458 On Evaporation and Dissociation. 



the value of b in our equation p = bt — a at the critical volume. 

 Until a mathematical expression is discovered, representing b 

 as a function of volume, the only means at our disposal for 

 ascertaining the true volume corresponding to b is by inter- 

 polation of the original curve by which the values of b were 

 smoothed. The common point of intersection of the three 

 pressure-temperature curves has been shown on p. 455 to lie 



at the temperature 193°*83. The value of — on the vapour- 

 pressure curve at this temperature, calculated by the formula 

 of which the constants have already been given, is 405 millim., 

 which is also the value of b at that temperature. The volume 

 corresponding to this value is 4*06 cub. centim. per gram ; 

 and the specific gravity of ether at its critical point is there- 

 fore 0-2463. 



Unfortunately, Dr. Andrews's measurements of the constants 

 of carbon dioxide are not sufficiently numerous to warrant an 

 attempt to obtain the critical temperature, pressure, and volume 

 by this method. The critical volume of carbon dioxide is 

 evidently less than 3 cub. centim. per gram ; but the values 

 of b below that quantity are unascertainable. It may be 

 noticed that the curves below volume 3 are inserted in broken 

 lines, showing a probable course ; but no reading from them 

 would be permissible. 



The two liquids, ether and carbon dioxide, have no chemical 

 analogy with one another ; and we therefore feel justified in 

 concluding that the law which is the subject of this paper is 

 generally applicable to all stable substances. We have, how- 

 ever, other less complete data for methyl and ethyl alcohols, 

 which, so far as they go, are confirmatory of the results 

 described. We have also data available for the examination 

 of acetic acid — a substance which differs from those men- 

 tioned, inasmuch as it undergoes dissociation when heated ; 

 and we hope shortly to be able to communicate an account of 

 its behaviour. 



Professor Fitzgerald, to whom we gave a short account of 

 this law, has recently communicated to the Royal Society a 

 paper in which its thermodynamical bearings are considered. 



Bristol, 12th February, 1887. 



