CHEMISTRY: A. W. C. MENZIES 
559 
by the above equation. This corresponds to temperature discrepancies 
of 1° at 0° and 0.8° at 50°. One regrets that such an elegant method is 
not more direct. Later, Knudsen^ appHed his admirable 'absolute 
manometer/ under certain difficulties, to measure this vapor pressure 
from —10° to +24.4°, and obtained results ranging from about 5% 
higher to about 10% lower in pressure, at the respective ends of this 
range, than the values furnished by our equation. 
In 1913 Villiers^ obtained a series of values from 60° to 100° which 
ran higher than ours by 2.7% of our calculated pressure at 60° to 7.5% 
at 100°. 
In 1914, Haber and Kirschbaum,^ adopting a suggestion of 
Langmuir's,^° measured the damping of a quartz fiber vibrating in 
mercury vapor at 20°, and obtained a single value by an independent 
method. This value Kes 3.8% in pressure below that given by our 
equation, or, otherwise, differs by 0.41° for the same pressure. 
In glancing over the results obtained by these various workers, here 
stated baldly without criticism, one is inclined to believe that our equa- 
tion of 1910 continues to stand the test of time by averaging their 
scattered results as well as can be hoped for. At the same time one is 
impressed by the pre-war activity in this field, especially in the lower 
temperature ranges. It appeared, therefore, to be of interest to obtain, 
by a method as direct as possible, new measurements below 250°, the 
lowest point of the range studied in 1910; and, in the work here described 
I have extended that range "from 250° to 120°. 
As the static isoteniscope is not especially well suited for the measure- 
ment of the rather low pressures with which we are here concerned, an 
entirely new plan was adopted. Two McLeod gauges of suitable capac- 
ities were calibrated and sealed to a central pressure reservoir. The 
gauges, which were operated by gas pressure and without rubber con- 
nexions, contained purified mercury. The reservoir and connected 
gauges could be charged with dry hydrogen at any pressure desired, and 
sealed off by mercury. The smaller of the two gauges was completely 
immersed in a riotously stirred oil bath whose temperature was measured 
by a mercurial thermometer whose thread was all submerged. The 
corrections of this thermometer were known^^ to tenths of a degree. The 
other, larger, gauge and the reservoir were maintained in baths at room 
temperature. On operating the two gauges simultaneously, different 
pressure readings were obtained, due to the condensation of the mercury 
vapor that contributed part of the total pressure in the hot gauge; and 
from this difference in reading the vapor pressure of mercury at the 
