38 ON THE FIRE-DAMP OF COAL-MINES, 



There is another circumstance which will give security to 

 these lamps, should it ever happen, from any unforeseen cause, 

 that a mixture of inflammable air were introduced, — the rare- 

 faction of the air within the lamp, and especially near the 

 flame. It is well known that mixtures of inflammable air with 

 atmospheric air, or even with oxygen, cannot be inflamed, if 

 the elastic fluid be in a certain degree of rarity. The experi- 

 ments of Grotthus with regard to this are important They 

 prove that the combustibility of the inflammable gases is so 

 much dependent on their density, that if a mixture of any of 

 them with oxygen gas be rarefied to a certain extent, either by 

 the air-pump, or by elevation of temperature, it could not be 

 kindled by the electric spark which kindled the same mixture 

 easily in its denser state. Hence, as he justly remarked, bo- 

 dies may be inflammable under pressure, the inflammability of 

 which is weak, or not apparent in a rarefied atmosphere ; and 

 in mixtures of different inflammable gases with atmospheric 

 air, there will be a certain degree of density within which only 

 the mixture can be inflamed. The inflammability of any mix- 

 ture of carburetted hydrogen with atmospheric air, is limited 

 to certain proportions, and in all of them is inconsiderable. Dr 

 Thomson states what is a proof of this, that he had never been 

 able to cause any mixture of it with atmospheric air to ex- 

 plode, it merely burnt rapidly ; its exploding in the mine, must 

 therefore probably be owing to the large mass of it inflamed, 

 and to the state of condensation in which it exists. Another 

 circumstance which shews, that even in the mine, its power of 

 kindling so as to explode, is not more than what just renders 

 it possible, is, that it is not kindled by the ignited sparks from 

 the collision of steel and flint, a machine producing these be- 

 ing used to give light in working or exploring the mine, when 

 much danger is dreaded, and having very seldom caused explo- 

 sion. 



