Young — Vapour-Pressures, ^'c, of Thirty Pure Substances. 379 



of the curves at their upper extremities must liave been iucorrect ; aud it 



became advisable to recalculate the coustatits for Biot's formula. 



In the original paper' it was pointed out that for any two of the four 



halogen derivatives of benzene, the ratio of the boiling-points (on the 



T 

 absolute scale) was evidently the same at all equal pressures, or -— = constant. 



The values of the ratios of the absolute temperatures of fluorobenzene to 

 those of the three other compounds were first ascertained ; and the boiling- 

 points of fluorobenzene, at a series of even pressures, were then recalculated 

 from those of each of the other halogen derivatives. The mean of the three 

 recalculated temperatures and the observed temperature of fluorobenzene at 

 each pressure was taken to be the correct boiling-point at that pressure. 

 Finally, from the corrected boiling-points of fluorobenzene, those of the 

 other three substances were recalculated by means of the constants. 



Comparing these recalculated temperatures with the observed boiling- 

 points, it appeared probable that there were small errors, chiefly at the 

 highest temperatures. 



Taking these probable errors into account, new constants for Biot's 

 formula were calculated, and although the agreement between the re- 

 calculated and observed pressures up to 270° was not quite so good as before, 

 yet the calculated critical pressures were now satisfactory, notwithstanding 

 the great extrapolation — in the case of iodobenzene from 270° to 448°C. 



The calculations of Dr. Mills had indicated tlie probable existence of 

 errors — either of experiment or calculation — in several other cases, and in 

 some of them it seemed not unlikely that the constants for Biot's formula 

 might be at fault. The constants were tlierefore calculated again for the 

 following substances in addition to chlorobenzene, bromobenzene, and 

 iodobenzene : — fluorobenzene, ethyl ether, ethyl propionate, methyl butyrate, 

 and methyl isobutyrate. 



The whole of the data, pressures, specific volumes, &c., have been carefully 

 revised, corrections being made where necessary ; and the collected data are 

 given in the tables at the end of this paper. 



It has not, however, been thought necessary to republish the results of 

 determinations of the compressibilities of the liquids or unsaturated vapours, 

 as they are very voluminous and are not affected by the alterations in the 

 orthobaric volumes. These data are to be found in the following papers : — 



1. Ethyl Ether, Ramsay and Young, Traus. Eoy.Soc, clxxviii., p. 57, 1887. 



2. Methyl Alcohol, U. and Y., ibid., clxxviii., p. 313, 1887. 



» Trans. Chem, Soc, Iv., p. 486, 1SS9. 



