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SCIENCK 



[N. S. Vol. IX. No. 219. 



timately with the supporter of combustion, 

 and it is also well recognized that many 

 explosions are due solely to very rapid com- 

 bustion, yet it is only within comparatively 

 recent times, and since manufacturing 

 operations have come to be carried on upon 

 a very considerable scale, that we have had 

 it strongl}^ demonstrated that ordinarily 

 combustible solids might, when finely di- 

 vided and mixed with air, give rise, on 

 ignition, to most violent and disastrous 

 explosions, and it seems especially notable 

 that the first well demonstrated cases of 

 this kind should have arisen from the ap- 

 parentljr harmless operations attending the 

 grinding graiu, and the more particularly 

 as flour is not looked upon as a very readily 

 combustible substance when compared with 

 other commonly used solids. 



Among the many instances of this kind 

 which we have now on record we will cite 

 that which occurred ou the 9th of July, 

 1872,* when the inhabitants of Glasgow 

 were startled by the report of an explosion 

 which was heard to a considerable distance 

 and which was found to have occurred in 

 some very extensive flour mills, the front 

 and back walls of which were blown out, 

 while the interior was reduced to ruins, and 

 speedily enveloped in flame which destroyed 

 the remaining buildings. Several persons 

 were killed, and a number of others were 

 severely, burned, or injured by the fall of 

 masonry. 



On May 2, 1878, a similar disaster oc- 

 curred in the enormous flour mills in Min- 

 neapolis, but in this case it was observed 

 that the explosion which originated in the 

 Washburn mill was communicated by flame 

 successively to the Diamond mill and to the 

 Humboldt mill. As a consequence of these 

 explosions, the walls of these mills, which 

 were solid masonry, six feet thick at the 

 base, were razed to the ground ; sheets of 

 corrugated iron roofing, two by six feet in 



*Abel, Eoy. Inst., March 12, 1875. 



area, were projected to a distance of more 

 than two miles; a wooden building fifty feet 

 from the center of explosion was burst open; 

 stout plate glass windows one-fourth of a 

 mile away were torn out bodily, sash and 

 all, and projected into the street; an im- 

 mense volume of smoke and flame was pro- 

 jected to an estimated height of six hundred 

 to eight hundred feet, and finally persons 

 by the edge of the adjacent river observed 

 a displacement of water, producing a wave 

 estimated to be eighteen inches high, before 

 they heard the report of the explosion. The 

 concurrent testimony of persons employed 

 in the mills, and of the experts who were 

 called, proved the absence in each case of 

 any of the so-called explosive substances on 

 the premises and that the boilers had not 

 burst, and from the facts brought out the 

 origin was conclusivelj' traced to the strik- 

 ing of fire by a pair of mill-stones, through 

 the stopping of the ' feed,' and the conse- 

 quent friction of their bare surfaces against 

 each other, with the result that the mixture 

 of air and fine flour-dust surrounding the 

 millstones became ignited. 



This ignition alone would not suffice to 

 develop any violent explosive effects ; for 

 similar ignitions which have been not in- 

 frequently observed in small mills, where 

 they have been caused by the stones ' strik- 

 ing fire' or by the incautious use of a burn- 

 ing lamp near the millstones, or the meal- 

 spout attached to them, have not been 

 attended by any serious results. But in an 

 extensive mill, where many pairs of stones 

 may be at work at one time, each pair has 

 a conduit attached, which leads to a com- 

 mon receptacle called an exhaust box ; into 

 this the mixture of air and very fine flour- 

 dust which surrounds the millstones is 

 drawn by means of an exhaust fan, which 

 is sometimes aided by a system of air-blow- 

 ers. The fine flour is allowed to deposit 

 partially in this chamber or exhaust box, 

 and the air then passes into a second cham- 



