146 Journal of the Mitchell Society. [Dec. 



SP 



THE -^7p AT THE CRITICAL TEMPERATURE. 



In the second paper we commented upon the difficulty of 



. . 8P 



obtaining- correctly the -k-=- from Biot's formula near the 



critical temperature. We entirely overlooked the fact that 



the constant b of the equation, p = bT — a, proposed by 



8 P 

 Profs. Ramsay and Young-, was also a -5-=- and that at the 



o 1 



8 P 

 critical temperature (but only at that point, see p. 166) the -«-= 



of the two equations became identical. In work done to 



8 P 

 establish the truth of the equation, p = bT — a, the -^7p 



was obtained for three substances at volumes practically iden- 

 tical with the critical volume. The results are such as to 

 confirm entirely and quantitively the view that the diver- 

 gences at this point in p were due to the Biot formula used to 



obtain the -pp. 



Thus isopentane gives at the critical temperature (volume 



8 P 

 4.266) the y~- = 367.8, calculated from Biot's formula. Prof. 



Young, found from drawn isochors (at the volume 4.3) the 



value 397, and a calculated value of 407. Therefore Biot's 



formula gives results about ten per cent, too low, an amount 



just sufficient to explain the variations in // near that point. 



For normal pentane Biot's formula at the critical volume, 



Sp 

 4.303, gives 364,8 for the ^-^-. Mr. J. Rose-Innes and Dr. 



o 1 



Young 2 obtained 407.3 from drawn isochors. Biot's formula 

 therefore shows results too low by about eleven per cent., an 

 amount which is sufficient to explain the low values obtained 



iProc. Phys. Soc, 1894-95, p. 650. 

 2 Phil. Mag., April, 1899. 



