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 litre18.5%oxygen
2nd litre20.6%”
3rd litre53.0%”
4th litre72.0%”
5th litre79.0%”
6th litre84.0%”

Table IX.

Liquid Gases.Boiling
Point.		Liquid Volume
of 1 gram at
Boiling Point
in c.c.		Latent Heat
in gram
Calories.		Volume of Gas
at 0° C. and
760 mm. per
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 1.7119.0 7.0
Oxygen−182.5 0.9 53.0 13.2
Nitrogen−195.6 1.3 50.0 15.9
Hydrogen−252.514.3125.0 88.9
Helium−269.0 7.0 13.0450.0

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.

Fig. 13.—Calorimetric Apparatus.

The apparatus (fig. 13) consists of a large vacuum vessel A, of 2 or 3 litres’ capacity, containing liquid air, in which is inserted a smaller vacuum vessel B, of 25-30 c.c. capacity, having sealed to it a long narrow tube G that projects above the mouth of A and is held in place by some loosely packed cotton wool. To the top of this tube the test tube C, containing the material under investigation, is connected by a piece of flexible rubber tubing D; this enables C to be tilted so as to throw a piece or pieces of the contained material into the calorimeter. An improved form of this receptacle, attached to B by a flexible tube at D′, is shown at C′. In this P is a wire movable through a cork Q and having at its end a hook by which a piece of the substance under examination can be pulled up and dropped into B. In the absence of other arrangements the substance is at the temperature of the room, but when lower initial temperatures are desired a vacuum vessel H containing solid carbonic acid, liquid ethylene, air or other gas, can be placed to envelop C or C′, or higher temperatures may be obtained by filling the surrounding vessel with vapour of water or other liquids. The gas volatilized in B is conveyed by a side tube E to be collected in a graduated receiver F over water, oil or other liquid. If liquid hydrogen is to be used as the calorimetric substance the instrument must be so modified as to prevent the ordinary atmosphere from entering G, and to that end a current of hydrogen supplied from a Kipp apparatus is arranged to flow continuously through D and E until the moment of making the experiment, when it is cut off by a suitable stop-cock. In this case the outer vessel must contain liquid hydrogen instead of liquid air.

Table X.

Substance. 18° to
−78° C.,
or, at
−30° C.		 −78° to
−188° C.,
or, at
−133° C.		 −188° to
−252° C.,
or, at
−220° C.
Diamond 0.0794 0.0190 0.0043
Graphite 0.1341 0.0599 0.0133
Ice 0.463* 0.285 0.146
* This is from −18° to −78° in the ice experiment.