172 Prof. Sydney Young on the 



The degree of deviation from constancy of the various 

 ratios is indicated by the values given at the foot of each 

 table, representing the ratio of the highest to the lowest ratio 

 in each vertical column. These values are collected together in 

 Table XVI. (p. 171), but forthe sake of greater clearness they are 

 given as percentage-differences between the highest and lowest 

 ratios in each case, the lowest ratio being always taken as 100. 



It will be seen that the halogen derivatives of benzene show 

 very much smaller deviations from constancy than the other 

 compounds ; and I have previously suggested (Trans. Chem. 

 Soc. 1889, p. 486 ; 1891, p, 125) that the generalizations of 

 Van der Waals do hold good for these bodies, in which case 

 the deviations of the ratios from constancy may be taken as 

 an approximate measure of the experimental errors to be 

 expected with other compounds. It must, however, be ad- 

 mitted that the mean ratios of the absolute temperatures differ 

 sensibly from the mean ratios of the molecular volumes of 

 liquid, though they should be identical if Van der Waals's 

 generalizations were strictly true, while the errors in these 

 determinations are probably very small. Indeed, in the com- 

 parison of bromobenzene with fluorbenzene the difference 

 between the mean ratios amounts to 1*5 per cent. (Table XXI.), 

 while the difference between the highest and lowest ratio in 

 either case is only 0*2 per cent. (Table XVI.). It is pro- 

 bable, therefore, that the generalizations of Van der Waals are 

 not quite, though very nearly, true for these bodies. 



The deviations from constancy are smallest in the compa- 

 risons of the absolute temperatures at corresponding pressures, 

 and of the molecular volumes of liquid at corresponding pres- 

 sures and temperatures ; and this is no doubt to be expected, 

 since the ratio of the highest to the lowest absolute tempera- 

 ture or volume of liquid for any one substance does not 

 amount to 3 : 1, whereas the ratio of the highest to the lowest 

 pressure is from 1700 to 10000 : 1, and of the highest to the 

 lowest molecular volume of saturated vapour from 3*7 : 1 in the 

 case of iodobenzene, where the range is limited, to 2670 to 1 

 with acetic acid. It is also impossible to determine the 

 volumes of saturated vapour with anything like the same 

 degree of accuracy as the volumes of liquid, the liability to 

 error by the new method being especially great at the lowest 

 temperatures. 



The eleven substances which are compared with fluorben- 

 zene may be arranged in three groups as below : — 



1. Chlorobenzene, Bromobenzene, Iodobenzene. — As already 

 pointed out, the generalizations of Van der Waals are very 

 nearly true for these bodies when compared with fluorbenzene. 



2. Benzene, Carbon tetrachloride, Stannic chloride, Ether. — 

 With these substances the generalizations may be taken as 



