﻿Volume, and 'Temperature of Rarefied Gases. 315 



for air at such low pressures. In addition to this, the surface 

 of glass is uneven and not of uniform porosity. Two sets of 



pv curve I. 



10 20 



64 

 pv curves II. and III. 



readings of the two gauges at the same vacuum were obtained 

 with carbon dioxide. These two series were calculated for 

 the same volume and were plotted against pressures. The 

 two curves crossed at a point (Nos. II. & III.). Now this 

 point, pressure and volume being the same, should be a point 

 of equal surface, but it is very far from being so. We see, 

 therefore, that equal pressures may condense quite different 

 volumes of gas on two equal surfaces of glass ; or, in other 

 words, that two equal surfaces of glass may have quite different 

 capabilities of absorbing a gas at a given pressure, o wing- 

 apparently to want of uniform porosity. A curve drawn for 

 a set of readings is, therefore, not to be depended upon. 

 Indeed, with some gauges, the readings are so erratic that it 

 is exceedingly difficult to draw a curve at all, and one of the 

 gauges made for these experiments had actually to be dis- 

 carded for this very reason. Such results as these put beyond 

 a doubt the total worthlessness of the gauge as a means of 

 measuring low pressures with air or with carbon dioxide. 



With hydrogen, however, the case is different. After the 

 apparatus was absolutely filled with hydrogen, the sets of 



Y2 



