102 Mr. F. A. Ahel [April 28, 



even different samj)les of magnesia, differing somewhat in lightness 

 from each other, api:>eared to possess the activity in different degrees. 

 These facts seem to favour the view that a dust possessing par- 

 ticular physical characteristics exerts a contact- or catalytic action 

 upon gas mixtures, similar to that known to be possessed by platinum 

 and some other substances under particular conditions. Thus, when 

 finely divided platinum, or even a clean recently heated siu-face 

 of the compact metal is brought into contact with mixtures of 

 hydrogen, or of a hydrocarbon gas or vapour, with oxygen or air, 

 oxidation of the hydrogen or hydrocarbon is at once established, 

 accompanied by the development of heat, whereby the temperature 

 of the metal is raised and chemical activity promoted, so that heat 

 speedily accumulates, raising the metal to a temperature sufficiently 

 high to bring the surrounding gas-mixture to the exploding point. 

 K the metal presents a very large surface, or is in a specially porous 

 condition, as in the form of sj^onge or very fine powder (platinum 

 hhicJ:), the explosion of the gas-mixture may follow very rapidly, or 

 almost instantly, upon the first contact of some portion with it.* 



In many of the experiments with calcined magnesia just referred 

 to, it was distinctly noticed that a dark space intervened between the 

 gas flame use^l as the source of heat and the flare produced by the 

 ignition of the gas mixture through the influence of the dust cloud 

 suspended in it, which would seem to indicate that the dust particles, 

 immc^iliately upon passing through the flame, established some amount 

 of oxidation of the fire-damp, which proceeded with increased rapidity 

 as the dust became more highly heated through the chemical action 

 developed, so that within a short distance from the point where the 

 heating commenced the dust became incandescent, and the ignition of 

 the gas-mixture followed. Further exjjeriments which are contem- 

 plated may elucidate the precise nature of this action of non-com- 

 bustible dust in promoting the ignition of gas-mixtures which, in the 

 absence of dust, are not inflammable ; there appears little doubt, 

 however, that it constitutes one element in the dangers arising from 

 the presence of dust in the air of a mine which contains a small 

 proportion of fire-damp, and in which a hirge body of flame is acci- 

 dentally produced, either by a blown-out shot, or by a fire-damp 

 explosion of local character. 



Numerous experiments similar to those of Marreco and IMorison 

 were made bv the lecturer at "VTi^an with mixtures of air and coal- 

 dust from Seaham and other collieries, in the complete absence of 



* This action of platiiium (or palladium) has recently received applications 

 bearing special reference to the existence of explosive gas mixtures in coal mines. 

 The one consists in an apparatus proposed by Mr. Komer for removing, by slow 

 combustion, local accumulations of fire-damp : the other is a very simple and 

 portable photometric apparatus, devised by Mr. G. H. Liveing, by which p>ro- 

 portions of fire-damp much lower than the smallest amount discoverable by the 

 Davy lamp in the hands of the most expert, can be readily and quickly detected, 

 and the amount estimated with considerable accuracy. 



