t 435 ] 



LI. On Evaporation and Dissociation. — Part VI.* On the 

 Continuous Transition from the Liquid to the Gaseous State 

 of Matter at all Temperatures. By William Ramsay, 

 Ph.D., and Sydney Young, D.Sc] 



[Plates VII., VIIL, IX., & X.] 



IT was proved by Boyle, in 1662, that the volume of a gas, 

 provided temperature be kept constant, varies inversely 

 as the pressure to which it is subjected ; this relation may be 



n 



expressed by the equation p=~, or pv = constant, where p 



and v respectively stand for pressure and volume. But sub- 

 sequent experiments by Van Marum, Oersted, Despretz, and 

 others showed that certain gases do not obey this law ; and it 

 is now well known that Boyle's statement is only approximate; 

 for it has been proved by experiment by Regnault, batterer, 

 and more recently by Amagat, that no gas, under high pres- 

 sures, is diminished in volume in inverse ratio to the rise of 

 pressure. Indeed Boyle's law could hold only on the assump- 

 tion that the actual molecules of matter possess no extension 

 in space and exert no attraction on each other. A gas, such 

 as hydrogen, at low pressures, and consequently at large 

 volumes, fills a space very great when compared with the 

 space occupied by the actual molecules ; and these molecules 

 are comparatively so distant from one another, that the attrac- 

 tion which they mutually exercise is inappreciable. But, on 

 compression, the actual space occupied by the molecules bears 

 an increased ratio to the space which they inhabit ; and, by 

 their approach, the attraction which they exert is also increased. 

 The gas, then, deviates appreciably from Boyle's law. 



Gay-Lussac, in 1808, enunciated the law that the volumes 

 of all gases increase by a constant fraction of their volume at 

 0° for each rise of 1° in temperature. It was subsequently 

 ascertained by Magnus, and confirmed by Regnault, that cer- 

 tain gases deviate from this law, expanding more rapidly than 

 others. Such gases, as a rule, are at temperatures not far re- 

 moved from those at which they condense to liquids ; that is, 

 their volumes are comparatively small, and the actual size of 

 the molecules and their mutual cohesion begin to manifest 

 themselves within the range of experimental observation. 

 Again, it is evident that no gas can perfectly follow Gay- 

 Lussac's law ; but the larger the volume it occupies the 

 smaller is the influence of the disturbing factors. The usual 



* Parts I. and II., Philosophical Transactions, part i. 1886, pp. 71 and 

 123; Part III., ibid, part ii., 1886, p. 1; Part IV., Trans. Chem. Soc. 

 1886, p. 790 ; Part V., in the hands of the Koyal Society. 



t Communicated by the Physical Society : read February 26, 1887, 



2 G2 



