18 BULLETIN 379, U. S. DEPARTMENT OF AGRICULTURE. 
agreed, however, that the dust must be fine and dry and in a state 
of suspension in the atmosphere, so that upon being brought in con- 
tact with sufficient heat or flame an ignition is caused. It generally 
is conceded that there must be a proper proportion in diffusion in 
order for the explosive mixture of dust and air to ignite with sufii- 
cient force to propagate an explosion. 
The conditions under which these thresher fires and explosions 
occur appear somewhat similar to those with other cereal dusts. 
During the threshing process the smut, which is a form of very 
fine, dry dust, is thrown into suspension in the air and forms a 
dangerously explosive mixture, which readily would produce an ex- 
plosion or fire if ignited. The mixture of smut dust and air may 
have limits of explosibility, and it is quite possible at times to have 
too much dust present, and at other times not sufficient, for an igni- 
tion. For this reason explosions may occur at a given time and under 
certain conditions and not occur at other times or under different 
conditions. 
AMOUNT OF DUST REQUIRED FOR EXPLOSION. 
Explosions have been produced at the Pittsburgh Testing Station 
of the United States Bureau of Mines when there was only 0.032 
ounce of coal dust suspended in each cubic foot of air, or 1 pound 
in 500 cubic feet. In order to produce complete combustion it takes 
all of the oxygen in 1 cubic foot of air to burn completely 0.12 ounce 
of the coal dust used.t Preliminary experiments have shown that 
smut dust is highly inflammable, and also that many of the cereal 
dusts have relatively a lower ignition temperature and produce 
higher pressures than coal dusts. It might be concluded that the 
explosive limits of cereal dusts would be lower than those of the 
coal dusts. 
In some of the thresher explosions at least two distinct reports 
were heard; the first beg sharp and quick and the second resem- 
bling a loud roar and lasting longer than the first and accompanied 
by more flame. An explosive mixture consisting of sufficient quan- 
tity of smut dust in suspension, ignited by sufficient source, would no 
doubt cause the sharp report usually heard first. This original igni- 
tion, possibly only an inflammation, might produce sufficient concus- 
sion to shake the dust that had settled on various parts of the sepa- 
rator into suspension in the air, thus forming an additional explosive 
mixture. The heat or flame from the first small puff or inflammation 
would cause an ignition of this newly formed mixture, and the 
explosion would propagate through the entire dust zone. This may 
serve to explain the loud rumbling sound sometimes heard, accom- 
panied by a large body of flame causing extensive damage. 
i. S. Dept. Interior, Bureau of Mines. Miners’ Circular No. 3, Coal-Dust Explosions, 
(1911), p. 7. Geo. S. Rice. 
