_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. [Illustration: FIG. 5.] TABLE VII. +---------------+---------------+--------+-----------+ | | Concentration |Pressure|Temperature| | Gas. |in cc. per grm.| in mm. | Absolute. | | | of Charcoal. | | | +---------------+---------------+--------+-----------+ | Helium | 97 | 2.2 | 20° | | Hydrogen | 397 | 2.2 | 20° | | Hydrogen | 15 | 2.1 | 90° | | Nitrogen | 250 | 1.6 | 90° | | Oxygen | 300 | 1.0 | 90° | | Carbon dioxide| 90 | 3.6 | 195° | +---------------+---------------+--------+-----------+ TABLE VIII.