Table VI.

Volume
of Gas
absorbed.		Occlusion
Hydrogen
Pressure.		Occlusion
Nitrogen
Pressure.
cc.mm.mm.
00.000030.00005
50.0228..
100.0455..
150.0645..
200.0861..
250.1105..
300.13390.00031
350.1623..
400.1870..
130..0.00110
500..0.00314
1000..0.01756
1500..0.02920
2500..0.06172

Charcoal Occlusion Pressures.—For measuring the gas concentration, pressure and temperature, use may be made of an apparatus of the type shown in fig. 5. A mass of charcoal, E, immersed in liquid air, is employed for the preliminary exhaustion of the McLeod gauge G and of the charcoal C, which is to be used in the actual experiments, and is then sealed off at S. The bulb C is then placed in a large spherical vacuum vessel containing liquid oxygen which can be made to boil at any definite temperature under diminished pressure which is measured by the manometer R. The volume of gas admitted into the charcoal is determined by the burette D and the pipette P, and the corresponding occlusion pressure at any concentration and any temperature below 90° abs. by the gauge G. In presence of charcoal, and for small concentrations, great variations are shown in the relation between the pressure and the concentration of different gases, all at the same temperature. Table VI. gives the comparison between hydrogen and nitrogen at the temperature of liquid air, 25 grammes of charcoal being employed. It is seen that 15 cc. of hydrogen produce nearly the same pressure (0.0645 mm.) as 2500 cc. of nitrogen (0.06172 mm.). This result shows how enormously greater, at the temperature of liquid air, is the volatility of hydrogen as compared with that of nitrogen. In the same way the concentrations, for the same pressure, vary greatly with temperature, as is exemplified by table VII., even though the pressures are not quite constant. The temperatures employed were the boiling-points of hydrogen, oxygen and carbon dioxide.

Fig. 5.

Table VII.

Gas.Concentration
in cc. per grm.
of Charcoal.		Pressure
in mm.		Temperature
Absolute.
Helium 972.2 20°
Hydrogen3972.2 20°
Hydrogen 152.1 90°
Nitrogen2501.6 90°
Oxygen3001.0 90°
Carbon dioxide 903.6195°

Table VIII.

Gas.Concentration
cc. per grm.		Molecular
Latent Heat.		Mean
Temperature.
Absolute.
Helium 97 483.0 18°
Hydrogen390 524.4 18°
Hydrogen 202005.6 78°
Nitrogen2503059.0 82°
Oxygen3003146.4 82°
Carbon dioxide 906099.6180°

Heat of Occlusion.—In every case when gases are condensed to the liquid state there is evolution of heat, and during the absorption of a gas in charcoal or any other occluding body, as hydrogen in palladium, the amount of heat evolved exceeds that of direct liquefaction. From the relation between occlusion-pressure and temperature at the same concentration, the reaction being reversible, it is possible to calculate this heat evolution. Table VIII. gives the mean molecular latent heats of occlusion resulting from Dewar’s experiments for a number of gases, having concentrations in the charcoal as shown. The concentrations were so regulated as to start with an initial pressure not exceeding 3 mm. at the respective boiling-points of hydrogen, nitrogen, oxygen and carbon dioxide.

Fig. 6.