2) ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Liquid air promises to be the sole industrial source of oxygen and 
nitrogen. The manufacture of these two gases at a very low price is 
a problem the solution of which has a very great importance in 
metallurgy and in the fertilizing industry. How we can derive 
these gases from liquid air is a problem that I am now going to 
consider. 
Oxygen and nitrogen are two bodies whose critical points are 
slightly different (—118° and 13 atmospheres for oxygen; —146° and 
33 atmospheres for nitrogen). The vapor tension curve of nitrogen 
is below that of oxygen. Ata lke temperature, below the lower of 
their critical temperatures, the two gases, considered separately, 
liquefy at very different pressures. The lquefaction of air—that 
is to say, a mixture of these two gases—presents, however, some 
peculiarities which are worth mentioning. 
If at a sufficiently low constant temperature, T, air is compressed 
in a closed chamber, the following phenomena may be observed: 
(1) At a predetermined point of pressure, P,, a first drop of liquid 
appears. This is what, after Duhem,*? we call the dew-point.” 
(2) If at the constant temperature T the volume of the air is 
diminished the pressure increases; at the same time the quantity of 
the liquid phase grows larger. If this increase of the pressure is 
continued all the air will pass into the liquid state, the values for 
P, and T, for pressure and temperature at that instant, characterizing 
what Duhem has denoted the boiling point. 
(3) The dew and boiling points obtained at different tempera- 
tures trace in the system P O T, on one hand the dew line and on the 
other the line of boiling of the gaseous mixture considered. 
(4) For each system of values (T, P) of temperature and pres- 
sure the two phases, liquid and gas, are in a state of equilibrium; in 
this state the composition of the two phases is different. 
(5) The percentage of oxygen (the more easily liquefied element) 
in either liquid or gaseous phase, we will term the content of this 
phase. 
The content of the liquid phase in a state of equilibrium is always 
greater than the content of the gaseous phase. At a constant tem- 
perature, when the pressure is increased, the contents of the two 
phases, liquid and gaseous, continue to diminish till the mixture is 
completely liquefied. Thus, when air is liquefied (volumetric con- 
tent 21 per cent oxygen) the first drop of liquid contains oxygen and 
nitrogen and its content is 47 per cent. This content continues to 
diminish as the volume of the liquid phase increases. A liquid with 
34 per cent oxygen can only be in equilibrium with gas of 12.5 per 
cent oxygen. But as long as there is a gas phase its content is con- 
siderably above zero. It remains above 7 per cent. 
@Duhem: Traité de Mecanique chimique, vol. 4, chap. 3. Paris, Hermann. 
