1876.] of Coal-dust in Colliery Explosions, 361 



At the whole of these velocities the mixture became instantly inflam- 

 mable when coal-dust was added to it, and filled the apparatus from B 

 to O with a large smoky flame, which set the wood on fire when continued 

 for more than a few seconds. At a rather higher velocity than the last, 

 the coal-dust did not make the mixture inflammable; I did not, however, 

 determine the exact point at which it ceased to have this effect. 



By other experiments I found that a mixture consisting of 1 volume of 

 this firedamp and 15 volumes of air is inflammable, and that with 16 

 volumes of air it is not inflammable at a temperature of 57° Fahr. 

 Making use of these data, then, to calculate the composition of the mix- 

 ture passing through the apparatus during the last of the second series 

 of experiments, we find that the relative volumes of gas and air were 

 9*375 and 140*625 at a velocity of 150 feet per minute; whereas they 

 were 9*375 and 1050*625 at 1060 feet per minute, or 1 firedamp to 

 112*06 air in the latter case. 



In order to obtain reliable results from these experiments, it was ne- 

 cessary to have some means of ascertaining that the whole of the fire- 

 damp entered the apparatus at the low velocities, and that the fan exerted 

 no exhausting force on the gas-pipe in the opposite cases. For this 

 purpose the 2-in. gas-pipe was terminated at a distance of 16 inches from 

 the fan, and the gas was conducted from this point to the centre of the 

 fan-chamber, through a l§-inch pipe fitted loosely into the former to a 

 distance of about a foot; a very delicate means of observing the state of 

 the pressure in the gas-pipe was also provided by having a small hole 

 punched in it at a distance of 15 feet from the fan, and keeping a jet of 

 gas continually burning at that point. 



When the velocity of the current was low, gas escaped through the 

 space between the two pipes, and discovered its presence when a light 

 was applied to it. It was then necessary to close some of the air-inlets 

 to the fan, either partially or wholly, until a flame held at the junction 

 of the gas-pipes was drawn inwards; at the same time the flame of 

 the jet 15 feet off had to be carefully observed, so that the closing of the 

 inlets might not be carried too far. "When the velocity was greatest, on 

 the other hand, all the air-inlets were opened ; and although air was 

 drawn through the junction of the gas-pipes, the height of the flame of 

 the jet already referred to did not diminish to an appreciable extent. 



Having ascertained in the manner just described that when air con- 

 tains only a very small quantity of firedamp (0*892 per cent, of its 

 volume) it is capable of forming an inflammable mixture with coal-dust, 

 we are naturally led to inquire into the means by which the presence of 

 firedamp is usually ascertained in mines ; and we find that the universal 

 practice at the present day is to carry a safety-lamp into the atmosphere 

 to be examined, and observe the indications of the flame. When the 

 atmosphere is explosive, or nearly so, these indications are unmistakable; 

 but when the proportion of firedamp is small, it is necessary to pull down 



vol. xxiv. 2 E 



