18 PRACTICAL FARM CHEMISTRY. 



given a good opportunity by exposure to heat, will 

 seize and devour them in fiery embrace. This pro- 

 cess in everyday life is termed burning. 



Oxygen also destroys vegetable and other sub- 

 stances in a slower way, under a rise of temperature 

 so slight that it generally escapes our notice. This 

 slow combustion (oxidation, burning) is commonly 

 called decaying, rotting or rusting. But whether 

 rapid with fire and flame, or so slow as to be hardly 

 perceptible, this decomposition is the result of the 

 same element and of the same process — a chemical 

 union with oxygen. 



Hydrogen — "trifles lighter than air," in fact, the 

 lightest known substance — forms only a little more 

 than one twentieth part of the dry substance of 

 plants. This gas, like oxygen and nitrogen, has 

 neither taste, smell nor color, but un- 

 y ogen. j.j^^ them, is very inflammable. Burnt 

 hydrogen (hydrogen combined with oxygen in the 

 proportion of one pound of the former to eight 

 pounds of the latter) is the common liquid we call 

 water. Combined with carbon we find this gas in 

 the common coal gas, used for illuminaf ing purposes^ 

 in petroleum, etc. 



Nitrogen, although forming four fifths of the 

 atmosphere, where it exists in mixture (not in com- 

 bination) with oxygen, and entering still more 

 lightly into the composition of plant 

 substance than does hydrogen, deserves 

 the study and attention of the farmer even more 

 than the three elements already named, for it is not 

 available as plant food in its simple form, and not 

 so easily or cheaply obtained in the desired com- 

 binations as other elements of plant food. 



