8 NEW METHOD FOR DETERMINING COMPRESSIBILITY 



The combination of stresses involved is not simple enough for certain 

 interpretation. 



The method which attempts to correct itself by measuring the ex- 

 ternal change in volume of a bulb subjected to internal pressure is 

 obviously faulty, as Amagat ! has pointed out, because the erroneous 

 assumption is made that the interior volume changes no more than 

 does the exterior volume. Moreover, such an apparatus can endure 

 but a small pressure, and it is difficult to enclose it in any other ap- 

 paratus capable of withstanding pressure without hiding the substance 

 under examination. Again, it is always possible that this substance 

 may dissolve some of the fluid used to transmit the pressure, or be dis- 

 solved by it. 



When a capillary glass tube is used to contain a liquid under exam- 

 ination, three serious errors are introduced. First, the bore and hence 

 the volume of the tube increases under pressure. Next, the non-con- 

 ducting nature of the walls prevents the compression from being 

 strictly isothermal, while the heat capacity of the walls prevents it 

 from being strictly adiabatic. From the work of Barus, 2 who has 

 carried out the most successful and comprehensive experiments accord- 

 ing to this method, it may be inferred that these errors in some cases 

 nearly counterbalance one another. Another cause of error is the 

 adhesion of the compressed liquid to the emptied tube, as the column 

 shortens under pressure, a circumstance which causes the compression 

 to appear too large. Moreover, the compressibility of liquids is so 

 slight that very small changes in length of column must be accurately 

 observed, when the liquid is arranged in a uniform thread. The most 

 serious causes of error having been reviewed, it is possible to describe 

 the forms of apparatus which we have used in order to avoid them. 



Apparatus. 



In order to obviate the uncertainty of temperature, and the last 

 mentioned difficulty of measurement, we decided to enclose the greater 

 part of the liquid to be examined in a thin glass bulb. Barus, indeed, 

 had seen the advantage of this arrangement, but he was unable to pre- 

 vent the fracture of the bulb under comparatively small pressures. In 

 order to obviate this difficulty, we surrounded the bulb with mercury, 

 and subjected the exterior to the same pressure as that applied to the 

 liquid within. The balancing of pressure prevented the bulb from 

 bursting, and the great thermal conductivity of mercury soon estab- 



1 Amagat, Am. Chem. Phys. (6), 22, 95 (1S91). 

 2 Bull. U. S. Geolog. Survey, No. 92 (1892). 



