BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 269 



for the equalization of the temperature, and then applying the last 

 few hundred kilograms. Differences of temperature at different times 

 of measuring the displacement did not prove great enough to intro- 

 duce perceptible error, since the difference of dilation between the 

 glass and the steel is small. To secure good results, it was found 

 necessary that the glass tube fit closely in the steel cylinder without 

 play sidewise. As it was found difficult to draw a tube accurately 

 enough, this desired freedom from play was secured by wrapping tin 

 foil at either end. 



Measurements were made in this way of the change of length of two 

 kinds of Jena glass : a hard combustion tubing No. 3883, and a very 

 fusible glass No. 3880 a. The results at first were discouragingly ir- 

 regular. After repeated trials, however, they settled down into a 

 fairly regular final form. It became evident on trial with different 

 pieces of glass that there is here, directly observed, the same seasoning 

 effect of successive applications of pressure that was noticed in meas- 

 urements of electrical resistance. The final behavior never became 

 entirely regular, however. The general effect of frequent applications 

 was to slightly increase, in a totally irregular fashion, however, the 

 observed change in length. In Figure 7 the observed changes of length 

 are plotted against pressures. The irregularity of the results is notice- 

 able, particularly for the hard glass; it approaches, or may some- 

 times exceed, 5 per cent of the total effect to be expected. The results 

 with the soft Jena glass were only one third as irregular. The explana- 

 tion suggests itself that the less regularity of the results with the hard 

 infusible glass is because of the greater internal strains set up in this 

 by the long temperature range through which it cools after passing 

 plasticity. 



In order to find whether there is any appreciable change in the 

 linear compressibility of glass when it is drawn down from larger 

 sizes, the above form of apparatus was modified by placing the com- 

 parison piece of steel inside, instead of outside, the glass tube. The 

 tubes tested were 1 cm. in diameter, which is the original size from 

 which the test pieces mentioned above were drawn down to 0.5 cm. 

 Within the limits of error, no variation of compressibility with abso- 

 lute size could be detected. 



The linear compressibility of the steel against which the glass was 

 compared was determined indirectly by finding the relative change of 

 length in the same manner as for the glass, of the steel and a piece of 

 aluminum cut from the rod whose absolute compressibility was de- 

 termined above. These readings of the relative change of steel and 

 aluminum are shown in the lower line in Figure 7. The points, except 



