350 Mr. Carl Barus on the 



the torsion-galvanometer*. To calibrate this instrument the 

 boiling-points of water and of mercury were first used. 

 Finally, I made extensive comparisons with the glass re- 

 entrant air-thermometer (§§ 35, 36). 



32. Pressure was measured as shown in Section I., and 

 the factor of the Tait Gauge frequently checked by aid of a 

 small sensitive 300-atmosphere Bourdon Gauge. About 20 

 or 30 minutes were allowed to secure isothermal conditions, 

 both for the gauge and the piezometer tube. Leaks must be 

 avoided ; otherwise the operator is apt to pump heat into the 

 liquids, Particular care was taken to redetermine the initial 

 pressure and temperature (17° and 68°, respectively) before 

 and after each high-temperature observation. This is a 

 certain and an essential check on the validity of the results. 

 At any high temperature at least four readings were made, 

 about five minutes apart. The last two of these, if coincident, 

 were taken. 



Obsekvations. 



33. In the Tables below the notation used is as follows : — 



ISy, deflexion in degrees of arc observed at the torsion- 

 galvanometer. 

 £, temperature of cold junction of thermocouple. 

 n e , correction by which N fl ' is reduced to 

 N e , the deflexion which would be observed at the torsion- 

 galvanometer, if £ = 20°. 

 0, temperature of the substance in the constant- volume 

 tube ; is computed from N and is identical with 

 the temperature of the vapour-bath surrounding 

 the piezometer. 

 Nj,, reading in centimetres at the capillary tube of the 

 pressure-gauge. 

 p, pressure corresponding to N p , in atmospheres. Hence 

 p is the pressure bearing on the substance in the 

 constant-volume tube. 

 A0\ temperature and pressure obtained by shifting the 

 Apj coordinates, so that the initial temperature and 

 pressure may be zero. 



34. For illustration I will give a full example of two of 

 the many series of results obtained for ether and the other 

 substances. Each series contains at least one observation at 

 the boiling-point of water, the importance of which will 

 presently be seen, § 35. Since the relation between p and 



* Cf. Barus, Phil. Mag. [5] xxix. p. 146 (1890). 



