Introduction. 9 



In ordinary cases of burning, the evolution of heat is readily 

 evident, but in some cases the combustion is so slow that the 

 heat evolved is carried away as fast as produced and very slight 

 or no elevation of temperature is apparent. In some cases of 

 slow combustion where the escape of heat is hindered from any 

 cause, the temperature may rise so as to be perceptible or even 

 dangerous. It may, under particularly favorable conditions, rise 

 sufficiently to start a rapid combustion with oxygen and flames 

 then result. Such cases of "spontaneous combustion" frequently 

 occur. Drying oils, as linseed or cotton-seed oil, especially when 

 spread on cotton waste, and fermentation changes in vegetable 

 matter as hay and tobacco are notable examples of these condi- 

 tions. 



Hydrogen (II). This element is rarely found in a free state 

 in nature, but is combined with carbon and oxygen as in animal 

 and vegetable matter, with oxygen to form water, and in a few 

 cases with some of the base elements to form hydroxides. It is 

 not found in large amounts in the soil and that which forms a 

 part of the tissues of plants and animals comes largely from the 

 hydrogen in water. It is a colorless, odorless gas and charac- 

 terized by its lightness. This fact has led to its use for filling 

 balloons, although coal gas is now more generally employed but 

 is not nearly so efficient. In a free state it has been found in 

 the gases escaping from volcanoes. 



Carbon (C) is the element most closely associated with plant 

 and animal life. It forms a large proportion of the solid matter 

 of all living beings; and the chemical processes of animal and 

 plant life are mainly those in which carbon plays an important 

 part. It exists in the combined state in many minerals as the 

 carbonates of calcium, magnesium, iron, zinc, and also in a small 

 but very important constituent of the air, carbon dioxide. The 

 carbon of the soil, where it exists as the main constituent of 

 organic bodies, takes no direct part in forming the carbon com- 

 pounds of the plant. It is not necessary to apply carbon fer- 



