COMPRESSIBILITY OF METALS. 173 



by them under the name of Type D. After machining it was heat 

 treated by quenching from 930° for a few seconds in water and then 

 finishing the quenching in oil. It was pressure seasoned before the 

 measurements by an application of 12500 kg., and then machined to 

 final size. No perceptible change of dimensions was produced by this 

 preliminary treatment. 



The measurements of the external change of length of the cylinder, 

 which were made at the same time as the measurements of the change 

 of relative length of cylinder and bar, were entirely satisfactory. The 

 relation between pressure and elongation was linear within the limits 

 of error, which were not more than a few per cent, and there was no 

 perceptible hysteresis, showing that the elastic limit had not been 

 exceeded. However, the absolute value of the elongation was ab- 

 normal in that it was only half that which would be computed from 

 the ordinary elastic constants of steel, assuming perfect homogeneity. 

 It is evident that the method of heat treatment must have introduced 

 considerable internal stress. 



Measurements were made at two temperatures, 30° and 75°, and 

 at both temperatures the runs were repeated. The two runs at each 

 temperature agreed within the limits of error. The magnitude of 

 the effect was sufficient to give a displacement of 40 cm. on the wire of 

 the potentiometer. The regular procedure was followed in making 

 readings, which were at intervals of even thousands of kilograms with 

 increasing pressure and at the odd intervals on decreasing pressure, 

 the maximum being 12000. There was no perceptible hysteresis. 

 At each temperature 27 readings were taken. Two of these had to be 

 discarded, the observed points lying off a smooth curve by slightly 

 over 1 cm. of the potentiometer wire; the average arithmetical devia- 

 tion of the remaining 52 points from smooth curves was 0.6% of the 

 maximum effect. The departure from linearity was perfectly well 

 marked and unmistakable; at the middle of the range it amounted 

 to 0.32%, and was sensibly the same at both temperatures. This 

 deviation from linearity is thus twice as great as the average error of a 

 single observation ; hence it is evident that no high degree of accuracy 

 can be claimed for the deviation from linearity (which determines at 

 once the pressure coefficient of compressibility), and certainly one is 

 not entitled to more than two figures in the pressure coefficient of 

 compressibility. 



With regard to the temperature coefficient of compressibility, the 

 change with temperature was certainly established to be much smaller 

 than had been previously found, but again in view of the smallness of 



