Thermodynamic Properties of Air. 301 



values of e at ordinary temperature ( + 16°) have been de- 

 termined with great care by Amagat with the aid of a huge 

 mercury-manometer. 



Consider now a closed vessel, capacity s, charged with 

 compressed air. under a pressure p, at t degrees. Since a 

 unit volume of air, exerting the pressure of one atmosphere 



(at + 16°) , gets reduced to v — — when submitted to p atmo- 

 spheres (by the last formula), then we infer that the air-mass 

 compressed in the closed vessel when liberated and submitted 



to unit-pressure would occupy the volume — ; or, in the 

 normal condition, when cooled to 0°, the volume : 



6(1+7*) 



It will be seen now that the apparatus described in the fore- 

 going paragraphs may be used also as a manometer, recording- 

 pressures to be calculated by the formula : 



M(l + 7 

 V = *— s > 



provided that t equals that temperature for which the values 

 of e were determined, or at any rate that the difference be 

 not too great. For this reason one of the bulbs of my appa- 

 ratus was always immersed in a water-bath at + 16°, together 

 with a stirrer and a mercury-thermometer divided in -fc of a 

 degree. Since the temperature r of the space a differed but 

 slightly from + 16°, we may write with sufficient accuracy : 



A(l+7*) ■" ,k\ 



r= 6 STV- (5) 



The method chiefly employed till now for measurements of 

 high pressures consists in the use of manometers charged 

 with a constant quantity of gas. I thought it interesting to 

 compare this method with the constant-volume method used 

 by myself. For this purpose I connected my apparatus with 

 a gas-manometer, charged at first with dry air, afterwards 

 with pure dry nitrogen. From a large number of comparisons 

 at various pressures, there resulted a slight but systematic 

 difference of results : the values of pressures, as determined 

 by my method, were constantly less, by several tenths per 

 ' Phil Mag. S. 5. Vol. 41. No. 251. April 1896. Y 



