i6o 



THE POPULAR SCIENCE MONTHLY. 



temperature were the same as before ; but it is a fact, which you have 

 all observed, that fuel in burning produces heat ; it is also a fact that 

 heat expands a gas, and it is this great amount of heat, taken up by 

 the carbonic acid formed, that produces the immense pressure in all 

 directions. 



Let us return to our log of wood. There is exactly the same amount 

 of heat and carbonic acid produced when complete combustion takes 

 place in each of the cases of burning, the only difference being as to 

 time. In the first case, the explosion or pushing aside of the surround- 

 ing air occupies a week, in the last only a second. 



Snow-flakes fall gently upon your shoulders, and you are reqviired 

 to perform an insensible amount of work to resist the crushing effect 

 of each flake ; but, suppose that all the snow that has fallen upon your 

 head and shoulders for the last ten years was welded together in one 

 solid mass of ice, weighing perhaps one hundred pounds, and that it 

 should descend with the velocity of a snow-flake upon you, an immense 

 effort would be required to prevent its crushing you, even if you were 

 able to withstand the shock at all. The work of many days would be 

 concentrated into an instant. 



So it is with burning wood : four or five cords of wood, and a large 



stove, will give you a roaring fire 

 all winter ; the work done is mani- 

 fested by the heat obtained, by the 

 rushing of hot gases up the chim- 

 ne\', and of air from outside into the 

 room through every crack. But, if 

 the wood were ground into a pow- 

 der and scattered through all the 

 house, and burned instantly, the 

 cracks, doors, window^s, and flues, 

 would not be sufficient to give vent 

 to the hot gas, and the roof and 

 sides of the house would be blown 

 to pieces. 



What is true of wood is also 

 true of grains ; also of vegetables, 

 with their products when they con- 

 tain carbon, with this exception : 

 grain, either whole or ground, will 

 not burn readily when in bulk. A 

 fire could be buUt upon a binful of 

 flour, and kept burning for half a day wdthout igniting the flour ; it 

 would char upon the surface, but it lies in such a compact mass that 

 the air does not get access to it readily, hence it does not burn. 



I wish to show you now how combustible dust will burn when blown 

 into the air by means of a pair of ordinary hand-bellows. 



Fig. 1. 



