1880.] 



On the G)*itical Point. 



203 



when in solution ; when dry, it is a red powder ; and when alcohol 

 containing eosine in solution is placed in one compartment of such a 

 tube, the other having its sides coated with a thin film of dry eosine, 

 no fluorescence takes place till time has been given for diffusion. In 

 fact, the rate of diffusion may be approximately measured by the 

 increase in intensity of fluorescence in that half of the tube originally 

 containing the vapour of the solvent. 



Three facts appear, therefore, to be demonstrated : — First, that at 

 the temperature at which the meniscus of a liquid disappears, and at 

 temperatures above that point, but not far removed from it, an in- 

 crease of pressure is required to cause diminution of volume, com- 

 parable with that necessary to compress a liquid at a temperature 

 somewhat below that at which its meniscus disappears ; second, that 

 when a mixture of liquid and gas is maintained at a certain volume, 

 the expansion of the liquid on raising the temperature, so long as it 

 is possible to distinguish liquid from gas, points to the ultimate 

 occupying of the whole space by liquid at temperatures above which 

 the meniscus becomes invisible ; and third, that under such circum- 

 stances the liquid retains its solvent powers, while the gas is incapable 

 of dissolving a solid. All these facts point to the conclusion, that at 

 or under such a volume the matter is really in the liquid state, whereas 

 at a greater volume, the matter must be viewed as consisting at least 

 partially of gas. 



No direct experiments have been made with a view to ascertaining 

 whether heat is evolved when a gas is converted into liquid by pressure 

 at such high temperatures. I hope to be able to execute some experi- 

 ments which promise some satisfactory answer to the question. 



It now remains to consider the condition of raising a mixture of two 

 liquids to such a temperature that the meniscus disappears. Isotherms 

 for a mixture of benzene and ether are given on Table C, and graphi- 

 cally represented on the diagram. 



The first isotherm at the temperature 215 0, 6 is at least 20 degrees 

 above the temperature at which the meniscus of pure ether disappears, 

 and yet the tension of ether vapour does not markedly appear. If 

 that curve be constrasted with Curve No. VI for ether alone at 207°' 1, 

 some 8 degrees lower, it is noticeable that diminution of volume in the 

 latter case is accompanied by a much greater rise of pressure than 

 in the former. The presence of benzene, therefore must exert some 

 marked influence on the pressure exercised by ether vapour, and 

 cause the mixture to behave to some extent as a single substance. But 

 at higher temperatures the influence of the ether becomes more 

 marked, and at the temperature 240°" 7 the critical point is nearly 

 reached. The tube then appeared full of mist, till the volume 40 was 

 reached, when the mist disappeared, and the tube appeared full of 

 liquid. The pressure at which ether becomes critical is situated about 



