150 CHEMISTRY. 



getlier,the heat thus developed and retained may be suffi- 

 cient to set fire to it. Gunpowder factories have sometimes 

 exploded from this cause. For the same reason spontane- 

 ous combustion may occur in a mixture of lamp-black and 

 linseed-oil, if the lamp-black be in excess, or if a portion of 

 it be dry. Any substances in which chemical action is apt 

 to take place, if heaped together so as to shut in the heat 

 which this action produces, may take fire spontaneously. 

 This is the case with oiled cotton and rags if there be in 

 them any drying oil, or even with damp goods packed to- 

 gether. Damp hay may take fire for the same reason. More 

 often, in this case, the combustion is imperfect, and the hay 

 is turned black that is, charred or changed into charcoal, 

 just as wood is in the coal-pit. Spontaneous combustion of 

 the human body, often referred to by ignorant people, is a 

 fiction. 



193. Combustion without Oxygen. We have seen that 

 ordinary combustion is the union of a substance with oxy- 

 gen, accompanied by the development of light and heat. 

 The presence of oxygen, however, is not indispensable to 

 combustion, for we have many examples of chemical com- 

 bination taking place, with such intensity as to generate 

 light and heat, where oxygen is absent. Thus carbon will 

 burn when heated in the vapor of sulphur, and a yellowish 

 green gas called chlorine supports the combustion of metals 

 and even of a candle. But of this we will learn more far- 

 ther on. 



194. Requisites for Combustion. The variations in the 

 readiness with which ordinary combustion goes on depend 

 chiefly on three things: 1. The comparative affinity of the 

 substance for oxygen. 2. The amount of oxygen supplied. 

 3. The temperature to which the combustible body is raised. 

 Thus in the case of phosphorus, the slight heat caused by 

 friction is sufficient to make it take fire. Wood, on the oth- 



