CHANGE FROM THE LIQUID TO THE SOLID STATE ON VAPOUR-PRESSURE. 477 
38. We have previously mentioned that in order to test the accuracy of our new 
method of determining vapour pressures, a careful set of determinations was made of 
the vapour-pressures of ice and water. The slight differences between the results 
obtained and those given in Pegnault’s tables were considered at the time to be due 
to experimental error, for they were so small as to appear insignificant. On comparing 
these results, however, with the curve calculated from theory, it was found that they 
much more nearly coincided with it, than with P,egnault’s curve. As they afford a 
striking confirmation of the results already given, they are here appended. 
Temperature (corrected). 
Pressure (observed). 
Pressure calculated 
from theory. 
Pressure calculated 
from Pegnault's formula. 
millims. 
millims. 
millims. 
- 2'6 
37 
3708 
3’75 
- 3-7 
3'2 
3-378 
3-45 
- 5-6 
2-85 
2-856 
2-96 
- 67 
275 
2-580 
2-72 
- 97 
1-95 
1-942 
2-13 
-111 
1-65 
1-688 
1-90 
-13-3 
1-45 
1-330 
1-60 
-15-2 
1-25 
1-066 
1-38 
-15-9 
1-00 
0-980 
1-29 
39. Description of the curves (Plate 19). 
No. I. Acetic acid. Curves drawn to follow the experimental results. 
(a) Vapour-pressure of liquid acid. 
(b) „ „ solid acid. 
No. II. Water and ice. 
(a) Curve for water calculated by PtENAULTS formula for pressures between 
0° and 100°. 
(b) Theoretical curve for ice, calculated from (a). 
Our results for water are given as coinciding with (a), for it was assumed that 
the temperature of the thermometer surrounded by water was correct. 
This was done, for the object was to show differences of temperature 
at certain points in the curve, rather than the absolute relationship 
between temperature and pressure at any one point. 
No. III. (a) Theoretical curve for ice. 
(b) Curve from Plegnault’s tables. 
(c) Curve for water as in (II.). 
The crosses denote the observations given in § 38 ; the circles the observations 
for ice given in § 33. 
40. In the foregoing pages it has been proved : (l) that with camphor and benzene, 
the former in a barometer tube, and the second in the “still,” the curve representing 
vapour-pressure of liquid above the melting-point is discontinuous with that of the 
solid below the melting-point: (2) that acetic acid shows this difference in a very 
