C. Barue — Fluid Vohirm end its Relation, etc, 483 



ments are made l>y observing the distance apart of the lower 

 meniscus of the upper mercury thread, and the upper meniscus 

 pf the lower thread, with Grunow'fl cathetometer. When the 



adjustment is perfect the motion of the upper mercury thread 

 is nearly zero. 



B. To heat the sample to any required temperature, I sur- 

 rounded the capillary tube abe, with one of my closed boiling 

 tubes* of clear glass, v. By connecting the latter with a 

 condenser, the ebullition may be prolonged indefinitely. The 

 whole length of thread of substance is thus virtually exposed 

 in a vapor bath. Methyl alcohol (65°), water (100°), aniline 

 (1S5°), and diphenylamine (310°) are available for boiling 

 points. The liquid shown at kk, is heated by Wolcott Gibbs's 

 ring burner RB. At high temperatures it is of course neces- 

 sary to jacket the boiling tube appropriately with asbestos 

 wicking (not shown), leaving only two small windows exposed, 

 through which the ends of the thread may be seen with the 

 telescope. When the tube has become fully heated, it is clear, 

 and condensation takes place only in the condenser, connecting 

 at D. By adjusting suitable white and black screens with 

 reference to the line of sight, the image of the meniscus may 

 be sharpened. Intermediate temperatures may be reached by 

 boiling the substances under low pressures. For this purpose 

 it is merely necessary to attach the open end of the condenser 

 to Prof. R. H. Richards'! pneumatic exhausting pumps. Pro- 

 vision has been made at 7 7 for the introduction of a suitable 

 thermometer, by which the temperature of the boiling tube 

 may be checked. 



To save space the description here given is a mere sketch. 

 It is scarcely necessary to state that the manipulations are 

 difficult throughout, and that with the best of care breakage of 

 tubes and other failures of experiment are a frequently recur- 

 ring annoyance. Special pains must be taken to select well 

 annealed glass ; otherwise the internal stress adds itself to the 

 applied stress, and the tube is easily broken. Hot vapor baths 

 must not be removed until the tube a h c is thoroughly cold. 

 The capillary does not outlast many series of experiments : for 

 sheared glass undergoes a gradual molecular change very analo- 

 gous to permanent set. Eventually the tube may be ruptured 

 at less than -J the original test pressure. 



9. Pressure measurement. — It is easily seen that the method 

 of experiment is necessarily such that temperature is kept 

 constant, while pressure is varied at pleasure. 



* Bulletin U. S. G. 8., No. 54, pp. 86 to 90, 1889. 



f Richards: Trans. Am. Inst. Mining Eng., vi, p. 1, 1879. 



