202 



Dr. W. Ramsay. 



[Dec. 16, 



based, lead to an opposite conclusion when correctly explained. The 

 heat in these experiments was imparted to the tubes through a large 

 block of copper, in grooves in which the tubes were placed. I have no 

 doubt that the temperature of the copper block represented accurately 

 that of the tubes ; but not that of the liquid contained in the tube. 

 When the temperature of the tube is raised, however slowly, evapora- 

 tion of the liquid in the tube ensues, and time is required for evapora- 

 tion. During this time, the temperature of the copper block is rising ; 

 and with a tube insufficiently filled, it appears necessary to allow a 

 longer time for evaporation, than with one containing more liquid. 

 On reading temperature when the meniscus vanishes in the former 

 case, it will appear higher than in the latter. The difference is ac- 

 counted for by the fact that during evaporation, the temperature of the 

 copper block is continually increasing. This would point to the con- 

 clusion that a considerable amount of heat must be absorbed, even 

 under such circumstances, in order to convert liquid into gas, and 

 thus that the latent heat of vaporisation is still considerable, even at 

 temperatures so near the critical point. 



The critical change for benzene occurs at a temperature of 291°" 7, 

 and at a pressure of 60*3 to 60*5 atmospheres, and the isotherm at this 

 point is represented in Curve VII of Table A. Between the volumes 

 of 90 and 60 it is evident that gas is being compressed, for pressure 

 rises regularly. But from 60 to 38 the rise of pressure required to 

 produce diminution of volume is smaller proportionately to the effect 

 produced, and after the volume 38 has been reached the pressure rises 

 much more rapidly. And, on referring to Table D and to the accom- 

 panying diagram, it is also to be remarked that, when the volume is 

 60, or greater, the curve of equivolume represents the total evapora- 

 tion of the substance. With a volume of 50, that particular propor- 

 tion of gas to liquid appears to be reached at which evaporation 

 almost exactly balances expansion, and neither total evaporation nor 

 total expansion takes place, but the ratio of gas to liquid appears to 

 remain unaltered. With volumes of 40 and 35 divisions of the tube 

 total expansion takes place, and the tube, above a certain temperature, 

 must become filled with liquid. Above the temperatures represented 

 in Table D, it becomes impossible to distinguish liquid from gas, for 

 the meniscus has disappeared. But I can see no reason for assuming a 

 particular state of matter under such circumstances. In the prelimi- 

 nary note already referred to, I described an experiment, in which 

 liquid and gas were kept separate for some time by means of a capillary 

 tube, and in which even after the meniscus of the liquid had disappeared, 

 a solid, adhering to the wall of the tubes containing presumably only 

 gas, refused to dissolve. This experiment has been frequently repeated, 

 with identical results ; it is, perhaps, most striking when the fluorescent 

 colouring matter, eosine, is used as the solid. Eosine fluoresces only 



