[Illustration: FIG. 12.] Charcoal enables a mixture containing a high percentage of oxygen to be extracted from the atmosphere. In one experiment 50 grammes of it, after being heated and exhausted were allowed to absorb air at -185° C.; some 5 or 6 litres were taken up in ten minutes, and it then presumably contained air of the composition of the atmosphere, i.e. 20% oxygen and 80% nitrogen, as shown in fig. 11. But when more air was passed over it, the portion that was not absorbed was found to consist of about 98% nitrogen, showing that excess of oxygen was being absorbed, and in the course of a few hours the occluded gas attained a new and apparently definite composition exhibited in fig. 12. When the charcoal containing this mixture was transferred to a vacuum vessel and allowed to warm up slowly, the successive litres of gas when collected and analyzed separately showed the following composition:-- 1st litre 18.5% oxygen 2nd litre 20.6% " 3rd litre 53.0% " 4th litre 72.0% " 5th litre 79.0% " 6th litre 84.0% " TABLE IX. +----------------+--------+--------------+-----------+-------------+ | | | Liquid Volume| |Volume of Gas| | |Boiling | of 1 gram at |Latent Heat|at 0° C. and | | Liquid Gases. | Point. | Boiling Point| in gram | 760 mm. per | | | | in c.c. | Calories. | gram Calorie| | | | | | in c.c. | +----------------+--------+--------------+-----------+-------------+ | Sulphurous acid| + 10°C.| 0.7 | 97.0 | 3.6 | | Carbonic acid | - 78.0 | 0.65 (solid)| 142.4 | 3.6 | | Ethylene | -103.0 | 1.7 | 119.0 | 7.0 | | Oxygen | -182.5 | 0.9 | 53.0 | 13.2 | | Nitrogen | -195.6 | 1.3 | 50.0 | 15.9 | | Hydrogen | -252.5 | 14.3 | 125.0 | 88.9 | | Helium | -269.0 | 7.0 | 13.0 | 450.0 | +----------------+--------+--------------+-----------+-------------+ _Calorimetry._--Certain liquid gases lend themselves conveniently to the construction of a calorimeter, in which the heat in weighed quantities of any substance with which it is desired to experiment may be measured by the quantity of liquid gas they are able to evaporate. One advantage of this method is that a great range of temperature is available when liquid air, oxygen, nitrogen or hydrogen is employed as the calorimetric substance. Another is the relatively large quantity of gas yielded by the evaporation, as may be seen from table IX., which shows the special physical constants of the various gases that are of importance in calorimetry. In consequence it is easy to detect 1/50 gram calorie with liquid air and so little as 1/300 gram calorie with liquid hydrogen. [Illustration: FIG. 13.--Calorimetric Apparatus.]