PHYSICAL CHARACTERS OF THE SOIL. 165 
Silk is sold in Europe by weight with suitable allowance 
for hygroscopic moisture, its variable conteut of which is 
carefully determined by experiment in each important 
transaction. It is plain that the circumstances of sale 
may affect the weight of wool to 10 or more per cent. 
§ 4, 
CONDENSATION OF GASES BY THE SOIL. 
Adhesion.—In the fact that soils and porous bodies gen 
erally have a physical absorbing power for the vapor of 
water, we have an illustration of a principle of very wide 
application, viz., Zhe surfuces of liquid and solid matter 
attract the particles of other kinds of matter. 
This force of adhesion, as it is termed, when it acts up- 
on gaseous bodies, overcomes to a greater or less degree 
their expansive tendency, and coerces them into a smaller 
space—condenses them. 
Absorbent Power of Charcoal, ete.—Charcoal serves 
to illustrate this fiict, and some of its most curious as well 
as useful properttes depend upon this kind of physical 
peculiarity. Charcoal is prepared from wood, itself ex- 
tremely porous,* by expelling the volatile constituents, 
whereby the porosity is increased to an enormous extent. 
When charcoal is kept in a damp cellar, it condenses so 
much vapor of water in its pores that it becomes difficult 
to set on fire. It may even take up one-fourth its own 
weight. When exposed to various gases and volatile 
matters, it absorbs them in the same manner, though to 
very unequal extent. 
De Saussure was the first to measure the absorbing 
power of charcoal for gases. In his experiments, boxwood 
charcoal was heated to redness and plunged under mer- 
* Mitscherlich has calculated that the cells of a cubic inch of boxwood have 
no less than 73 sauare feet of surface. 
