4 MITSCHERLICH ON CHEMICAL REACTIONS 
face of 73 square feet, or of 10,512 square inches, the thickness 
of the layer of carbonic acid clothing the walls of the cells must 
be represented by 0:000002 inch. This layer is more dense 
when the experiment is made with ammoniacal gas, hydrochloric 
acid, or sulphurous acid, which do not require such enormous 
pressure to become liquid, and which are absorbed in far greater 
quantity. Since all porous bodies which offer a considerable 
surface to the gas act in the same manner as charcoal, it ne- 
cessarily follows that the gases in contact with solid bodies 
must be in a particular state, such as they do not present when 
removed from them; and moreover, since the thickness of the 
layer of condensed gas varies, the attraction is not manifested 
solely on the gas which is in immediate contact with the solid 
body; it acts at variable distances. But porous bodies do not 
act solely by their surface; for if this were the case, the absorp- 
tion of different gases by different substances ought to take 
place im the same ratio. Such is not the case, for, according to 
M. de Saussure, wood condenses proportionally more carbonic 
acid than charcoal; in the same way, asbestos, meerschaum, 
cotton and silk tissue, absorb the various gases in other propor- 
tions than box-wood charcoal. 
The absorbing power of pulverulent substances has up to the 
present time been very little studied; platinum black, prepared 
by Davy’s method, greatly surpasses all others; 10 grains con- 
dense 0°550 cube inch of oxygen, that is to say, one cube inch 
would condense 253,440 cube inches (Chimie du Platine, by 
Dobereiner, }». 64); but it is impossible to decide what is the 
volume which the platinum occupies when it has condensed oxy- 
gen, because it is in a state of powder. The property which 
some bodies, such as silica, possess of condensing the humidity 
of the air, may authorize our concluding that they are adapted 
for condensing gases. 
In the same manner that solid bodies attract gases, so are they 
also capable of exercising this attraction upon liquid bodies or 
other solid bodies; thus we may, by means of box-wood char- 
coal, separate from alcohol the potatoe oil which it holds in solu- 
tion; on boiling the charcoal with water, the oil passes off un- 
altered in company with the latter: this force of attraction takes 
up solid colouring matters from liquids which hold them in 
solution. 
Some precipitates have the property of carrying down a part 
