248 Observations on the Absorption of [Ocr. 
bable that the relation between the absorption of the gases, and the 
height of the barometer, will be discovered. I have not prosecuted 
the subject ; nor have I made any trials upon the change produced 
in the absorption by equal increments of temperature. 
The same cause, which makes the charcoal become hot when it 
absorbs gas, must produce a diminution of temperature on the sepa- 
ration of this gas. A cylinder of box-wood charcoal, 3°15 inches 
long, and 1-57 inch in diameter, was, with a thermometer fixed in 
it, placed in a very small receiver; and then its air was driven off 
by means of the air-pump, that it might be saturated with carbonic 
acid gas. When I afterwards separated this gas from the charcoal 
by means of the air-pump, the thermometer fell in a few minutes 
7°2°.* The same experiment was repeated with common air: the 
thermometer fell from 5° to 7°. 
5. The Property of condensing Gases ts common to all Bodies which 
possess a certain degree of porosity. 
That the property of absorbing gases has hitherto been observed 
only in charcoal is owing partly to experiments having been made 
with no other substance of the requisite texture, and partly to no 
accurate observations on the absorption having been made. I have 
found no body which possesses the property in so high a degree as 
charcoal. 
Jn all the experiments which I have made on this subject, I have 
made use of the air-pump to free the porous bodies from atmos- 
pherical air, and to make them capable of absorbing gases. The 
method of heating to redness succeeds well only with charcoal, be- 
cause, on account of its combustibility and its small specific heat, it 
may be taken out of the fire and plunged under mercury while still 
white hot. The other porous bodies, which are not combustible, 
suffer on that account, when they are small, very various degrees of 
cooling during their passage from the fire to the mercury, which 
have a sensible effect upon their power of absorbing gases. Besides, 
the air-pump put it in my power to employ animal and vegetable 
substances, which are totally destroyed in the fire. 
Lixper. 1. Absorption of Gases by Spanish Meerschaum.+—As 
* By the absorption of carbonic acid gas by charcoal freed from air, the tem- 
perature was raised 25°, 
+ The yariety of meerschanum employed came from Valecas, near Madrid. The 
specific gravity, porosity, and proportion of water contained in it, vary in diffe- 
rent specimens. The piece which I used lost 0°23 of its weight in a red heat. Ets 
specific gravity, ascertained by plunging it in mercury, which did not penetrate 
into the pores, was 0 $26. Mvcerschaum from Nat{olia, according to Klaproth, is 
composed of > 
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AL ODTAC BRIG) 656, <0" oe.apdsas ed cas DERE a)0 
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Dos ee ea ike pists any DAES ba dane ae Wr Weieeh 
100:00 
