Youne— Vapour-pressure of a pure Liquid at constant Temp. 101 
88 determinations above 180°, the mean observed fall in pressure 
with expansion is less than 1 in 2,000; and for the whole 
240 observations, it is less than 1 in 6,000. Taking all the stable 
substances together, the mean fall in pressure for the 257 de- 
terminations up to 180° is less than 1 in 10,000; above 180°, the 
fall for 236 observations is 1 in 1,450; and for the whole 
493 determinations, it is only 1 in 2,700. 
There are 21 substances on the list; frequently more than one 
specimen was employed; and, as a matter of fact, this part of 
the investigation involved the preparation and purification of 
82 specimens, and the filling of the experimental tube 45 times.* 
Assuming that the vapour-pressure is really independent of 
the volume, the theoretical value of A, 0:000, could only have 
been obtained if each of the 32 specimens had been absolutely pure 
and stable, if in each of the 46 fillings air had been completely 
expelled from the liquid and from the walls of the tube, and if 
there had been no vaporisation of mereury during any of the 
493 determinations (or if the partial pressure of the mercury- 
vapour had been the same at the largest and smallest volumes in 
every case). 
With a number of substances—the two pentanes, di-isobutyl, 
benzene, methyl formate, ethyl acetate, and, perhaps, stannic 
chloride—success seems to have been fully attained, and for 
several others the differences up to 180° are inappreciable; a 
complete absence of errors in the whole series of determinations 
could, however, not be expected. 
1 In this number a few cases are not included in which the liquid was found 
to contain air, and one in which a trace of non-volatile impurity was accidentally 
introduced. 
[Group II. 
