340 
PHYSICS: C. BARUS 
ties like 0.1 microfarad fail to influence the vibrator though to the ear 
the sound is quite loud. The capacity should have to be increased to 
10 or 20 microfarads for equal effects on telephone and vibrator. In 
another experiment the closed circuit gave 20 scale-parts. The inser- 
tion of 4 microfarads decreased this to 4 scale-parts, which is again a 
demand of about 20 microfarads for an equality of behavior. On the 
other hand, while the telephone responds for a phenomenally small 
capacity, it soon ceases to increase in loudness (for 1, 2, 3, 4, mf., or 
resistances), whereas the deflections of the vibrator increase regularly. 
* Advance note from a Report to the Carnegie Institution of Washington, D. C. 
2These Proceedings, 5, 211-217, (1919). 
3 These Proceedings, 4, 328-333, (1918). Carneg. Publ. No. 249, 3, 1919, chap. v. 
ON THE PRESSURE VARIATION OF SPECIFIC HEAT OF 
LIQUIDS 
By C. Barus 
Department of Physics, Brown University 
Communicated June 13, 1919 
1. Introductory. — The measurement of the specific heat of a liquid in 
its relation to pressure is surrounded by so many difficulties, that any 
method which gives a fair promise of success deserves to be carefully 
scrutinized. During the course of my recent work on interferometry, 
I have had this in view, and the plan which the present paper proposes 
is particularly interesting as it seems to be quite selfcontained. 
2. Equations. — If 0, p, p, c, denote the absolute temperature, the 
pressure, the density, and the specific heat at constant pressure, re- 
spectively, of the liquid, and if a = (dv/v)/dd is its coefficient of 
volume expansion, the relation of these quantities may be expressed 
by the well known thermodynamic equation 
dO/dp = a'e/Jpc (1) 
where J is the mechanical equivalent of heat, and the transformation 
is along an adiabatic. The main difficulty involved would therefore 
be the measurement of the temperature increment; for dp could be 
read off on a Bourdon gauge after a partial stroke of the lever of my 
screw compressor, with facility. It is my purpose to find dd by the 
displacement interferometer. To fix the ideas; suppose the Hquid in 
