418 PROCEEDINGS OF THE AMERICAN ACADEMY, 



Volumes, together with the correction for the expansion of the glass, 

 can easily be measured with the required accuracy. The temperatures, 

 also, of melting ice and of steam, are certainly known to within 0.03° C. ; 

 but in the pieces of apparatus previously used, not all of the gas which 

 was being measured has been at the temperature of the bath, except in 

 some constant volume experiments. In the apparatus of Regnault,* the 

 bulb which contained the gas to be measured was connected by a capil- 

 lary tube to a graduated cylinder, where the expansion was read by the 

 rise and fall of the mercury which closed the lower end of this cylinder. 

 Some doubts may be entertained as to the accuracy of the correction to 

 be applied for the temperature of the gas in the capillary tube, and also 

 as to the perfect equality of temperature throughout the water bath which 

 surrounded the graduated cylinder. 



With the exception of Magnus f (who used an apparatus, modelled 

 after Gay-Lussac's, which proved very unsatisfactory because of leakage), 

 other investigators, as MendeleefF, | Andrews, § Kueuen and Randall, || 

 and Callendar, ** have employed methods similar to that of Regnault. 

 Callendar, instead of correcting for the amount of gas outside the bath, 

 introduced a system of automatic compensating tubes. 



The attainment of the required accuracy in the reading of pressures 

 is a far more difficult matter than the problem offered by constancy of 

 temperature. Regnault states his belief that the height of the barometer 

 can be read only to one-tenth of a millimeter. Chappuis, tt however, 

 has devised an apparatus, open to the atmosphere, for measurement of 

 increased tension in constant volume, by which he makes record of preS' 

 sures to the thousandth of a millimeter. Of course, this extra care 

 would have been needless if the barometer had been at fault. Cal- 

 lendar tX has overcome the difficulty by using a constant artificial at- 

 mosphere, attained by keeping a large gas reservoir packed in ice, and by 

 using sulphuric acid instead of mercury in the manometer. The very 

 recent work of Travers §§ also attained great uniformity of condition. 

 Unfortunately none of these investigators have made any extended 



* Memoires de I'Academie des Sciences, 21, 112-120 (1847). 



t Poggendorf Annalen, 55, 1-27 (1842) ; 57, 177-199 (1842). 



t Abstract in B., 8, 1681 (1875) ; B , 9, 1311 (1876); B., 10, 81 (1877). 



§ Pliilosophical Transactions, 166,421-449 (1876). 



II Proceedings of tlie Royal Society, 59, 00-65 (1895). 

 »* Proceedings of the Royal Society, 50, 247-251 (1891). 

 tt Travaux et Memoires du Bureau Internat. des Poids et Mesures, 16 (1888). 

 tt Proceedings of the Royal Society, 50, 247-251 (1891). 

 §§ Abstract in Proc. Roy. Soc, 70, 484 (1902) ; also (Miem. News, 86, Gl. 



