208 Mr. J. Rose-Innes and Dr. S. Young on the 



r> 



the effect of moving electrified bodies still unanswered, at 

 least satisfactorily ; for example, the motion of a dielectric in 

 a uniform field ; also the reverse of the above experiment. 

 These problems are also to be attacked. 



It is a source of great satisfaction to me that the above 

 investigation was practically completed before the death of 

 Prof. Rowland. His advice and criticism throughout the 

 work were invaluable. 



To Prof. Ames also I am indebted for much valuable 

 advice. I also take this opportunity of thanking Mr. N. E. 

 Gilbert for his valuable assistance in the early part of tbe 

 work, and Messrs. J. T. Barrett and Jas. Barnes for their 

 help in taking the final readings. 



Physical Laboratory, 



Johns Hopkins University, 



June 10, 1901. 



XY. The Thermal Properties of Isopentane compared ivith 

 those of Normal Pentane. By J. Rose-Innes, M.A., JB.Sc, 

 and Sydney Young, I), So., F.R.S.* 



[Plate IV.] 



IN a paper read before the Physical Society in December 

 1898, attention was drawn to the great interest attaching 

 to the comparison of the isothermals of two isomeric sub- 

 stances (Phil. Mag. xlvii. p. 366). An experimental investi- 

 gation of the relations between the temperatures, pressures, 

 and volumes of isopentane had been carried out by one of us 

 some time previously ; and in the paper above referred to 

 the results of a similar investigation into the thermal properties 

 of normal pentane were given . A preliminary comparison was 

 instituted between the thermal data belonging to the two sub- 

 stances respectively, and the conclusions provisionally arrived 

 at were stated (Joe. cz£. pp. 366-367). "We have since under- 

 taken a more exhaustive examination of the experimental 

 results, and the conclusions now reached form the subject of 

 our present paper. 



If we could treat normal pentane and isopentane as perfect 

 gases, we should be able to write jw = RT, where all the letters 

 have their usual signification. In practice it is found that pv 

 changes as p diminishes along any isothermal, in such a way 

 that the limiting value of pv for p = is equal to RT ; but any 

 actual value of pv lies below RT within the limits of tempe- 

 rature of the experiments. The quantity RT— pv, which 



*. Communicated by the Physical Society ■ read May 10, 1901. 



