GRAIN-DUST EXPLOSIONS. 5 
The Bureau of Chemistry is now conducting laboratory investiga- 
tions of these factors, and has obtained data on the relative inflam- 
mabilities of various grain dusts, as well as on the use of inert gases. 
Briefly, the conclusions reached thus far are as follows: 1 
1. Most of the carbonaceous dusts have a higher degree of inflam- 
mability than Pittsburgh coal dust. 
2. An inert gas mixture containing 12 per cent or less of oxygen 
will prevent a dust explosion from starting or propagating. 
In 1913 R. V. Wheeler, 2 chief chemist for the Explosion in Mines 
Committee, England, published the report of his investigations of 
the relative inflammabilities of various dusts. He classified dusts 
as follows: 
1. Dusts which ignite and propagate flame readily, the source of heat required for 
ignition being comparatively small, for example, a lighted" match. 
2. Dusts which are readily ignited, but which, for the propagation of flame, require 
a source of heat of large size and high temperature, such as an electric arc, or of long 
duration, such as the flame of a Bunsen burner. 
3. Dusts which do not appear to be capable of propagating flame under any condi- 
tions likely to obtain in a factory, because they do not readily form a cloud in air, 
or are contaminated with a large quantity of incombustible matter, or the material 
of which they are composed does not burn rapidly enough. 
Class 1. 
Sugar. Grain (flour mill): 
Starch. Maize. 
Rice meax and sugar refuse. Grain (grain storage). 
Wood flour. Rape seed. 
Malt. Corn flour. 
Oat husk. Flour (flour mill). 
Rice milling. Grist milling. 
Castor oil meal. Corn meal. 
Offal grinding (bran). Mustard. 
Class 3. 
Spice milling. Sack cleaning. 
Cotton seed. Rape seed (Russian). 
Cotton seed and soy bean. Grain cleaning. 
Available data show quite conclusively that there is a great simi- 
larity between gas and dust explosions. The following extract 
from an article 3 by II. H. Brown, of the Bureau of Chemistry, is of 
interest in this connection: 
It will be noted that just as high pressures have been observed in coal-dust explo- 
sions as in gas explosions. However, the extremely high velocities attained by the 
flame in certain gas mixtures have not been reported as observed in dust explosions. 
Such high velocities could not be expected, for even the finest dust particles are many 
times larger than gas molecules, and so, even in the most dense dust clouds, the 
particles could not be as close together nor as intimately mixed with the oxygen as are 
gas molecules. Therefore, the heat of combustion of one dust particle can not be as 
readily transmitted to the next particle as it can in a mixture of gases. But the finer 
the dust the more nearly will it approach the size of a gas molecule. Therefore, it 
1 References 36 and 37, Bibliography. 2 Reference 26, Bibliography. 3 Reference 36, Bibliography. 
