42 BULLETIN 681, IT. S. DEPARTMENT OF AGRICULTURE. 
CARBON ARC IN DUST ROOM. 
The location of the arc in the dust room, at the end of the screw 
conveyor, did not cause an explosion (Series 4). This fact, however, 
merely indicates that the many conditions involved in an explosive 
mixture did not happen to be met in this particular case. Probably 
the dust cloud was not dense enough. In any event, it would be well 
to draw the following conclusion, especially in view of past explosions 
in mills, and those obtained by the arc located near the grinding 
discs of the attrition mill : 
It is always very dangerous to allow an open flame (or any source 
of ignition) in the vicinity of a dust-laden atmosphere. 
EXPLOSIBILITY OF VARIOUS GRAINS. 
The summary of all the explosions of various materials obtained 
with the electric arc (except those in the demonstrations, Series 5) is 
found in Table 16. From the explanation of the meanings of the 
terms of the fractions appearing in this table it would seem that each 
fraction ought to indicate the inflammability of the material. As 
a matter of fact, these fractions, especially those of the same series, 
serve as rough indications of the relative inflammabilities of the 
various materials; but too much stress should not be laid upon this 
point, as most of the conditions of testing were beyond control. Rela- 
tive inflammability can properly be determined only by careful 
laboratory methods which control the many factors entering into the 
question. 
In view of the fact that the grinding of oat hulls constitutes an 
important part of the work done by attrition mills, it is interesting 
to note that oat hulls alone (Mixtures 1, 13, and 30) were not found 
to be very inflammable. Only extremely fine oat hulls, or those with 
considerable dust, were capable of giving inflammable mixtures, in 
spite of the fact that in series like 11 and 12 oat hulls were fed into 
the mill and recirculated continuously for several hours without a 
resulting explosion, except in the case of Explosion 3 of Series 12. 
A study of Table 16, together with a knowledge of the conditions 
of testing, makes possible the following conclusions : 
1. Elevator dust, flour, wheat scourings, and malt sprouts seem to 
produce explosions the most consistently. 
2. Oat hulls do not appear to give very inflammable mixtures, 
unless they contain a considerable amount of fine dust. 
3. Every precaution should be taken to remove dust as fast as it 
is formed, and to prevent it from coming in contact with airy possible 
source of ignition. 
