4 BULLETIN 681, tJ. S. DEPARTMENT OF AGRICULTURE. 
the States of Washington, Idaho, and Oregon. 1 During the past 
season the investigation has been extended to Colorado, Texas and 
other grain-growing sections. 
THE EXPERIMENTAL ATTRITION MUX. 
The investigations conducted with the experimental attrition mill 
were undertaken for (1 I the determination of the possible causes of 
explosions, and (2 I the testing of various preventive measures that 
have been suggested during the progress of the work. The entire 
scope of the investigation can be seen in the following outline of the 
various phases of the problem that were touched upon: 
1. Sparks emitted by foreign substances. 
2. Naked flame. 
3. Carbon arc in attrition mill, 
4. Carbon arc in dust room. 
5. Explosibility of various grains. 
6. Static electricity in attrition mill. 
7. Static electricity as cause of explosions. 
8. Effect of atmospheric humidity. 
9. Effect of moisture content of materials. 
10. Revolving dampers or fire traps. 
11, Relief valve and pipe. 
PRINCIPLES UNDERLYING GRAIN-DUST EXPLOSIONS. 
On account of the general lack of knowledge concerning the theory 
of dust explosions it may be advisable to insert at this point a brief 
summary of the chief facts and principles involved. 
It is generally conceded that two main conditions must be present 
hi order that a dust may produce an explosion: (1) Fine, dry dust 
must be in a state of suspension in an atmosphere containing the 
proper amount of oxygen. (2) A source of sufficient heat must be 
brought in contact with the dust. 
It should be noted that carbonaceous dusts are not considered 
capable of igniting spontaneously, for an outside source of heat is 
required. The following sources of heat have been listed as the 
probable causes of many of the explosions in milling plants throughout 
the country: 2 
1. Use of open lights, or naked names, such as lamps, torches. 
gas jets, lanterns, candles, and matches. 
2. Entrance of foreign material in grinding machines. 
3. Electric sparks from motors, fuses, switches, lighting systems. 
-I. Static electricity produced by friction of pulleys and belts, 
grinding machines, etc. 
As for the first condition, many factors are involved, such as the 
chemical composition, moisture content, fineness, atmospheric 
humidity, density or degree of diffusion, and presence of inert gases. 
1 U. S. Dept. Agr. Bui. 379. 2 Reference 27, Bibliography. 
