niYSK'AL CHAnAC'TEUS OP THE SOIL. 1 C5 



Silk is sold ill Europe by weight with suitable allowance 

 for hygroscopic moisture, its variable conti-nt of which is 

 carefully (lelt-rmined 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 liave a physical absorbing power for the vapor of 

 water, we have an illustration of a principle of very wide 

 application, viz., The surfaces of liquid and solid matter 

 attract the particles of other kinds of matter. 



This force o? 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, etc. — Charcoal serves 

 to illustrate this tact, and some of its most curious as well 

 as useful properties 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 Sanssure was the first to measure the absorbing 

 power of charcoal for gases. In his experiments, boxwood 

 chai'coal was heated to redness and plunged under mer- 



* Mitscherlich lias calculated that the cells of a cubic inch of boxwood have 

 no less than 73 square feet of surface. 



