GKAIN-DUST EXPLOSIONS. 39 
DISCUSSION OF RESULTS AND CONCLUSIONS. 
SPARKS EMITTED BY FOREIGN SUBSTANCES. 
Many attempts were made, in both the preliminary and the regular 
tests, to produce dust explosions in an attrition mill by introducing 
with the feed various kinds of foreign substances, such as nails, small 
pieces of stone and flint, and matches, but not once did an explosion 
occur. Various kinds of grain products, with different degrees of 
fineness, and both normal and minimum moisture contents, were fed 
during these attempts. Evidently, however, the intensity and amount 
of the heat generated by the sparks from these foreign substances were 
not sufficient to ignite the dust, although an explosive mixture of air 
and dust may have been present, as manifested by the regularity of 
the explosions obtained by the electric arc under identical conditions. 
In rare cases, and especially with very low humidity and moisture 
contents, it might be possible for a nail or other hard substance to be 
caught in such a way that a very long succession of sparks would be 
emitted with sufficient intensity to cause ignition. The results of 
these tests, however, point strongly to the following conclusions: 
1. Sparks emitted by foreign substances passing through an attri- 
tion mill do not in general appear to be hot enough to ignite an 
inflammable dust. 
2. Nevertheless, to take care of the exceptional case, every pre- 
caution should be taken to keep all foreign substances from entering 
the grinding machines. 
NAKED FLAME. 
Only two attempts were made to obtain explosions by means of a 
naked or open flame, but both were entirely successful (p. 11). Safe 
conclusions are as follows: 
1. Naked flames can readily ignite inflammable" dust mixtures. 
2. A naked flame should never be allowed in the vicinity of dust. 
CARBON ARC IN ATTRITION MILL. 
The summary of all results obtained with the use of the electric 
arc is shown in Table 16. It is to be noted that in practically two- 
thirds of the attempts with materials which gave explosions at one 
time or another, explosions were obtained by means of an electric 
arc located near the grinding discs. In every one of the 201 explosions, 
except Explosion 3 of Series 12, it was necessary to feed the material 
in the amount of at least a handful to produce an explosion. Appar- 
ently, a fairly dense cloud of dust was required to produce an ex- 
plosion with even the most intense source of ignition available, the 
electric arc. The following conclusions can be drawn : 
1. An attrition mill is capable of producing mixtures of dust and 
air which can be easily ignited by a sufficiently intense source of heat. 
2. There is much less danger of an explosion, when the rate of feed- 
ing is less than the .amount which the mill can take care of. 
