CHEMISTRY: A. W. C. MENZIES 
561 
Since the values thus found for the vapor pressures of mercury are 
obtained, broadly, by subtraeting the reading of the smaller and less 
accurate hot gauge from those of the larger and more accurate cold 
gauge, it will be seen that the greatest precision (so far as gauge readings 
are concerned) is to be expected when the vapor pressure most closely^^ 
approaches P. This is borne out by the results as tabulated. 
It is evident that the results found agree with those calculated almost 
as closely as can be expected, even having regard to the temperature 
error (0.1°) alone. For this reason one may state, meantime, that the 
equation (R) may be applied, without alteration of its constants, to 
give the vapor pressure of mercury over the range 120° to 250° as well 
as from 250° to 435°, as previously found. A table of the values given 
by this equation for every two degrees from 0° to 458° has already been 
published.^ Further experimental extension of the temperature range, 
as well as greater precision, may be attempted later, should facilities be 
available, either by this or else by another method shortly to be described. 
It is, perhaps, of interest to consider the upper temperature limit of 
this vapor pressure curve, namely the critical temperature of mercury. 
This has been the subject of much speculation.^^ Recently, in 1916 and 
1917, J. J. van Laar, employing certain theoretical considerations, has 
computed 969°C . and later 899°C . f or the critical temperature ; while, in 
1918, E. Aries, from other considerations, arrives at 1077°C. In such 
cases, a little experiment is worth much theory. At the Minneapolis 
meeting of the American Chemical Society in 1910, I described^^ 
some experiments in which I had heated mercury in a thick-walled 
capillary tube of quartz glass of 0.2 mm. bore, and continued to see, 
through a telescope, liquid and gas phases distinct up to 1275°C. At 
this temperature the meniscus disappeared; but this was found to b.e 
due, not to the attainment of the critical temperature, but to the slight 
blowing out of the quartz capillary with consequent enlargement of its 
volume. The tube was never ruptured, but the very viscous hquid quartz 
glass had yielded somewhat to relieve the rather high pressure of several 
hundred atmospheres. In 1912, Koenigsberger,^^ using a quartz capil- 
lary of dimensions very similar to mine, reported that he had reached the 
critical temperature about 1270°C; but I was able to point out^^ the cause 
of Koenigsberger's mistake. Owing to lack of facihties, I have not been 
able to develop this work further; but this has now been done, by the 
same capillary quartz tube and telescope method, by Bender^^ at Frei- 
burg, who finds that the critical temperature lies between 1500° and 
1600°C. 
