664 Mr. H. W. Morley on the Vapour-Pressure 
is very nearly saturated. If, at a given temperature, the 
liquid is brought to the same level in the two arms of a 
differential manometer containing only the saturated vapour 
of the liquid in one arm, and containing some gas in the 
other, a measurement of the pressure of the gas determines 
that of the saturated vapour. The uncertainty of the observa- 
tions was 0:02 mm. An interpolation formula was computed 
from the observations at temperatures from 89° to 206°, and 
it would be difficult to improve his results until the very 
small quantities which represent the vapour-pressure of 
mercury at ordinary temperatures are made to depend, not 
on a somewhat remote extrapolation, and an extrapolation from 
values whose errors are many times as large as the quantities 
sought, but on direct measurement. 
The measurements of Ramsay and Young are also very 
satisfactory for temperatures above 100°. The ratio of the 
absolute temperatures of water and of mercury having the 
same vapour-pressures varies so regularly, that if it is deter- 
mined for a few temperatures, it is known for all temperatures. 
From this ratio and from the well-known vapour-pressures of 
water, they computed the vapour-pressure of mercury for 
temperatures from 135° to 520°. Below 135° the method 
could not be used, because the corresponding vapour-pressures 
of water are not well known; they therefore determined an 
extrapolation formula from the values for 160°, 220°, and 
280°, from which they computed values for temperatures 
down to 40°. 
We have, therefore, a determination by Hertz covering 
the interval from 0° to 100°, and one by Ramsay and Young 
for the interval from 40° to 100°. From 50° to 100° the mean 
difference between the two values is only 6°5 per cent. The 
mean difference between the values of the same experimenters 
from 120° to 220° is 8°6 per cent.; so that the agreement 
from 50° to 100° is satisfactory. But at 40° the difference 
is 27 per cent. 
The observations of van der Plaats were made at the 
temperatures in question, and were numerous and careful. 
He passed a known volume of an inert gas through pure 
mercury in such a way as to saturate it with the vapour of 
mercury. The mercury was then collected by absorption and 
weighed. It is not easy to suggest a cause tending to 
make values obtained in this way larger than the truth. But 
it was and is impossible to accept them. The results at 0° 
and 10° are twenty-five and sixteen times as large as those 
of Hertz. Hertz found it impossible to detect the vapour- 
pressure of mercury below 50°. But if the values of van der 
