GRAIN-DUST EXPLOSIONS. 13 
SERIES 2. 
Object.— -Same as that of Series 1. 
Conditions. — Similar to those of Series 1,; except that one of the 
disks was blocked so as to permit foreign substances, such as nails, to 
be in longer contact with the revolving disk and thus to give off more 
sparks. Definite amounts, usually 1 pint, of feed were measured out 
for the purpose of determining the limits of explosibilit}" of various 
mixtures. 
Results. — 
1. In 21 attempts with the electric arc and various grains and 
mixtures, 16 explosions were obtained. 
2. A mixture of 1 part elevator dust to about 8 parts of oat hulls 
appeared to be the limit of explosibility of these grain products. 
3. When there was no opening the explosion was propagated to the 
outside end of the screw conveyor, as indicated by puffs of smoke 
(E in PL III, fig. 1, and in fig. 4). 
4. Blocking one disk appeared to give results not different from 
those obtained when both disks were running. 
5. Other results corroborated those of Series 1. 
SERIES 3. 
Object. — The primary object was to determine how far the flame 
was propagated from the source of the explosion. 
Conditions. — The conditions of the test were similar to those of 
Series 2 (one disk being blocked), but the humidity was very high, 
inasmuch as there was a steady rain during a large part of the test. 
A 4 by 4 inch hole was cut in the upper part of the horizontal screw 
conveyor, about 1 foot from the chute leading down from the attri- 
tion mill and about 5 feet 4 inches from the shaft of the mill. The 
various kinds and mixtures of feed were prepared and measured 
into small paper bags, the contents of one of which were dumped 
into the feeding hopper at each attempt. 
Results. — 
1. Out of 39 attempts with the electric arc, 36 explosions occurred, 
in spite of the very damp atmosphere. 
2. Explosions were obtained with elevator dust and oat hulls, and 
elevator dust, flour, graham flour, wheat scourings, floor sweepings, 
corn, and various mixtures of these substances. 
3. The following classification of the 36 explosions may be made 
(PL III, fig. 1, and text fig. 2) : 
27 flashes were observed at B (5 feet 4 inches from shaft of mill). 
9 flames were observed at E (12 feet 6 inches from shaft of mill). 
3 flames were observed at F (17 feet from shaft of mill). 
4. During the course of the tests the conveyor continually brought 
out smoldering lumps of ground products, which could be readily 
fanned into a red glow. 
