478 



NATURE 



\ April i^, 1875 



explosive effects ; such ignitions, though occasionally observed 

 in small mills, being caused either by the striking of fire by the 

 stones, or by the incautious application of a light near the mill- 

 stones, or the meal-spout attached thereto, have not in these 

 instances been attended by any serious results. But in an exten- 

 sive mill, where many pairs of stones may be at work at one 

 time, each pair has a conduit attached to it, which leads to a 

 common 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, sometimes aided by a system 

 of air-blowers. The fine flour is allowed to deposit partially in 

 this chamber or exhaust-box, and the air then passes into a second 

 chamber called a stive room, where a further quantity of dust is 

 deposited. It follows that when the mill is at work these cham- 

 bers and the channels or spouts connecting them with the atmo- 

 sphere immediately surrounding each millstone, are all filled with 

 an inflammable mixture of the finest flour-dust and air, and that 

 consequently the application of a light to any one of those channels, 

 or the striking of fire by any one of the millstones, by igniting some 

 portion of the inflammable mixture, will result in the exceedingly 

 rapid spread of flame throughout the confined spaces which are 

 charged with it, and will thus develop an explosion. The vio- 

 lence of such explosions depends much upon details of construc- 

 tion of the exhaust-boxes and stive rooms, and upon the dimen- 

 sions of the channels of communication ; it must obviously be 

 regulated by the volume of inflammable mLxture through which 

 fire rapidly spreads and upon the extent of its confinement. 



The subject of flour-mill explosions, though it has attracted 

 little if any attention in this country previous to the Tradeston 

 explosion, is discussed in continental treatises on flour-mills, and 

 the results of Professors Rankine and Macadam's inquiries have 

 demonstrated that accidents of this kind are actually of ordinary 

 occurrence in mills, especially since the introduction of the 

 exhaust arrangements. Those gentlemen point out that it 

 appears scarcely possible to guard against such accidents alto- 

 gether, although the frequency of their occurrence may probably 

 be much reduced by adopting efficient precautions to prevent, as 

 far as possible, a stoppage of the "feed" to the millstone, or 

 the accidental introduction of nails between them together with 

 the grain, and by prohibiting the employment of naked lights in 

 the vicinity of the mills and the dust passages. In order to reduce 

 as far as possible the damage and risk of sacrifice of life resulting 

 from such explosions, it is important that all receptacles into which 

 the dust-laden air is drawn from the mills should be fixed outside 

 the buildings, and constructed so as to ofter as little resistance 

 as possible to the sudden expansion resulting from the ignition of 

 the inflammable mixture. The conduits leading from the mills 

 to the exhaust chambers should, moreover, be of small dimen- 

 sions, and there should be no other communication between the 

 interior of the building and the dust receptacles, which must not 

 be opened while the mill is at work. By adopting precautions 

 of this kind the mill-owner may succeed, at any rate, in reducing 

 the mischief resulting from an accidental ignition of flour-dust 

 at the millstones to such limits that the mill itself and the lives 

 of those engaged in it will not be endangered. 



