360 



SCIENCE. 



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



The lower ' limit ' of inflammable gas 

 rej)resents the minimum proportion M'hich, 

 when mixed with air under ordinarjr con- 

 ditions, will burn rapidly, and will, under 

 certain conditions, produce explosions. If 

 the proportion of inflammable gas mixed 

 with the air is less than this in amount the 

 mixture will only bum in the immediate 

 neighborhood of the kindling flame, and 

 will not burn throughout. If, on the other 

 hand, the propoi'tion of inflammable gas in 

 the air exceeds the maximum ' limit ' the 

 gas will only be kindled and burn where it 

 is in contact witli an additional supplj^ of 

 air. 



All proportions of gas intermediate be- 

 tween these limits are explosive when 

 mixed with air, consequently the chance of 

 an explosion resulting from the presence of 

 one of these gases in the air is the greater, 

 the more widely the ' limits ' are apart, 

 since this gives rise to the possibility of a 

 larger number of explosive mixtures being 

 produced. Therefore, the danger of explo- 

 sion is least with methane and greatest 

 with acetylene. Methane is a safer gas 

 also because it has a high temperature 

 and a slow rate of ignition. All of these 

 condititions tend to lessen the number of 

 colliery explosions. It is to be noted that 

 mixtures that cannot be ignited when the 

 flame is applied to their upper surface may 

 be fired from below, and this is the method 

 of firing most probable to occur in coal 

 mines. 



Few of the gases mentioned occur singly 

 under conditions likely to give rise to dan- 

 ger. More commonly the combustible gases 

 are present in a state of mixture, as in water 

 gas and in coal gas. 



In giving ' limits ' it is assumed that the 

 tempei'ature of the mixture is not above 

 18°C. and that the pressure does not exceed 

 76 cm., for a gaseous mixture which is not 

 inflammable under these conditions may 

 become inflammable under increased tem- 



perature or pressure, and also that a mix- 

 ture that by ordinary test appears unin- 

 flammable will propagate flame if a con- 

 siderable volume of gas be projected into it, 

 owing to the resulting increase in tempera- 

 ture and pressure. 



It will be observed that Clowes' detector 

 reveals the presence Of gas in proportions 

 much below the danger point and gives 

 timely warning. 



The ignition of the fire damp has been 

 frequently caused by the gunpowder and 

 ' straw' used in blasting, for the outbursts 

 of gas from the shaken coal and the outru^sh 

 of flame and incandescent particles from 

 the blast were often coincident. The use 

 of electi'ic primers and detonators remedied 

 entirely the evils following the use of straws 

 and naked fuse, and the employment of the 

 high explosives gave greater immunity by 

 reducing the frequency of the blasts. 

 Greater security still has followed the use 

 of the flameless explosives made from nitro- 

 substitution compounds, or dynamites in 

 which crystalline salts, like sodium carbon- 

 ate and alum, containing a larger amount of 

 water of crj^stallization, are incorporated in 

 the mass, or water cartridges, in which the 

 explosive in the bore holes is placed in a 

 water bag or surrounded by moss, or other 

 porous substances, saturated with water. 



The occurrence of these mining accidents 

 has caused the authorities grave concern, 

 and several of the European governments, 

 notably Prussia, Fi-ance and England, have 

 appointed many commissions, some tem- 

 jDorary and others continuous, to investi- 

 gate the reasons for the accidents and the 

 methods of prevention. Many of the most 

 prominent chemists of these countries 

 have been called to serve uj)on the 

 commissions, and their reports have 

 proved not only useful in the solution of 

 the problem in hand, but have been val- 

 uable contributions to chemical science. 

 One of the more recent consequences of 



