44 new method for determining compressibility 



Change of Compressibility with Pressure. 



A glance at the above table shows that all the substances studied, 

 like all those examined by Barus, show a decrease in compressibility 

 with increasing pressure. This decrease is by no means a simple 

 function, however. Leaving out of consideration the cases of chlorine 

 and iodine, which cannot claim accuracy enough for serious considera- 

 tion in a discussion of this kind, the other substances show the follow- 

 ing percentage decrease in their compressibilities between 100 and 500 

 atmospheres: CHC1, 29, CC1 4 , 26; Br, 21; CHBr 3 , 20.6; H,0, 

 13 ; Hg, 4. This order is arranged according to the magnitude of 

 the compressibility, and it exhibits a steady decrease ; hence one may 

 infer that, other things being equal, the greater the compressibility, the 

 greater is its percentage decrease with increasing pressure. That other 

 circumstances influence this relation is shown however by the fact that 

 chloroform and carbon tetrachloride manifest different second differen- 

 tial quotients although their first differential quotients are exactly iden- 

 tical at 150 atmospheres. Moreover, bromoform and water have 

 almost the same compressibility, and yet the change of this compres- 

 sibility with the pressure is noticeably different. Such differences as 

 this must be referred to the specific natures of the component ele- 

 ments, and the internal pressure relations within each substance. 



When the theorizer goes further than such a comparison as this, and 

 attempts to determine the mathematical expressions for these curves, 

 he is met by a serious obstacle. The departure from the perfectly 

 linear equation x = ay is not sufficiently greater than the possible error 

 of the gauge to make its somewhat subtle nature clearly manifest. 

 One should point out also the probability that the parabolic equations 

 proposed by Barus for the organic liquids studied by him are subject 

 to an even greater experimental uncertainty ; so that it is safe to say 

 that no data now known to us afford a satisfactory basis for the deter- 

 mination of the law underlying the change of compressibility with 

 pressure. It is our hope, by more accurate experiments made upon 

 larger quantities of material and with a more perfect gauge, to pro- 

 ceed further in this direction. 



In conclusion, it is a pleasure to express our great indebtedness to 

 the Cyrus M. Warren Fund of Harvard University for assistance in 

 the early part of this investigation, and to the Carnegie Institution of 

 Washington for assistance in the latter part. 



