BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 271 



comparison piece of steel determined as above, the compressibility of 

 the glass was found to be: 



for Jena glass No. 3880 a 2.17 X 10 -6 kgm. per sq. cm. 

 for Jena glass No. 3883 2.23 X 10 -6 kgm. per sq. cm. 



The hard glass, contrary to what one might expect, is therefore the 

 more compressible, a result that has already received confirmation by 

 measurements of electrical resistance. 



Beside these determinations of the compressibility of glass, it was 

 also necessary to find the compressibility of mercury, in order to find 

 the pressure coefficient of the molecular conductivity of mercury. None 

 of the data at hand reach over a sufficient pressure range for the pur- 

 pose of this paper, and the data had, therefore, to be extended up to 

 6800 kgm. The correction introduced by the compressibility of mer- 

 cury is only 10 per cent of the total change of resistance, so that a 

 highly accurate value of the compressibility was not necessary. The 

 interest of this determination lay rather in finding whether there is 

 any marked decrease of compressibility over the pressure range used. 

 To make this determination, a method was adopted which gives 

 promise of being a considerably better means of determining com- 

 pressibility even at comparatively low pressures than those methods 

 at present in common use. 



The compressibility of mercury at low pressures has been the sub- 

 ject of a number of investigations, and the results which have been 

 obtained recently have been fairly concordant. It is a common feature 

 of all earlier determinations that the mercury has been enclosed in a 

 glass piezometer, the correction for the compressibility of which is 

 60 per cent of the total effect. The correction for the glass is unusu- 

 ally large in this case because of the comparatively small compressibil- 

 ity of the mercury. For many liquids, the correction for the piezom- 

 eter is considerably less (6 per cent for water, for example), and the 

 objections urged in the following have proportionally less weight. 

 This correction may be determined in various ways, depending in 

 general on the theory of elasticity, which makes, among others, the 

 assumption of the uniform compressibility of the glass in all directions. 

 Too often, however, the compressibility of the glass has been merely 

 assumed from the work of other investigators on a glass presumably 

 of the same general character as the glass used in the experiments. 

 The correction for compressibility determined by elastic experiments 

 on the same or other pieces of glass seems doubtful in view of the 

 large correction involved. Thus if the behavior of the glass were as 



