﻿On the Internal Pressure of a Liquid. 955 



I may add, in conclusion, that I have recently * constructed 

 a displacement interferometer combining the desideratum of 

 lightness and portability with rigidity, and provided with long 

 arms of invariable length (water circulation within), and with 

 sufficient height of mirrors, &c. above the arms to admit of 

 the insertion of bulky objects like a fog-chamber into the 

 interfering beams of light. The adjustment of fig. 8 is par- 

 ticularly useful, the distance M to N being one or more 

 metres. The apparatus is especially adapted for the measure- 

 ment of the refraction of gases at all temperatures and con- 

 versely (in view of the large coefficient) for the measurement 

 of high temperature in terms of the refraction of gases. It 

 is also adapted for the measurement of the pressure variation 

 of the refraction of a gas and conversely for pressure mea- 

 surement in terms of refraction, particularly in case of the 

 adiabatic transformations of a dry or a wet gas, at all tempe- 

 ratures. For the displacement of ellipses is instantaneous 

 and dependent on the ratio of pressure to absolute tempe- 

 rature. Tests made with the fog-chamber have given results 

 which promise to be fruitful, as will be shown elsewhere. 



Brown University, 

 Providence, K.I. 



XCV. On Mr. McC. Lewis's Papers on the Internal 

 Pressure of a Liquid. , 



To the Editors of the Philosophical Magazine. 



Gentlemen, — 



Mr. McC Lewis, in his voluminous and interesting 

 papers t, departs from Dupre's proposition, according to 

 which the latent heat of vaporization of a unit of volume 

 is equal to the internal pressure, and concludes in applying 

 the second law of thermodynamics to this case, that " appa- 

 rently the only accurate relation, the thermodynamic one/' is 



K-J*T(fK I ...... (1, 



" But,"" according to Mr. McC. Lewis, " this does not allow 

 us to go further unless some new assumption be made with 



* Am. Journ. Science, xxxiii. p. 109 (1912). 



t Phil, Mag. xxii. pp. 193-197, and pp. 208-276 (1911). Trans, of 

 Faraday Soc. vii. pp, 9-1 115 (1911). Zeitschr.f. phys, Chcm. Bd. lxxviii. 

 pp. 24-38 (1911). 



