$32 Observations on the Absorption of [Nov. 
charcoal is greater or smaller than that of the gas into which it is 
put, the atmosphere surrounding the charcoal is increased whereby 
cold is produced, or diminished whereby heat is disengaged. We 
have seen, for example, that charcoal absorbs much more carbonic 
acid gas than hydrogen gas. When a piece of charcoal saturated 
with carbonic acid is put into hydrogen gas, the bulk of the gas in- 
creases very remarkably, and the charcoal becomes colder. There 
is absorbed only a very small quantity of hydrogen gas into the pores 
of the charcoal, while a far greater proportion of carbonic acid gas 
is disengaged; and this small quantity of hydrogen occupies in the 
pores of the charcoal exactly the same space as the carbonic acid 
gas disengaged did. Suppose, on the contrary, that a piece of 
charcoal saturated with hydrogen gas is put into a receiver filled 
with carbonic acid gas, the bulk of the gas is diminished, and the 
charcoal becomes warmer. A considerable proportion of carbonic 
acid gas is absorbed by the charcoal, while only a very small quan- 
tity of hydrogen gas is disengaged ; and the former occupies exactly 
the space which the latter left. Oxygen gas (according to para- 
graph 1) is absorbed in greater proportion by charcoal than hydrogen 
gas. These two gases, therefore, exhibit the same phenomena. A 
piece of charcoal saturated with oxygen gas being put into hydrogen 
jets a greater proportion of the former gas go than it absorbs of the 
latter. Hence the bulk of the gas is increased, and cold produced. 
On the other hand, when charcoal saturated with hydrogen is put 
into oxygen gas, the volume of air is diminished, and heat pro- 
duced. In this way, from the table given in paragraph 1, of the 
rate of condensation of the pure gases by charcoal, the consequence 
may always be foretold in every one of these experiments. ‘The 
absorbed gas in these cases separates itself from the charcoal pre- 
eisely as it does from water impregnated with the gas, when that 
liquid is placed in contact with another species of gas. 
(B)—The volume of gas expelled from charcoal by another gas 
varies according to the proportion in which both gases exist in the 
unabsorbed residue. The quantity expelled is always the greater, 
the more there is an excess of the gas which produced it. Yet it is 
not possible in close vessels to expel the whole of one gas out of 
charcoal by means of another; a small quantity always remains in 
the charcoal. 
(C)—Two gases united by absorption in charcoal often experience 
a greater condensation than each would in a separate state. For 
example, the presence of oxygen gas in charcoal facilitates the eon- 
densation of hydrogen gas; the presence of carbonic acid gas, or 
of azotic gas, facilitates the condensation of oxygen gas; and that 
of hydrogen gas, the condensation of azotic gas. Yet this effect 
does not take place in all cases with the four gases now mentioned ; 
for the presence of azotic gas in charcoal does not promote the ab- 
sorption“of carbonic acid gas. 
(D)—When the absorption of one of the four named gases has 
been facilitated by another of them, ‘no perceptible combination 
