318 ABSORPTION OF GASES BY SOLIDS. 



Charcoal absorbs not only carbonic acid but also air, althong 

 in a less degree. The pores of common charcoal which has bee 

 prepared for some time are hence always filled with condensed ai 

 which evidently must be removed before the charcoal is capable 

 manifesting distinctly its absorptive power for carbonic acid. This 

 done by holding the piece of charcoal to be used (cut about 12 < 

 15mm i on g an( j 6 or I2 mm thick) with the forceps over the point 

 the flame of a spirit lamp or a Bunsen's burner until it is rt 

 hot : the heat causes the absorbed air to escape. To prevent tl 

 piece of coal from absorbing air again, it is at once placed red 

 into the mercury under the mouth of the test-tube, which toge 

 with the retort stand is slightly raised for this purpose so that tl 

 piece of coal may be pushed into the gas. Care must be taken n 

 to raise the tube above the surface of the mercury, or the carbor 

 acid would escape while air would enter. The stand is then imm 

 diately lowered again, and the absorption of the gas proceeds 

 rapidly that in a few minutes the mercury fills the tube at the t< 

 of which the charcoal may be seen. 



Charcoal may be purchased in suitable pieces, or prepared 

 putting a small log of soft wood, but not too small, into the fire, a: 

 letting it burn until it is completely reduced to glowing coal a 

 no longer burns with flame. It is then withdrawn from the fire a 

 closely covered with ashes or sand, until it is extinguished ; or 

 may be put into water, but in that case it must be first complett 

 dried before cutting a piece suitable for the present experime: , 

 because wet charcoal bursts when heated. In the experiment ak 

 described, the pores of the charcoal become partially filled with m 

 cury : the mercury so absorbed may be recovered by pounding 1 

 charcoal in a mortar, stirring up the powder with water, and po' 

 ring it off from the globules of mercury. 



The surfaces of all solid bodies, are, under ordina 

 circumstances, covered with an invisible layer t 

 condensed air, and of condensed vapour of water, whii 

 is always contained in atmospheric air. Clean a gk 

 plate and let it stand a few hours, so that a layer of; 

 may be condensed upon it. Write upon it with a pie 

 of hard wood or a brass or iron point (not a steel 01 . 

 which scratches glass). The adhering air is tl 

 removed from those places which have been written 

 breathe upon the plate, and the writing will < 



