28 Professor Macaire on the 



longing to the nature of the absorbing bodies themselves. Thus, 

 coals and meerschaum absorb more azote than hydrogen, and 

 woods, on the contrary, condense more hydrogen than azote. 



In order that the absorption may be as great as possible, it 

 is necessary that the body should have the greatest number of 

 pores, and that these should be of the smallest dimensions, and 

 as empty as possible. Thus the charcoal of the box-tree, 

 when pulverized, does not absorb above the half of what the 

 same weight absorbs when left unbroken ; the charcoal of cork, 

 which has a specific gravity of only O'l, scarcely absorbs air at 

 all ; the charcoal of the pine, which has a specific gravity of 

 0'4, condenses four times and a half of its volume ; that of the 

 box, which has a specific gravity of 0*6, absorbs seven times 

 and a half of its volume ; finally, the Rastiberg coal, the specific 

 gi*avity of which is 1-326, makes ten times and a half its 

 volume of air disappear. There is, however, a limit to this 

 increase ; and if the charcoal is too dense, as, for example, 

 that which is obtained by making the essences pass through a 

 red-hot tube, the gases cannot penetrate into its pores, and 

 there is consequently no absorption. Humidity diminishes 

 the absorbing faculty of porous bodies for gases, and we may 

 expel a portion of what they have condensed by means of water. 



The charcoal of box is, of all porous bodies, that which ab- 

 sorbs, in general, the greatest quantity of all the gases. There 

 are, however, some exceptions. Experiments were made with 

 fifteen porous bodies of different natures. 



The condensation of the gases by the charcoal of box causes 

 a feeble disengagement of caloric ; and this fact explains the 

 spontaneous combustion of magazines or boats filled with coal, 

 a phenomenon which we have frequent occasion to observe. This 

 temperature becomes very high when the absorbed gas is sul- 

 phuretted hydrogen. It is such that charcoal becomes very hot, 

 if conveyed, when impregnated with this gas, into the air or 

 oxygen. The gas is then decomposed, and forms water and sul- 

 phur by the combination of the oxygen with the hydrogen 

 which enters into its composition. Theodore de Saussure like- 

 wise found that the oxygen of the air, absorbed by charcoal, 

 gradually combines with it, and forms carbonic acid, even at 

 the ordinary temperature. In the space of a year, moist char- 



