44 ON THE FIRE-DAMP OF COAL-MINES, 
steel and flint, in the séeel-mill, as it is called, a person could 
advance to any spot, empty a bottle of water there at any par- 
ticular height from the floor, cork the bottle, and immerse the 
mouth of it, inverted in water. A portion of the air would 
thus be withdrawn, and at the bottom of the shaft, or at the 
mouth of the pit, it might be tried whether it were explosive. 
or not. Even if it did not kindle, the degree of the approach 
to danger from the intermixture of a certain portion of the in- 
flammable air, might be ascertained. ‘That a mixture of car- 
buretted hydrogen and atmospheric air should be capable of 
being inflamed, they must be in certain proportions to each 
other. Not less, as Dr Tomson has remarked, of the carburet- 
ted hydrogen than one-twelfth of the volume of common air 
must be present ; and when it exceeds one-sixth of the volume 
of the air, it ceases to explode. All mixtures in the propor- 
tions between these will explode, but beyond these extremes 
will not. If the air withdrawn from the mine for examination 
does not explode, it may be discovered how near it is to the 
first proportion at which this. will happen, by adding to differ- 
ent portions of it, certain proportions either of hydrogen or at- 
mospheric air, until an explosive mixture is formed. If the’ 
addition of a small proportion of hydrogen, for example, were 
sufficient for this purpose, this would indicate the near ap- 
proach to danger, by shewing that.a very little farther inter-~ 
mixture of fire-damp would render the air in that part of the 
mine explosive. An excess of hydrogen, if it were present, 
would always be hazardous, for although it might not form 
properly speaking, an explosive mixture, still it would be in- 
flammable. ‘I hese experiments are so simple, that the more: 
intelligent miners or superintendents might be easily taught to 
perform them. One of the most frequent causes of the unfor- 
tunate accidents that have occurred, seems to have been the: 
want, 
