Scientific Lectures. 213 



clkcumstances which affect combustion. 



We have not time to discuss all the conditions which regulate or 

 affect combustion ; but a few of them merit attention. 



As the rapidity with which oxygen is brought in contact with a 

 burning body hastens its combustion, we make use of bellows to 

 increase the supply of oxygen. Still, too rapid a motion of the air 

 rnay extinguish a flame by cooling the burning body below 

 its kindling point. When we blow out a candle, we send a blast of 

 air across the wick which cools the wick below its bnrning point. 

 But if we do not cool the body too much, the increased supply of 

 oxygen increases the combustion. We avail ourselves of our know- 

 ledge of this fact, in supplying furnaces with oxygen on a large scale 

 by means of blowing engines. Tons of air are forced through a 

 blast furnace where iron is produced every twenty-four hours. In 

 fact, more air is employed in the production of iron, than of any 

 other substance ; for the weight of the ore, the fuel and the flue com- 

 bined, is not equal to that of the air which produces the combustion. 



Another plan for hastening the supply of oxygen, is to employ a 

 high chimney. All gas is made lighter by heat on account of the expan- 

 sion. So, if we provide a long chimney, we obtain the advantage of the 

 buoyancy of the heated air in it. The cold air outside, from its 

 greater weight, presses the hot air up the chimney, with a rapidity 

 depending on the height of the chimney ; the combustion being 

 increased in proportion. 



A third method is by a common blower, which does not act exactly 

 by either of these methods. In the ordinary open grate, the greater 

 part of the air passes over the fire, and consequently does not affect 

 the combustion. If we want a quicker fire, we put up the blower, 

 which prevents the air from passing into the chimney over the fire, 

 but forces it through the fire. A friend of mine asked Bret Harte if 

 life was not rather fast on the Pacific coast. " Yes, " he replied ; 

 " people out there always live with the blower on." 



Bodies require a certain temperature to bring them into a state 

 of combustion. This is the reason we make use of kindlings. 

 Shavings take fire readily, because it is easy to bring a shaving to the 

 required temperature. By a match we ignite the shavings. They 

 set fire to the charcoal or kindling wood, which burns readily ; and, 

 by means of their heat, we finally bring up the anthracite coal to its 

 kindling point. If bodies do not burn with sufficient rapidity to 

 produce the necessary amount of heat to keep them at a temperature 

 above their kindling point, they cease to burn. 