The production of explosions by mixtures of air with marsh 

 gas, coal gas, petroleum vapours, or a finely divided iiiflammable 

 solid such as flour, has been shown to be due to the application 

 of sufficient heat to some portion of the mixtuie to cause the 

 atmospheric oxygen to combine with the combustible constituents 

 of the gas, vapour or solid, the results being the development of 

 chemical action, the formation of gaseous products, and their 

 expansion by the heat developed. It need scarcely be said that 

 the same explanation applies to the producton of exploaions by 

 that class of so-called explosive agents which is prepared by 

 intimately mixing combustible or inflammable solids with a solid 

 oxidising agent (i.e., an oxygen compound which rtadily yields 

 up a part or the whole of that gas under the influence of heat, 

 and with the co-operation of chemical force, to carbon, hydrogen, 

 or other readily oxidisable elements). Distinct from these explo- 

 sive mixtures as regards their nature, but quite analogous to 

 them in their behaviour and the effects they produce when sub- 

 jected to heat or other disturbing influences, are explosive com- 

 pounds. The majority of these contain carbon, h)drogen, and 

 oxygen as the most important components ; they are more or 

 less susceptible of sudden or extremely rapid transfonnation into 

 gases or vapours, attended by development of great heat, in 

 consequence either of their resolution into their elementary con- 

 stituents, or generally of the rearrangement of these into com- 

 paratively simple forms of combination. Some of these explosive 

 compounds are of such unstable character that they are liable to 



undergo change from very slight inciting causes, such as the 

 existence in them of minute quantities of foreign substances of 

 active chemical character ; or they may even be prone to abso- 

 lutely spontaneous charge. In such substances decomposition 

 may be in the first instance estabhshed only to a very minute 

 extent, but this decomposition, by the products to which it gives 

 rise, and by the attendant development of heat, however small, 

 may speedily promote further and more rapid change in the 

 mass of the substance, so that eventually decomposition of 

 violent nature may be established, and the principal portion of 

 the compound may suddenly undergo the same transformation 

 into gases or vapours, attended by the same development of 

 heat, as though any one of the agencies {i.e., fire, friction, or 

 percussion) ordinarily employed to determine the explosion of 

 these bodies had been applied. Cases of so-called spontaneous 

 explosion thus brought about are more familiar to scientific and 

 manufacturing chemists than to the general public, but accidental 

 explosions of very alarming, and, in a few instances, of very 

 calamitous character, are on record which, though not actually 

 of spontaneous nature, in the strict appHcation of the term, have 

 been brought about without any apparent appHcation of external 

 inciting agencies, and have hence, from a practical point of view, 

 not been incorrectly classed as spontaneous explosions. 

 {To be continued^ 



SOCIETIES AND ACADEMIES 



London 



1 



Mathematical Society, April 8.— Prof. ,H. J. S. Smith, 

 F.R.S., president, in the chair. — Mr. G. H. Darwin gave an 

 account of two applications of Peaucelliei's cells, first, to "the 

 mechanical description of equipotential lines" ; and secondly, to 

 " a mechanical method of making a force which varies inversely 

 as the square of the distance from a fixed point." In this latter 

 case, let be the fixed pivot of a cell, and suppose the cell to be 

 in equilibrium under the action of two faces, P and P, acting at 

 D and B. Then by the principle of virtual velocities — 



F .l.oD ^ P.Z.oB - 0. Now, oD.oB = oA- - AD- 



oD oB 



P' (0 A'' - A D"-] 



whence 



P = 



oB^ 



If then P' is a constant force acting away from 0, P is an attrac- 

 tive force varying as oB--. Mr. Darwin stated that the idea 

 was the joint production of his 

 brother Horace and himself, and 

 that he entertained the hope that 

 it would be possible to construct 

 a toy to give an ocular proof of 

 elliptic motion. A rough model 

 was exhibited. Sir W. Thomson, 

 F.R. S. , expressed his pleasure at 

 having heard the communication, 

 as he had himself failed in ti7ing 

 to get a mechanical means ol 

 making such a force. — Sir W. 

 Thomson then made two commu- 

 nications to the Society : one on 

 the integration of the equations for 

 the motions of a system acted on 

 by forces expressed by linear func- 

 tions of the displacements and velocities ; the other on the vibra- 

 tions ol a stretched string of gyrostats (dynamical theory of 

 Faraday's magnetic rotation of the plane of polarisation). — 

 Prof. Cayley, F.R.S., made a few remarks on some integrals 

 connected with the theory of attractions. — Mr. Tucker, hoji. 

 sec, then read a portion of a paper by Prof. Wolstenholme. 

 The problem discussed in this paper is thus enunciated : — A tube 

 of fine uniform bore is bent into the form of a regular polygon of 

 « sides, and filled with equal volumes of n different fluids which 

 do not mix ; it is then closed, and held in any position in a 

 vertical plane. The sides of the polygon formed by joining the 

 common surfaces of the different fluids will always have constant 

 directions ; but if two conditions be satisfied, every position will 

 be one of equilibrium. He applies his results to a few simple 

 cases ; thus, if « = 3, and the densities be in arithmetical pro- 

 gression, the straight line joining the ends of the fluid of 



