130 
ON SOME OF THE PROPERTIES OF WATER AND OF STEAM. 
first series, 149*1, is a little more than twice as great as the largest volume in the 
second; therefore, as the tube was cylindrical, the surface on which condensation 
could take place was about twice as great. If the condensation is not much affected 
by the rise of pressure from 194 5 millims. at the largest volume of the first series to 
42’05 millims. at the largest volume of the second, and if the high vapour-density is 
entirely due to condensation, we should expect the error to be about half as great 
in the second as in the first series, and this is approximately the case. It will be 
noticed that the weight of the water in both sets is nearly the same. 
The tube was again heated to 100°, and the external pressure so arranged that the 
volume of vapour remained about 56 cub. centims. After cooling, the tube was 
again heated to 40°, and a reading was taken with the following results :— 
Tempera¬ 
ture. 
Pressure. 
Volume. 
Vapour- 
deusity. 
o 
millims. 
c.c. 
40 
52-3 
54-87 
10-00 
The result is not much lower than at the corresponding volume in the last 
experiment, but the gain of surface was not very great, and the pressure, 52*3 millims., 
was very near the condensing pressure, 54*9 millims. ; and it is to be noticed that 
the vapour-density rises rapidly as this pressure is approached. At the same pressure 
in the first series, the vapour-density observed was 1047—a much higher value, 
The experiments show that the high vapour-densities are due to condensation on 
the walls of the tube, and that the error due to this cause may vary considerably, 
especially at pressures near the true vapour pressures. 
Similar experiments were not made at high pressures, but on constructing isothermal 
curves, it is seen that as the volume diminishes, and as the vapour-pressure is 
approached, the curves instead of cutting the vapour-pressure line so as to form an 
angle, as is the case with the other liquids which we have examined, gradually turn 
and run nearly parallel to the vapour-pressure line at a somewhat lower pressure. 
When a considerable amount of liquid has condensed, the true vapour-pressure is 
reached. This behaviour is analogous to that of a mixture of a condensible vapour 
with a gas like air; but it caunot be attributed to such admixture, for the last 
bubble of steam left on raising pressure w r as easily and quickly absorbed, and without 
any perceptible rise of pressure. Its cause must, as already explained, lie in the 
adhesion of the vapour to the glass, causing condensation before the true vapour- 
pressure is reached. 
