OP ARTS AND SCIENCES. 313 



XVII. 



NOTE ON THE VARIATION OF MOLECULAR 



PRESSURE. 



By Carl Barus. 



Presented December 9, 1891. 



1. The Method. — If a quautity of heat, 8 Q, be imparted to a sub- 

 stance, the result is usually expressed as 8 f^ + 8 U', where hU '\% 

 the internal, and 8 C/^' the external work done. For liquids at at- 

 mospheric pressure, 8 W is negligible. 



Regarding 8 U from an experimental point of view, two salient 

 considerations present themselves. If I measure latent heats (A), 

 8 U includes both the dissociation energy and the expansion energy 

 necessary to effect fusion, as follows : — 



a. The dissociation energy per gram necessary to change the 

 solid molecule into the liquid molecule, isothermally, without change 

 of volume, 



h. The energy per gram for the isothermal expansion of the 

 molecule. 



c. The energy per gram for the isothermal expansion of the con- 

 figuration of molecules which constitutes the given substance from 

 solid to liquid. 



If I measure specific heats isopiestically, 8 U includes the purely 

 thermal energy necessary to effect changes of the body's temperature 

 and the expansion energy, viz. : — 



d. The purely kinetic energy per gram per degree, by which the 

 solid is appreciably changed as to temperature. This is a temperakire 

 function only. 



e. The energy per gram for the corresponding isothermal expansion 

 of the molecule. 



/. The energy per gram for the corresponding isothermal expansion 

 of the configuration of molecules ; with similar quantities, o?', e\ f, 

 for the solid state. 



The permissibility of thus separating 8 U into a thermal (tem- 

 perature) and a volume and thermal component is given, if the iso- 



