174 BRIDGMAJST. 



the effect no great accuracy can be claimed for the temperature 

 coefficient itself, and in fact not more than one significant figure could 

 be obtained here. 



After applying all corrections, including one due to the cubic com- 

 pressibility not being exactly three times the linear, the following 

 formulas were found to give the change of volume produced by any 

 pressure p, expressed in kilograms per square centimeter. 



AV 

 At 30° — = - 10- 7 (5.87 - 2.1 X 1(H p) p 



I 



o 



AV 

 At 75° -jr = - 10- 7 (5.93 - 2.1 X 1Q- 5 p) p. 



f 







The T'o of these formulas is in each case the volume under atmos- 

 pheric pressure at 30°; the difference between the atmospheric 

 volume at 30° and 75° would produce a change of somewhat less than 

 one unit in the last place. 



The fairest comparison of these results with those previously found 

 is to be obtained by computing from the above formulas the average 

 compressibility to 10000 kg., since the range of the previous work was 

 10000 kg., and no departure was found from linearity. The above 

 formulas give as the average to 10000, 5.6G X 10- 7 at 30° and 5.72 X 

 10~ 7 at 75°. These are materially smaller than the previous values, 

 which were 5.83 X 10- 7 at 0° and 6.01 X 10~ 7 at 50°. Probably part 

 of the difference is to be explained by the greater purity of the present 

 specimen, it being consistent with a number of my other observations 

 that alloying iron increases its compressibility, but doubtless the 

 important part of the difference is to be ascribed simply to the im- 

 provement in the method; certainly the present determination of the 

 temperature effect is to be preferred to the previous one. 



The new value for the compressibility of iron will not affect at all 



the compressibilities given by Richards, which involve my previous 



value for iron indirectly through the value for mercury, but will lower 



the absolute values of compressibility given by A. W. J. by 0.25 X 



10 -7 , since their values also depend on my value for iron. The fact 



that the compressibility of iron is now found to decrease somewhat at 



high pressures will also of course somewhat increase the estimate of 



A. W. J. of the decrease of compressibility with pressure. 



1 dV 

 The instantaneous compressibility, which I define as 77 — -, may 



be found from the above formulas by differentiation. The change of 



