348 Mr. Carl Barus on the 



however, in limiting these experiments to 2000 atmospheres 

 was to avoid straining the gauges, § 17. I add in con- 

 cluding, that among the special facilities of the above screw- 

 compressor is the micrometric accuracy with which pressure 

 can be raised to and maintained at a given value, for any 

 reasonable length of time. 



II. — The Isometrics* of Ether, Alcohol, Thymol, Diphenyl- 

 amine, and Para-toluidine. 



23. By aid of the machine, described in the foregoing 

 section, I made the following investigation of the isometrics 

 of liquids. The nature of the problem is clearly defined by 

 the results of an earlier paper f , where it appears conformably 

 with the inferences of Dupre, Levy, Kamsay and Young, 

 and Fitzgerald, that liquid isometrics are lines of small 

 curvature. 



Apparatus. 



24. The apparatus to be constructed is such that the 

 criterion for constant volume is given by a reliable electric 

 contact. Figs. 7 and 8 show the tube of constant volume. 

 It consists of three parts: — the reservoir ef (diameter *3 centim. 

 to '4 centim.), and the two capillary stems ed and da (diame- 

 ter about *08 centim.), separated by the enlarged portion d, 

 into which a platinum terminal c penetrates. Other dimen- 

 sions are given in fig. 7. Fig. 8 (diagram) shows the tube 

 ready for use. The end g has been sealed, holding the 

 platinum wire (the other terminal) mhn, which passes 

 through fe into the capillary. The substance to be tested 

 quite fills ef and about one half the adjoining stem. The 

 remainder of the tube is filled with mercury I k. 



The tube thus adjusted is inserted into the piezometer U U, 

 fig. 5, and the terminal c put in metallic connexion with the 

 walls of the steel tube near V. The terminal m o is suitably 

 insulated by surrounding it with a glass tube, and after 

 passing quite through the piezometer into the barrel, fig. 1, 

 is there put in metallic connexion with one of the screws M. 

 The constant-volume tube ag, fig. 8, is thus held in horizontal 

 position within the steel piezometer, by aid of its two tense 

 terminals. 



* The word "isometric " is used by J. Willard Gibbs in bis beautiful 

 researches on graphic methods in the thermodynamics of fluids (Trans. 

 Connecticut Acad. II. [2] p. 311, 1873 ; cf. ibid. p. 382). It has the 

 advantage of priority if not of expressiveness, and I do not feel the need 

 of withdrawing " isometric" in favour of "isochore," as is done by 

 Ramsay and Young (Phil. Mag. [5] xxiii. p. 435, 1887). 



t Barus, American Journal [3] xxxix. pp. 497 et seq. (1890). 



