154 Prof. Sydney Young on the 



favourable, the notion of " corresponding " states has received 

 wide acceptance ; and, indeed, the generalizations regarding 

 corresponding temperatures, pressures, and volumes might 

 still be true even though the formula on which they were 

 originally based required some alteration. 



In order to study the relations, for instance, between the 

 specific volumes of different substances, determinations were 

 made in the first place at the same temperature, generally at 

 0° C. ; later on it was considered that the conditions would 

 be more uniform if the comparison were made at the boiling- 

 points of the substances under normal atmospheric pressure. 

 It is now, however, usually admitted that in order to obtain 

 the best results the volumes should be determined at " corre- 

 sponding' ' temperatures — that is to say, at absolute temperatures 

 proportional to the absolute critical temperatures of the various 

 bodies— or at their boiling-points under corresponding pres- 

 sures — the two methods of comparison being, according to 

 Van der Waals, identical. 



During the last four years I have been engaged in a research 

 on the vapour-pressures and specific volumes — both in the 

 liquid state and as saturated vapour — of the following sub- 

 stances : — benzene, fluorbenzene, chlorobenzene, bromoben- 

 zene, iodobenzene*, carbon tetrachloride and stannic chloride f ; 

 the vapour-pressures and specific volumes of methyl J, ethyl §, 

 and propyl alcohols || , and of ethyl ether H have also been 

 determined by Dr. Ramsay and myself. We obtained the 

 same constants for acetic acid** up to 280°; and I have 

 recently extended the observations with this substance up to 

 the critical point (ibid. lix. p. 903). 



The methods employed for the determination of the vapour- 

 pressures of benzene and its halogen derivatives and for the 

 specific volumes of these bodies in the liquid state were, with 

 slight modifications, the same as those made use of by Ramsay 

 and myself in our researches on the alcohols and ether. 



Carbon tetrachloride, however, acts on mercury at high 

 temperatures, and stannic chloride renders it unfit for use — 

 though the chemical action is very slight — even at low tempe- 

 ratures ; a considerable alteration in the method of determining 

 vapour-pressures was therefore made (Trans. Chem. Soc. 

 lix. p. 917). 



For the determination of the specific volumes of stannic 

 chloride in the liquid state and of all the substances in the 

 condition of saturated vapour an entirely different method was 



* Trans. Chem. Soc. lv. p. 486, and lix. p. 125. f Ibid. lix. p. 911. 



% Phil. Trans, clxxviii. A, p. 313. § Ibid. 1886, part i. p. 123. 



|| Ibid, clxxx. p. 137. % Ibid, clxxviii. A, p. 57. 



** Trans. Chem. Soc. xlix. p. 790. 



