GRAIN-DUST EXPLOSIONS. 11 
At first the hood of the attrition mill Was removed, so that the 
sparks emitted could be observed. When a single nail (eightpenny, 
but clipped to a length of about one-half inch) or small piece of stone 
was passed through, the mill, sparks were, in nearly every case, 
emitted from different points of the periphery of the disks. Feeding 
1 to 2 ounces of these clipped nails, small pieces of flint, or other hard 
materials, either singly or in combination, resulted in a large number 
of sparks, as in the case of an emery wheel. These sparks were thrown 
a distance of from 10 to 16 inches from the periphery of the disks. 
The sparks were emitted radially when both discs were running, and 
tangentially when one disk was stationary. When matches were fed 
into the mill, usually no sparks were emitted from the periphery of 
the disc, as the rapidly revolving arms of the disk generally ignited 
the matches before they had fairly entered the eye of the runner. 
After making these observations of the sparks produced by foreign 
materials passing through the attrition mill, the hood was replaced, 
so that various kinds of grain products could be fed into the mill, 
with pieces of flint, nails, matches, etc. Three to five quarts of oat 
hulls, corn, elevator dust, flour, -wheat scourings, wheat shorts, and 
floor sweepings were fed into the mill, at first singly, then in various 
combinations, with about 2 ounces of foreign materials, a quantity 
which probably would greatly exceed the amount of such materials 
finding their way into an attrition mill with the grain during a com- 
paratively long period, under ordinary milling conditions. The 
disks were set very close in order to grind as fine as or even finer than 
is usual. Repeated attempts with such materials as flour, dried ele- 
vator dust, and dust settlings failed to produce a single explosion. 
Further efforts were made to obtain explosions by duplicating, as 
far as possible, the actual conditions obtaining in an attrition mill, by 
feeding the grain at normal rate until several bagfuls had been passed 
through. At frequent intervals pieces of nails, flint, etc.,. were added 
with the grain, but never was there the slightest evidence of an 
explosion. 
The first explosion in the experimental attrition mill was obtained 
shortly after the preceding series of runs at the time when J. K. 
Clement, of the Bureau of Mines, was present to witness the tests. 
The bottom of the hopper was closed by the slide so as to retain the 
ground material and not permit any dust to escape except through 
the hand hole, which was open. When a quantity of elevator dust 
and ground oat hulls was fed into the mill, a rather dense cloud of 
dust issued from the hand hole, due to the fan-like action of the grind- 
ing disks. Upon placing a gasoline blow torch near this hand hole, 
the dust ignited and a flame, 6 feet long, shot out toward the wall. 
The hopper was immediately withdrawn, and about a quart of the 
same materials thrown into the mill, while the torch was introduced 
