104 CHEMISTRY FOR AGRICULTURAL STUDENTS 



Fill an inverted tube with ammonia gas (supplied) 

 by mercury displacement, and insert a lump of 

 charcoal previously heated to redness (Fig. 42). 

 Note the almost complete absorption of the 

 ammonia. 



Owing to its power of absorbing gases, wood charcoal 

 is used as a deodoriser of sewer gases, etc. Not only 

 does it absorb noxious emanations, but, by condensing thenri 

 in its pores in close contact with the atmospheric oxygen, 

 also absorbed, it oxidises and disinfects them. As a rule, 

 Fig. 42. t.]^e more condensible and soluble a gas, the more is it 

 absorbed. Owing to its power of adhesion to dissolved 

 solids, animal charcoal is used as a decoloriser ; for instance, in sugar 

 refining. Blocks of animal charcoal are also employed as the filtering 

 medium in water filters, and, when fresh, serve to remove, and perhaps 

 oxidise, the microscopic living organisms in the water. 



When organic matter decays, or when submitted to destruc- 

 tive distillation, a compound is produced of carbon and hydro- 

 gen. The bubbles of gas that rise from decaying vegetable 

 matter at the bottom of stagnant pools chiefly consist of this 

 compound, which has therefore received the name of " marsh 

 gas." It also constitutes the " fire-damp " in coal seams, to 

 which explosions in coal mines are generally due. Heat some 

 wood in the flask of the apparatus figured on p. %(i. 

 Immerse the U tube in a beaker of cold water in order to 

 condense the creosote and other liquid products of distillation, 

 and collect the gas evolved in inverted test-tubes or cylinders 

 over water. Preserve the U tube and its contents for future ex- 

 amination. Apply a light to the gas in one of the cyHnders, 

 and observe that it is combustible, and burns with a flame some- 

 what bluer than hydrogen, and slightly luminous. By the usual 

 methods it can be shown that pure marsh gas produces carbonic 

 acid gas and water when burnt, and therefore is a compound 

 of carbon and hydrogen ; but, produced as above, the gas is 

 not pure, but is mixed with free hydrogen, and with other com- 



