666 
Explosions from Combustible Dust. [October, 
III. EXPLOSIONS FROM COMBUSTIBLE DUST* 
By Prof. L. W. Peck. 
t WISH to demonstrate to you this evening, by a few 
simple experiments, the fa(5t that all combustible ma- 
terial, when finely divided, forming a dust or powder, 
will, under proper conditions, burn with explosive rapidity. 
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 Humboldt 
Mills of this city, on the night of May 2 nd, — upon which 
occasion the rapid burning of hundreds of tons of flour, 
bran, &c., completely demolished the solid masonry walls 
(6 feet thick) of the mills, and threw sheets of iron from the 
roof of the Washburn so high into the air that they were 
carried 2 miles by the wind before striking the ground. 
Let us now see 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 by friction, concentration of the sun’s rays, chemical 
adtion as from a match, or in any other way — they combine 
to form carbonic acid gas. This chemical adtion produces 
a large additional amount of heat, which keeps up the adtion 
as long as there is any carbon and oxygen left to unite, and 
* Le&ure delivered June i, 1878, at Association Hall, Minneapolis, Minne- 
sota, at the request of the millers of the city. 
