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 warm 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.