﻿Volume, and Temperature of Rarefied Gases, 321 



pressure too high, for carbon dioxide, being evolved from the 

 glass with decrease of pressure, would mix with the gas under 

 experiment and increase the apparent pressure. It is prob- 

 ably the reason why the product of pressure and volume; has 

 so often been found to increase with reduction of pressure. 



To measure the expansion of hydrogen below the pressure 

 of 2*5 millim. the thermal method was adopted. One of the 

 two gauges was jacketed at a known temperature, and the 

 expansion of the gas thus determined. Headings were taken 

 on both the gauges, and the ratio of the two sets of values of 

 pv obtained. When the mercury is lowered in the gauges 

 the gas expands, and though its expansion below 2*5 millim. 

 may be abnormal, this ratio of the values of pv remains the 

 same. One of the gauges is then jacketed and the other 

 read. This reading multiplied by the ratio before obtained 

 gives the pv of the hot gas in the other gauge. The hot 

 gauge is then shut off and allowed to cool, when it is read. 

 The ratio of the readings of the gauge when hot and 

 when cold is equal to the ratio of the absolute temperatures. 

 To give an idea of the accuracy of the method we quote an 

 example : — 



Preliminary Readings to obtain the Gauge Ratio. 



Gauge E. Gauge F. 



pv. pv. 



376-2 395-1 



375-4 394-6 



376-3 394-5 



376-5 395-0 



375-6 394-8 



Mean = 376'0 Mean =394-8 



Ratio ~ =1-05. 



Hi 



Gauge F was then jacketed with aniline vapour. Barometer 

 764-9 at 0° 0. 



Headings after jacket. 

 E. F. 



pv. pv. 



379'84 247-60 



379-71 247-41 



380-11 247-94 



379*78 248-25 



379-17 247-71 



Mean =379-71 Mean =247*782 



Temperature =12°"5 C. 



