iSyg.] Explosions from Combustible Dust. 667 
also makes the temperature of the gas which is formed 
very high. 
As the space occupied by the carbonic acid gas and that 
occupied by the oxygen which entered into the combination 
is the same at the same temperature, there would be no 
bursting if, after combination, the temperature were the 
same as before ,* but it is a fa6t, which you have all observed, 
that fuel in burning produces heat : it is also a fa( 5 t 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 surrounding air 
occupies a week, in the last only a second. 
Snow-flakes fall gently upon your shoulders, and you are 
required to perform an insensible amount of work to resist 
the crushing effeCl 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 manifested by the heat obtained, by the rushing 
of hot gases up the chimney, and of air from outside into 
the room through every crack. But if the wood were 
ground into a powder and scattered through all the house, 
and burned instantly, the cracks, doors, windows, 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 grain ; also of vege- 
tables, with their products when they contain carbon, with 
this exception — grain, either whole or ground, will not burn 
readily when in bulk. A fire could be built upon a binful of 
flour, and kept burning for half a day without 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 
