Isometrics of Liquid Matter ; 349 



25. The method of experiment is easily inferred. The 

 glass tube being in place and quite surrounded with oil, 

 piezometer and barrel are put in a simple circuit containing 

 a battery and a galvanoscope. The electric contact, made or 

 broken by forcing the screw S S T in or out, is sharp and 

 thoroughly reliable, even at 2000 atmospheres and 200°. 



26. In passing from one constant temperature to another, 

 there is danger either of forcing the mercury thread, I k, out 

 of the tube ag, or into the reservoir ef. Hence when the 

 temperature of uu is increasing, the pressure is to be in- 

 creased in such a way as to keop the electric contact just 

 made ; and when temperature decreases the pressure is to be 

 relieved in such a way as to keep the electric contact just broken. 

 In these adjustments the screw and ratchet is very serviceable. 



27. When a series of isometrics is to be mapped out, the 

 volume of liquid between meniscus k and the free end n is 

 suitably changed. This volume may be conceived to be 

 either positive or negative. 



28. The operation of filling the glass tube ag calls for 

 some skill ; but a description is beyond the scope of the 

 present paper. I make extensive use of filamentary glass 

 funnel-tubes, and of filamentary glass connexion-tubes com- 

 municating with the mercury air-pump. 



Solid substances, like diphenylamine, must be kept fused 

 after filling in a hot- water-bath, and the tubes ag are to be 

 introduced into a hot piezometer. 



29. The greatest care must be taken to have all parts of 

 the tube clean, and to avoid rubber connexions or other 

 material impregnated with sulphur. When the mercury 

 appears dead, the tube is worthless. 



30. Seeing that the piezometer tube is a good conductor, 

 constancy of temperature cannot be guaranteed nor its value 

 measured, unless the vapours of the bath be directly in con- 

 tact with the tube, § 7. Methyl alcohol, 66°, water, 100°, 

 amyl alcohol, 130°, turpentine, 160°, naphthaline, 215°, ben- 

 zoic acid, 250°, diphenylamine, 310°, phenanthren, 340°, are 

 available for boiling. They form a convenient series. A low 

 (fiducial) temperature is given by ice, or by circulating- 

 water. It is essential that the vapour-baths slide on and off 

 the piezometer easily. Jarring may break the thread of the 

 constant-volume tube within. 



31. To measure the temperatures I used a platinum/platinum- 

 iridium thermocouple, with its junction in contact with the 

 piezometer tube, near the centre of the vapour-bath. Thermo- 

 electric force was expressed in terms of the twist of the 

 platinum fibre which suspended the given astatic system of 



Phil. Mag. S. 5. Vol. 30. No. 185. Oct. 1890. 2 B 



