Thermal Properties of Isopentane, 209 



should be zero if Boyle's law were correct, maybe considered 

 as measuring the error of Boyle's law, and we may term it 

 44 the departure from Boyle's law " for the particular volume 

 and temperature in question. It then appears that the 

 departure from Boyle's law in the case of isopentane bears a 

 constant ratio to the departure for normal pentane at the same 

 volume and temperature*. This simple law is found to yield 

 numerical results so nearly in accordance with the experimental 

 facts that the error incurred by assuming the law as entirely 

 correct is comparable with the errors of experiment. 



We will next proceed to describe our method of testing the 

 law. Take any volume and temperature, v and T say ; and 

 let p H and p t denote the pressures of normal pentane and of 

 isopentane respectively at the said volume and temperature. 

 Then the departure from Boyle's law for normal pentane is 

 HT — p n v, while the departure from Boyle's law for isopentane 

 is RT— p t v. The proposed law tells us that 



,,,,. ^ — = a constant 



111— pnV 



= X, say. 

 Hence we easily obtain 



p lV = \j> n v+{l-\)RT. 



By examining the experimental data in the neighbourhood 

 of the critical point of normal pentane, it was conclude:! that 

 X might conveniently be put equal to '9463. By the last 

 formula it was then possible to calculate p t v for any volume 

 and temperature whenever p n v for the same volume and 

 temperature was known. In this way a large number of 

 values of p t v were found for the isothermals 280° C, 240 3 C, 

 200° C, 160° C, 120° C. These were plotted in a diagram 

 against v~§, and the dots so drawn distinguished by means of 

 a cross. The values of p t v found by direct experiment for the 

 same temperatures were also plotted against r~*, and the 

 resulting dots distinguished by means of a small circle. As 

 both sets of dots were very numerous, and occurred at small 

 intervals of u~^we were well able to judge by the eye whether 

 the curve determined by the one set might be considered as 



* The connexion between chemical problems and the departure from 

 Boyle's law lias also been investigated by M. Daniel Berthelot. (.See 

 Comptes Rendm, 1898, cxxvi. pp 954, 1030, & 1415.) ^ His immediate 

 object, however, was different from ours. He confined himself to 

 studying how the departures from Boyle's law might be allowed for in 

 deducing the true molecular weights of substances by physical methods ; 

 and he nowhere pays special attention to isomeric bodies. 



Phil. Mag. S." 6. Vol. 2. No. o\ Aug. 1901. P 



