COMPRESSIBILITY OF METALS. 183 



nearly as regular as those with the longer wire, which indeed is not 

 surprising in view of the smallness of the effect. The average arith- 

 metical departure from a smooth curve of the 28 observed points with 

 the short specimen (no discards) was 2% of the maximum pressure 

 effect, and the departure from linearity, which was perfectly well 

 marked, was 3% at the maximum. The series of readings at the two 

 temperatures gave the same departure from linearity to one significant 

 figure. It is obvious that a high degree of precision cannot be expected 

 for the pressure coefficient of compressibility of this specimen. The 

 average arithmetical departure from the smooth curve of all the read- 

 ings on the wire (no discards) was 0.7% of the maximum pressure 

 effect, but here the deviation from linearity, although perfectly evi- 

 dent, was only two thirds as great as the error of a single reading. 

 The deviation from linearity at the higher temperature was about 50% 

 greater than at the lower. The average value of the compressibility 

 obtained from the wire sample may be expected to be relatively more 

 accurate than that obtained from the massive specimen, but the 

 differences are beyond the errors of the measurements, and point to a 

 real difference between the two specimens. The pressure coefficient 

 of the difference of compressibility between iron and platinum can- 

 not obviously claim any great accuracy, but the actual pressure coeffi- 

 cient, obtained by combining these readings with the absolute values 

 for iron, may be expected to be somewhat more accurate. 

 The results found are reproduced by the following formulas: 



AV 

 Wire, At 30° — = - 10~ 7 (3.G0 - 1.8 X 10^p) p 



At 75° — = - 10- 7 (3.64 - 1.8 X l(Hp) p 



I 



ii 



Rod, At 30° — = - 10- 7 (3.05 - 0.0 X 10-^p) p 



I 







AV 

 At 75° — = - 10- 7 (3.09 - 0.0 X I0~ b p) p. 



I 



o 



It is of course not likely that the compressibility of the rod actually 

 does not change with the pressure, but at any rate the change is small. 

 The initial compressibility of platinum has been found by Richards 

 to be 3.7 X 10~ 7 . The agreement with the value found above for the 

 wire is well within the limits of error. Richards also obtained his 

 material from Baker and Company. He found for the density at 20° 



