44 ON THE FIRE-DAMP OF COAL-MINES, 



steel and flint, in the steel-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 thus be withdrawn, and at the bottom of the shaft, or at the 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 

 ox 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 Thomson 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 If the air withdrawn from the mine for examination 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. 'Ihese 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 



