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 Grorrnus with regard to this are important. They 
prove that the combustibility, of the inflammable gases is so 
much dependent on their density, that if 2 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 mflammability 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 
Tuomson 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 mme, must 
therefore probably be owing to the large mass of it inflamed, 
and to the state of condensation 4 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 
mauch danger is dreaded, and having very seldom caused explo- 
sion. 
