146 METALLOIDS. 



light and heat. Metals which will not burn in the 

 air, because it is diluted oxygen, burn brilliantly, as 

 has been seen, in pure oxygen. 



342. PREVIOUS HEAT REQUIRED. In or- 



Why is heat _ _ 



required to der that most substances may burn, they 

 Uon/ mbHS ' must first be heated > to increase their affin- 

 ity for oxygen. Take carbon, as an exam- 

 ple. Before heating, its affinity for oxygen is not suf- 

 ficient to bring about the requisite combustion. In this 

 condition it may, therefore, lie for any length of time, 

 in the air, or oxygen gas, without uniting with it. 

 But heat stimulates the tendency to combination, and 

 the bit of charcoal previously ignited, goes on burning, 

 until it is consumed. The first particles obtain the 

 necessary stimulus of heat, from the previous igni- 

 tion, the next from the burning of the first, and so on. 



343. UNCOMBINED OXYGEN REQUISITE. 

 What kind of __ '. 



oxygen is re- Mere presence of oxygen is not sufficient 

 amlnrtion? for. combustion. It must be free, or un- 

 combined oxygen. After burning char- 

 choal in oxygen gas, the vial contains just as much 

 oxygen as before, but being already combined, it has 

 no affinity, or appetite, for more carbon, and there- 

 fore will not produce a new combustion. 



344. EACH PARTICLE IN TURN MUST BE, 



If each parti- 



cle is not heat- HEATED. If the first particles that com- 



bme ' do llot communicate sufficient heat 

 to the next, then the combustion stops. 

 This may be illustrated by lighting a tightly wound 

 roll of paper, and holding the flame upward. It is 

 soon extinguished, because the heat that is produced 



