
of Mercury at Ordinary Temperatures. 663 
These results, at first sight, seem to leave the whole matter 
very doubtful. The figures for 0°, for instance, differ in the 
ratio of 1, 25, 75, and 100. 
The case is altered when we examine critically the different 
series of experiments. Regnault’s values may first be dis- 
missed from consideration. His value for O° is simply 
assumed as differing trom the truth by a negligible quantity ; 
but the precision needed now is greater. Values for tempera- 
tures other than 0° depend on an interpolation formula 
computed from vaiues for 0°, 128°, 256°, 384°, and 512°. 
But Regnault had much difficulty with the observations at 
high temperatures, and the determinations were few and not 
concordant. Even if the true form of the function which 
expresses the relation between temperature and vapour- 
pressure were known, Regnault’s determinations would not, 
in the case of mercury, give values for the constants of the 
formula accurately enough for present needs; and he used 
only an empirical interpolation formula. Admirable as was 
his work, we have here to do with quantities which are smaller 
than the limits of accuracy which he claimed for such 
measurements. 
Hagen’s measurements may also be disregarded. He de- 
termined the difference of level between the two arms of an 
exhausted syphon-gauge, one of which was connected with a 
vessel kept at a temperature at which the vapour-pressure of 
mercury may be considered negligible. He made numerous 
experiments as nearly as convenient to the temperatures of 
0°, 50°, 100°, 150°, and 200°. By least square computations, 
he obtained the pressures corresponding to these precise 
temperatures ; from which normal values an interpolation 
formula was calculated. The determinations for the two 
higher temperatures were known to be in error on account 
of the rapidity of the evaporation from the surface of the 
mercury. The work satisfactorily proved that the values 
given by Regnault below 100° were much too large. But 
Hagen’s values cannot be accepted, if for no other reason, at 
least because his interpolation formula is too inconsistent with 
what we know of the behaviour of saturated vapours. The 
percentage increase of pressure due to an increase of tempera- 
ture by ten degrees diminishes with increasing temperature ; 
but Hagen’s table gives an increase of 46 per cent. between 
90° and 100°, while that from 0° to 10° is only 17 per cent. 
The experiments of Hertz were made with care, and their 
principle was satisfactory. When a liquid evaporates into a 
gas whose pressure is greater than that of the saturated vapour 
of the liquid, the vapour very near the surface of the liquid 
