x879-] 
Explosions from Combustible Dust . 
671 
passing to the open air. It is in this comparatively dead- 
air space that the dust settles, and can be collected from the 
floor. Here is some of this material, which, as you see, 
when blown into the air, produces a vivid flash, extending 
from the table to the wall. 
The evidence taken before the coroner’s jury shows very 
clearly that it was this material that started the great explo- 
sion of May 2nd. Just how the mill took fire will probably 
never be known, of course, but in all probability the stones 
either ran dry — that is, were without any meal between 
them — or some foreign substance, such as a nail, was in the 
feed, producing a train of sparks such as is produced by an 
emery wheel or a scissors-grinder’s wheel. These : sparks 
set fire to small wads of very hot dust, which, as soon as 
they were fanned into a blaze, communicated it to the spout 
and house full of dust. An eye-witness of the explosion 
first saw fire issuing from the corner of the mill where this 
flour-dust spout was situated, the end of the spout having 
probably been blown out. This fire was followed instantly 
by a quick flash, seen through all the windows of the floor 
upon which the flour-dust houses were situated, followed 
instantly by a flash in the second story, then the third, and, 
in rapid succession, fourth, fifth, and sixth stories ; then 
followed the great report produced when the immense stone 
walls were thrown out in all four directions, and the roof 
and part of the interior of the mill shot into the air like a 
rocket. 
It would seem that a blaze is necessary to ignite the mix- 
ture, for I have tried powerful eleCtric sparks from a machine 
and from a battery of Leyden jars ; also incandescent pla- 
tinum wire in a galvanic circuit, and glowing charcoal, 
without producing any fire, however thick the dust might 
be. Perhaps, however, under more favourable conditions 
the dust would ignite direCtly from sparks, but it seems very 
improbable. 
Let us continue now with the process through which the 
ground wheat is made to pass. From the stones it is con- 
veyed to the bolting-reels, where the very finest is sifted out 
first, and we obtain a grade of flour ; after the finer material 
is sifted out it goes to a coarser bolt, where the “ middlings,” 
as it is called, passes through, leaving the bran which comes 
out at the end of the reel. The middlings, as it comes from 
the bolts, has fine bran and dust in it, and, to purify it, it is 
subjected to an operation similar to that of cleaning the 
wheat, — that is, in the middlings purifiers it is subjected to 
a draught of air which takes away all the light bran and 
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