CARBON AND CARBONIC ANHYDRIDE. 81 



96. Absorbing Power of Charcoal. Most, if not all, of the 

 effects above mentioned are attributed to the absorbing 

 power of charcoal. This power is very great. Charcoal 

 will absorb of some gases from eighty to ninety times 

 its own bulk. This constitutes a protection to substances 

 which are covered with charcoal, for gases are the grand 

 agents in decay. Absorbed by the charcoal, they are put 

 out of the way ; and not only so, but they constitute a part 

 of the wall of defense together with the charcoal, filling up 

 as they do all its spaces. Charcoal thus saturated with 

 gases defends the substance that it covers from access of 

 the air. When decay has already begun before the char- 

 coal is applied in the work of purification, it absorbs all the 

 gases tli at have been produced in the decay, and thus puts 

 a stop to the process. 



97. Explanation. The question arises as to what gives 

 this power of absorption to charcoal. It is generally sup- 

 posed that it is owing to its great porosity. Charcoal is 

 full of minute spaces, and is therefore intersected by num- 

 berless partitions. If these were spread out they would con- 

 stitute a surface perhaps a thousand times larger than the 

 external surface of the charcoal. As every point of this sur- 

 face is a point of attraction, it is supposed to account for 

 the enormous accumulation of gases in the spaces of the 

 charcoal. But this accounts for it only in part. If it were 

 the only cause of the absorption, there should not be such a 

 great difference in absorbing different gases. Of some gases 

 it absorbs nearly fifty times as much in bulk as it does of 

 some others. When great quantities of gases are absorbed, 

 there must be great condensation, and this would hardly 

 come from mere common attraction. There must be some 

 peculiar power in the charcoal to change in some way the 

 condition of a gas of w r hich it absorbs ninety times its own 

 bulk. And, besides, it seems to show some sort of affinity 



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