CHANGE FROM THE LIQUID TO THE SOLID STATE OH YAPOUR-PRESSURE. 469 
19. As these results were still capricious the tube was filled by a third method, which 
ensured complete absence of air. The shape of the barometer-tube was modified, as 
shown in c, Plate 18, fig. 1. The tube, after exhaustion, was filled to within about an 
inch of the branch, and boiled out as usual. Air was then admitted, and about 1 cubic 
centimeter of acetic acid was introduced. The tube was again exhausted, and the 
acetic acid was gently warmed, so that its vapour might expel the last traces of air. 
While the pump was in action, mercury was admitted, the tube being placed in such 
a position that the acetic acid rose into the closed end of the tube near the junction. 
Disconnecting from the pump, the tube was inverted in mercury, when the acetic acid 
rose to its upper end. The absence of air was so complete that the liquid adhered for 
some time to the top of the tube, the mercury standing at several centimeters above 
the barometer level, and falling only after being violently shaken. This series includes 
only a few measurements below the melting-point, the majority being taken at higher 
temperatures for a different purpose. 
20. Series III. 
Temperature. 
Pressure. 
State. 
millims. 
14-95 
8-5 
Liquid 
4-30 
4-5 
Solid 
10-80 
6-7 
5 J 
In this series it was found impossible to prevent the solidification of the acid below 
the temperature 14°‘95, and as the results with the solid were still capricious the 
experiments were not continued. It may be mentioned, however, as a proof that no 
air was evolved during solidification, that after the acid in contact with the mercury 
had frozen, the mercury adhered to the solid even after lowering the level of the 
mercury in the trough. 
21. It may be remembered that Pegnault attributed his discordant results to the 
presence in his acetic acid of water, or of acetone, the former causing too low, the 
latter too high vapour-pressures. The acid which we used for these experiments was 
a portion of a stock of glacial acid, obtained for laboratory purposes several years ago. 
The liquid portion had been poured off from time to time, as required for laboratory 
use; a very complete series of fractionations has thus unwittingly been carried out, 
and it is now so pure that, at temperatures slightly below its melting-point, it is com¬ 
pletely solid. This acid was fractionated; a portion containing water came over 
below 1I9°T (pressure =750'3 millims.) ; but after this temperature had been attained 
the whole of the liquid distilled with absolute constancy. Fractions were taken as 
required from time to time, as the liquid was found very apt to absorb moisture. The 
purity of this acid is best guaranteed by the absolute concordance of the determinations 
of vapour-pressures by the distillation method already given, as well as of those at 
