FIRE DAMP. 47 



into it. The combustion of 1 cub. ft. of methane renders 

 about 40 cub- ft. of air unfit for respiration. The ignition of 

 fire damp has been variously determined as 650deg. C. and 

 780deg. C. As a rule no ignition is produced by a glowing 

 iron or an occasional spark caused by striking a steel tool 

 against some hard substance, but a flame or electric spark, or 

 a series of sparks is necessary. This is said to be due to the 

 fact that the gas requires time to attain ignition temperature, 

 and that with limited heat the gas is first warmed, then 

 ascends before the ignition temperaure is reached. 



