THE EEASON WHY. 79 



" Oh the depth of the riches both of the wisdom and knowledge of God ! how 

 unsearchable are his judgments, and his. ways past finding out." BOM. xi. 



Because the intense heat disengages a small volume of the gases 

 of which water is formed. 



310. Which gas do we (in this instance) recognise by the 

 smell ? 



The hydrogen gas. Oxygen gas possesses no odour. 



311. What is Spontaneous Combustion ? 



Spontaneous combustion is that which occurs in various bodies 

 when they become highly heated by chemical changes. 



312. Why is heat developed during chemical changes ? 

 Because, as all bodies contain latent caloric, the disturbance of 



the atoms of which those bodies are composed, during the new 

 combinations that constitute chemical changes, frequently sets the 

 caloric free, and an accumulation of caloric produces spontaneous 

 combustion. 



313. Does a match ignite spontaneously when drawn over 

 a rough surface ? 



No. Because in this case the combustion arises from heat 

 applied by friction. 



314. Does phosphorous ignite spontaneously when held 

 in a iv arm hand ? 



Phosphorous will ignite when held in a warm hand, but it does 

 not then produce spontaneous combustion, because it ignites through 

 the agency of applied heat. 



315. But if a piece of dry phosphorous be sprinkled with 

 powdered charcoal it will ignite, without the application of 

 heat. Why is this ? 



Because the carbon (charcoal) absorbs oxygen from the air, and 

 conveys it to the phosphorous. Here are chemical changes which 

 develope heat, and produce spontaneous combustion. 



316. Why do hay-stacks sometimes take fire ? 



Because the hay, having become damp, decays, and passes on to 

 a state of fermentation, ; .n which chemical changes occur, during 



