422 SMITH'S INTERMEDIATE CHEMISTRY 



with such charcoal reduced the concentration of all toxic gases 

 employed in the war (see Chapter XL) below the danger limit. 

 Toxic smokes, however, were not satisfactorily adsorbed. The 

 explanation is similar to that advanced in a preceding chapter 

 (p. 263) for the persistence of the fog obtained when a mixture of 

 sulphur trioxide and oxygen is bubbled through water. The 

 molecules of a gas are in such rapid motion that they are prac- 

 tically certain to strike the surface of the charcoal while passing 

 through the canister, and to be adsorbed on this surface if the gas 

 is easily condensible. The dimensions of the solid smoke par- 

 ticles, however, are much larger than molecular, and the particles 

 are relatively stationary. Most of them, in consequence, are 

 able to pass through the air channels between the charcoal granules 

 without touching. 



Pulverized charcoal, when shaken with a liquid, is also able to 

 extract from it any dissolved substances, and to concentrate them 

 upon its surface. Other finely-divided materials, such as soil 

 particles (p. 415), possess the same power of adsorbing substances 

 from solution. Salts are, in general, only partially taken up; 

 organic solutes are removed more completely. This property 

 of charcoal is made use of in water purification and in sugar re- 

 fining (p. 402). Charcoal is used, also, in making gunpowder, in 

 reducing ores, and as a fuel (smokeless). 



Coal. When wood burns with a plentiful supply of oxygen, 

 it gives nothing but carbon dioxide, water, free nitrogen, and a 

 certain amount of ash (oxides and carbonates of the metals). 

 What happens when it is heated in absence of oxygen, we have 

 just seen. In nature, however, the intermediate case of slow 

 decomposition of vegetable matter, without much heating and 

 without access of oxygen, takes place on a large scale. Clay and 

 sand, or even simply water, cover the vegetation and exclude the 

 air, and the products are anthracite coal, bituminous coal, or peat. 

 Little is known of the actual compounds contained in coal. We 



