340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



cant because it necessitates the grading of the material by air flota- 

 tion rather than by sifting. 



It is probably not widely known that the shells of diatoms can be 

 used to stop dust explosions. These explosions have long been a 

 serious problem in coal mines and grain elevators, flour mills, wood- 

 planing mills, and other plants, and they have become increasingly 

 numerous with the growth of industrial processes. Often they are 

 very violent and do great damage. Dust explosions may occur wher- 

 ever particles of a combustible dust — coal dust, textile fibers, paper 

 j)ulp, soybean meal, flour, soap, cereal dust, wood meal, or other finely 

 divided organic matter — becomes suspended in the air in a mine or 

 manufacturing plant in considerable abundance, though perhaps not 

 sufficiently dense as to be noticeable. Each particle is surrounded by 

 air but is close enough to its neighbor so that when it ignites from 

 a possible spark or flame (as, for instance, the spark created when 

 the heel of one's shoe strikes a rock) and combines with the air 

 around it, the heat can be absorbed by the adjacent particle, which 

 in turn ignites, increasing the heat, which is passed on to still another, 

 and so the combustion grows, spreading so rapidly as to become an 

 explosion. 



This all happens quicker than it takes to tell it, the explosion 

 increasing rapidly to a peak of violence and then quickly declining 

 in force. Such an explosion is usually localized first in some smoll 

 corner of a mine. The impact of this small local explosion com- 

 presses and disturbs the air in the whole enclosed space communi- 

 cating with it, raising and scattering dust from every corner and 

 surface of the mine or room. Hence the whole place is immediately 

 filled with clouds of combustible dust of the same nature as the first, 

 and slightly later, but almost simultaneously with the initial explo- 

 sion, there occurs throughout the whole mine or building a second- 

 ary and much more violent and destructive general explosion which 

 does the major damage. 



There are two ways to combat dust explosions: (1) By preven- 

 tion — that is, by using measures to keep the air clean or free from 

 combustible dust, by washing and spraying dusty walls, etc. This 

 is not very satisfactory for it is difficult to keep the air clean, and 

 one can never be sure at any particular time that the air is safely free 

 of dust. (2) By "rock dusting", which consists of dusting all walls, 

 ledges, and exposed places with fine diatomaceous powder. If this 

 has been previously done, or is regTilarly practiced, the primary 

 impact of a dust explosion will scatter the diatoms throughout the 

 air, and the second and violent explosion will not occur at all. This 

 is because of the interposition of the fine noncombustible, heat- 

 absorbing diatom shells among the particles of combustible dust. 



