UTAH ACADEMY OF SCIENCES 221 
governing this relation is capable of mathematical expres- 
sion and may be stated in words as follows: The logarithm 
of the vapor pressure is a linear function of the recip- 
rocal of the absolute temperature. That is, if we plot the 
logarithms of a set of vapor pressures of any substance 
against the reciprocals of the corresponding absolute 
temperatures, the curve connecting the various points 
in order will be a straight line. Since two points only 
are necessary to define a straight line we need know the 
pressures ‘corresponding to two temperatures only, in or- 
der to determine the pressure at any temperature. Al- 
though the law thus stated is only approximate the ap- 
proximation is so close as to be of great value. If, now, 
having determined from the above plot the pressures cor- 
responding to a wide range of temperatures, we plot pres- 
sures as ordinates directly against temperatures as abscis- 
sas, the curve rises imperceptibly from the origin at abso- 
lute zero until at a certain temperature it begins to diverge 
and from there it rises at a constantly increasing rate. 
In other words, the pressure-temperature curve is at 
first coincident with the temperature axis, and then con- 
cave upwards. This means that if we wish to volatilize 
any substance we must heat it to such a degree that its 
vapor pressure becomes appreciable and that relatively 
small increases of temperature above that point will pro- 
duce constantly increasing rates of evaporation. 
The vapor pressures of several metals have been ex- 
perimentally determined and those of others have been 
estimated by assumptions of greater or less validity. J. 
Johnston‘ has tabulated these and finds the temperatures 
at which the vapor pressure is equal to that of one mil- 
_ limeter mercury, barometer column to be as follows: Sil- 
ver 1320°C; copper 1520°C; iron 1590°C; lead 960°C; 
zinc 500°C; mercury 123°C. 
To obtain vapor pressures of 50 mm. the tempera- 
tures must be about 400° higher in the cases of silver, 
copper and iron, 300° higher for lead and 200° for zine. 
These temperatures seem too high for practical ap- 
plication of a distillation process except in the cases 
of zinc and mercury. However, the vapor pressures of 
4Jour. Indus. and Eng. Chem., Vol. 9, 1917. p. 874. 
