EXPLOSIONS FROM COMBUSTIBLE DUST. 159 

 EXPLOSIONS FKOM COMBUSTIBLE DUST.' 



By Professor L. W. PECK. 



I WISH to demonstrate to you this evening, by a few simple experi- 

 ments, the fact that all combustible material when finely divided 

 forming a dust or powder, will, under proper conditions, burn with 

 explosive rapidit3\ 



If a large log of wood were ignited it might burn a week before 

 being entirely consumed ; split it up into cord-wood, and pile it up 

 loosely, and it would burn in a couple of hours ; again, split it into 

 kindling-wood, pile loosely as before, and perhaps it would burn in 

 less than an hour ; cut it up into shavings and allow a strong wind to 

 throw them into the air, or in any way keep the chips comparatively 

 well separated from each other, and it might be entirely consumed in 

 two or three minutes ; or, finally, grind it up into a fine dust or powder, 

 blow it in such a manner that every particle is surrounded by air, and 

 it would burn in less than a second. • 



Perhaps you have noticed that shavings and fine kindlings will 

 sometimes ignite so quickly in a stove that the covers will be slightly 

 raised, the door forced open, or perhaps small flames will shoot out 

 through the front damper. You have, in such a case, an explosion on 

 a very small scale similar to that of the Washburn, Diamond, and Hum- 

 boldt Mills of this city, on the night of May 2d — upon which occasion 

 the rapid burning of hundreds of tons of flour, bran, etc., completely 

 demolished the solid-masonry walls, six feet thick, of the mills, and 

 threw sheets of iron from the roof of the Washburn so hiarh into the 

 air that they were carried two miles by the wind before striking the 

 ground. 



Let us see now why such explosions occur. Wood has in it a large 

 amount of carbon, the material of which charcoal is composed, and the 

 air is about one-fifth oxygen. Now, at the ordinary temperature, the 

 carbon of the wood and the oxygen of the air do not combine ; but, 

 when they are heated, as hj friction, concentration of the sun's rays, 

 chemical action as from a match, or in any other way, they combine 

 to form carbonic-acid gas. This chemical action produces a large addi- 

 tional amount of heat which keeps up the action as long as there is 

 any carbon and oxygen left to unite, and 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 



' Lecture delivered June 1, 1878, at Association Hall, Minneapolis, Minnesota, at the 

 request of the millers of the city. 



