GBAIN-DUST EXPLOSION'S. 17 
dentally the mill was run several hours while oat hulls were fed 
at as great a rate as possible. The ground oat hulls were conveyed 
back into the mill in order to produce as much dust as possible. 
For over an hour the electric arc was turned on, but riot a single 
explosion resulted. 
Results. — 
1. During the operation of the attrition mill under severe con- 
ditions as to rate of feeding and source of ignition no explosion 
could be obtained with oat hulls during a period of over an hour. 
2. Out of six explosions with the dampers in service two flames 
shot past B and D (fig. 4). 
3. In two instances with the dampers in service smoke was 
observed issuing from the end of the conveyor, but without any 
force. 
SERIES 12. 
Object. — Same as that of Series 11. 
Conditions. — Similar to those of Series 11, except that the oat 
hulls used during the preliminary run had been fairly well dried 
by being spread out in shallow pans, which were then placed on top 
of radiators for a period of several days. Thus the moisture content 
was reduced from 9.9 to 3.2 and 5.7 per cent. 
Results. — 
1. Only one explosion was obtained during the continuous grind- 
ing of fine oat hulls. 
2. In each of the four explosions with the double damper in 
service a flame got past the damper and in three cases extended 
as far as D. 
3. In no instance was a flame observed at the end of the con- 
veyor. 
4. Smoldering lumps of ground products were found to be very 
dangerous, in that they set fire to the wooden base of the attrition 
mill. 
On account of the fire hazard involved in these smoldering lumps 
of grain, special emphasis should be placed on this result of Series 12. 
During this series of tests it was observed that smoldering lumps, 
resulting from the explosions, were frequently brought out by the 
screw conveyor. Some of these lumps happened to be placed on 
several layers of asbestos paper. After several minutes it was ob- 
served that a hole had been burned through the asbestos paper. 
Even when this smoldering material was buried under several inches 
of grain it continued to burn, although not as .well as when it was 
spread out in a thin layer and exposed to the atmosphere. That this 
smoldering material was to be regarded as a great source of danger 
was very forcibly demonstrated when on the following morning it 
50220°— 18— Bull. 681 3 
