48 BULLETIN 6S1, U. S. DEPARTMENT OF AGRICULTURE. 
quantitative side of the problem could be taken care of only by care- 
ful laboratory investigations such as are being conducted by the 
Bureau of Mines and the Bureau of Chemistry. The chief purposes 
of an experimental mill, such as that installed at the college, are to 
show that explosions can be produced in the grinding mill and propa- 
gated a considerable distance, and to test various preventive devices 
recommended by manufacturing concerns or by the Government. 
Both of these purposes have been served to some extent. 
Although it has not been demonstrated that static electricity, or 
sparks emitted from foreign substances, are capable of producing 
explosions hi an attrition mill, it has been positively proved that 
more intense sources of ignition, such as an open flame or electric 
arc, can easily ignite dust mixtures, and that the resulting explosion 
can be propagated through fire traps and the screw conveyor. Con- 
sequently, the safest plan is to guard against every possible source 
of ignition. 
As to the second use, only three preventive devices have thus far 
been tested — a single revolving damper, a double revolving damper, 
and a relief valve. It would seem, however, that the present equip- 
ment would warrant further tests of preventive measures in coopera- 
tion with the Government. 
SUMMARY OF CONCLUSIONS. 
The final conclusions reached after studying grain-dust explosions 
in an experimental attrition mill, together with the other results 
obtained by Government investigators, may be summarized as 
follows : 
1. Every effort should be made to collect and remove the dust 
from the grinding mill and- surrounding atmosphere. 
2. In some cases it may be advantageous to use inert gases to 
decrease the oxygen content and thus prevent the formation of an 
explosive mixture of air and dust. 
3. Every possible source of heat should be eliminated where there 
is any danger of having a dust-laden atmospnere. 
4. Every precaution should be taken to eliminate sparks due to 
static electricity. 1 
5. Greater use should be made of sheet iron on account of the 
very great danger from smoldering lumps of grain (Series 12, p. 17). 
6. Revolving dampers, as installed here and elsewhere, appear to 
be of some value as preventive measures for the propagation of 
explosions. 
7. The principle of the automatic relief valve should receive more 
attention as a possible remedy to apply for the partial prevention of 
the propagation of the flame. 
i References 38 and 39, Bibliography. 
