Makch 10, 1899.] 



SCIENCE. 



347 



ber, called a stive room, where a farther 

 quantit}' of dust is deposited. It follows 

 that Avhen the mill is at work, these cham- 

 bers and the channels are all filled with aa 

 inflammable mixture of the finest flour-dust 

 and air, and that the ignition of any por- 

 tion of the inflammable mixture will result 

 in the exceedingly rapid spread of the flame 

 throughout the whole, and will thus develop 

 an explosion. The violence of such explo- 

 sions depends much upon the details of con- 

 struction of the exhaust boxes and stive 

 rooms, and upon the dimensions of the 

 channels of communication ; it must obvi- 

 ously be regulated by the volume of the in- 

 flammable mixture through which the fire 

 rapidly spreads, and upon the degree of 

 confinement. In the case of the catastrophe 

 at Glasgow the production of a blaze at a 

 pair of millstones was observed to be fol- 

 lowed by a crackling noise as the flame 

 spread rapidly through the conduits lead- 

 ing to the exhaust box upon an upper floor, 

 and a loud report from that direction was 

 almost immediately heard. Professors Ran- 

 kine and Macadam, who carefully investi- 

 gated the cause of this accident, report* 

 that other flour-mill explosions which they 

 had inquired into had been observed to 

 have been attended by a similar succession 

 of phenomena to those noticed upon this 

 occasion. The bursting open of the ex- 

 haust box by a similar though less violent 

 explosion, attended by injury of workmen, 

 the blowing out of windows and loosening 

 of tiles, appears to have taken place on a 

 previous occasion at these particular mills. 

 In the last and most disastrous accident, 

 however, the more violent explosion appears 

 to have been followed by others, the flame 

 having spread with great rapidity to distant 

 parts of the mills through the many chan- 

 nels of communication in which the air 

 was charged with inflammable dust, result- 

 ing from the cleansing and sifting opera- 

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



tions carried on in different parts of the 

 building, and rapidly diffused through the 

 air by the shock and blast of the first ex- 

 plosion. 



In the experimental investigation of the 

 Minneapolis explosion by Professor S. F. 

 Peckham * it was shown that compacted 

 masses of flour which had become heated 

 and charred ignited readily and smouldered, 

 but were inflamed only with considerable 

 difficulty, though the atmosphere of the 

 conduit from the stones, through which a 

 strong current of air is being continually 

 drawn and which is filled with a dense 

 cloud of very fine particles of flour heated 

 to a maximum temperature of 140° F, 

 could be inflamed with comparative ease. 

 White-hot wires and glowing charcoal were 

 incapable of producing this inflammation, 

 and only burned the particles in actual con- 

 tact with them, and the only means bj' which 

 the mixture, in the best proportions, could 

 be made to burn explosively was by contact 

 with flame. 



The danger in the' process was found to 

 arise from the friction of the stones heat- 

 ing the last portion of the grist that re- 

 mained between them to a temperature 

 sufficient to char it or to convert it into a 

 substance resembling tinder, which would 

 readily ignite from a spark produced from 

 the stones striking together. Although 

 this burning mass could not inflame the 

 dust-laden atmosphere, it did ignite wood, 

 which a strong draught of air readily forced 

 into a blaze. Uader the conditions de- 

 scribed with a draught of air passing 

 through the dry stones strong enough to 

 convey the pellets of smouldering tinder 

 into the wooden conductor an explosion was 

 a necessary consequence. 



Knowing the chemical composition of 

 flour, we may calculate approximately the 

 mechanical work which a given mass of 

 flour can perform, and find that the con- 



*Am. J. Sci. 16 (3), 301-306 ; 1878. 



