, which 

 was 8 cm. long, was placed in a steel sleeve in order to adapt it to the 

 apparatus, and mounted as a compression specimen in the apparatus 

 for direct measurement without magnification. 



The regular series of runs at 30° and 75° was made. The average 

 arithmetical deviation from a smooth curve of the 29 observed points 

 (one discard) was 0.49% and the maximum deviation from linearity 

 was 3.8% of the maximum pressure effect. The maximum deviation 

 from linearity was the same at the two temperatures. Within the 

 limits of error the deviation from linearity is symmetrical about the 

 mean pressure so that it was possible to represent the results by a two 

 constant formula. 



In addition to the measurements on this pure sample, in the early 

 stages of the development work several series of readings were made 

 on an extruded sample of commercial magnesium of unknown purity 

 obtained from Eimer and Amend. These results agreed within the 

 limits of error, two or three per cent, with those found for the pure 

 sample. This makes it probable that small impurities do not exert 

 an important effect on the compressibility, and also lends much 

 plausibility to the contention that the compressibility of this material 

 is the same in different directions. 



The final results are given by the formulas: 



At 30° — = - 10- 7 (29.60 - 20.3 X 10" 5 p) p 



I o 



At 75° — = - 10- 7 (29.97 - 18.0 X 10- p) p. 



^0 



