RICHARDS AND MARK. THERMAL EXPANSION OF HYDROGEN. 131 



Although the change in pressure of the gas with change in temperature 

 must be determined with the utmost accuracy, the actual pressure which 

 the gas exerts need be known only approximately. An inspection of 

 the formula for calculation on p. 135, where this value Centers both the 

 numerator and the denominator, makes this fact obvious. An accuracy 

 to within half a millimeter was shown by calculation to be quite suffi- 

 cient. Since the pressure of the gas is measured on the Lord Rayleigh 

 barometer by the height of the mercury meniscus (m) above the lower 

 point (1), and since the distance from the meniscus to the upper point 

 (n) is determined during the course of the experiment, it only remains 

 to find the distance between the two points. In making this measure- 

 ment, the cross-hair of the telescope was set upon one of the points in 

 question. The telescope standard was then revolved so that on a vertical 

 axis this height could be directly read on the silvered brass meter-bar, 

 which had been placed at the proper distance from the object glass. 

 The height of the other point was determined similarly. Two deter- 

 minations, giving 7G1.33 and 761.35 when corrected to the international 

 meter, agreed in showing the distance between these points to be 

 70 1.34 mm. This measurement, like all the other measurements con- 

 cerning the pressure, was made at 20°. 



The actual pressure of the gas in the tubes leading to the bulb was of 

 course indicated hy the barometric column thus read, plus the very small 

 pressure (usually 0.03 mm.) of the residual gas in the Torricellian vac- 

 uum. This latter was kept as constant as possible by a good Sprengel 

 pump, and read from time to time with a McLeod gauge. 



The McLeod gauge was so made that a difference of pressure read in 

 the fine tube was one hundred times as great as the real pressure being 

 determined. It was only necessary, therefore, to read this gauge to an 

 accuracy of 0.1 mm. Since, moreover, the only requisite is that this 

 pressure shall remain about constant within a small known range through- 

 out the course of an experiment, a possible inaccuracy in registration of 

 the gauge due to adsorption * would make no difference, applying alike 

 to all measurements, and cancelling from the final result. 



The manner of making the pressure readings was the same as that 

 previously used, except for the improvement in the background already 

 described. The following typical day's record shows the constancy of 

 reading attained and the usual magnitude of the corrections which were 

 applied. 



* Ramsay and Baly, Phil. Mag. [5], 38, 314 (1894). 



