6 Prof. Sydney Young on a Corresponding" 



It was pointed out in the earlier paper on this subject 

 (Phil. Mag. xxxiii. p. 172) that the compounds investigated 

 might be arranged in groups, and this is confirmed by the 

 method of comparison now adopted. The twenty-two com- 

 pounds appear to fall into four chief groups : — 



I. Benzene and its halogen derivatives, carbon tetra- 

 chloride, stannic chloride, ether. 

 II. The three alcohols (methyl alcohol differing, however, 

 considerably from the others). 



III. Acetic acid. 



IV. The ten esters. 



Ratios of Absolute Temperatures at Corresponding Pressures 

 to Absolute Critical Temperatures.- — The ratios for the com- 

 pounds in groups I. are the lowest ; those for the esters are 

 mostly higher; acetic acid comes next in order; whilst the 

 values for the alcohols are much higher than for any of the 

 other substances. 



In the first group the differences are not great, but the 

 ratios for benzene and carbon tetrachloride are below, and 

 those for stannic chloride and ether at low pressures are 

 somewhat above the average. The influence of molecular 

 weight and constitution on the ratios for the esters is fully 

 discussed in the paper by Mr. Thomas and myself (Trans. 

 Chem. Soc. lxiii. p. 1252), and it will be sufficient here to 

 mention that there is a marked rise as the molecular weight 

 increases. This is not the case, however, with the ratios of 

 the volumes of liquid or of saturated vapour to the critical 

 volumes. All the ratios appear to depend to some extent on 

 the constitution of the esters. 



Ratios of Volumes of Liquid at Corresponding Pressures to 

 Critical Volumes. — The differences in this case are small ; in 

 general the ratios for acetic acid are the highest, and those 

 for the esters the lowest. At the lowest pressures the values 

 for the alcohols are higher than for the esters, but at high 

 pressures they are lower than for any of the other substances, 

 and this is especially the case with methyl alcohol. 



Ratios of Volumes of Saturated Vapour at Corresponding 

 Pressures to Critical Volumes. — The grouping of the com- 

 pounds is well seen in this case, the differences being naturally 

 most marked at low pressures. Acetic acid stands quite 

 alone with the lowest ratios, whilst the alcohols, and especially 

 methyl alcohol, have much higher ratios than the other 

 compounds. Of the remaining substances the esters have 

 higher values than the members of the first group. 



There is little doubt that the low ratios for acetic acid are 

 due to the existence of complex molecules in the saturated 



