THE SAFETY-LAMP. 285 



for us briefly to point out the true qualities of the 

 lamp. 



In the Davy lamp a common oil-light is surrounded 

 by a cylinder of wire-gauze. When the air around 

 the lamp is pure the flame burns as usual, and the 

 only effect of the gauze is somewhat to diminish the 

 amount of light given out by the lamp. But so 

 soon as the air becomes loaded with the carburetted 

 hydrogen gas generated in the coal-strata, a change 

 takes place. The flame grows larger and less lumi- 

 nous. The reason of the change is this : The flame 

 is no longer fed by the oxygen of the air, but is 

 surrounded by an atmosphere which is partly in- 

 flammable ; and the inflammable part of the gas, so 

 fast as it passes within the wire cylinder, is ignited 

 and burns within the gauze. Thus the light now 

 given out by the lamp is no longer that of the com- 

 paratively brilliant oil-flame, but is the light resulting 

 from the combustion of carburetted hydrogen, or 

 " fire-damp," as it is called ; and every student of 

 chemistry is aware that the flame of this gas has very 

 little illuminating power. 



So soon as the miner sees the flame thus enlarged 

 and altered in appearance, he should retire. But it 

 is not true that explosion would necessarily follow if 

 he did not do so. The danger is great, because the 

 flame within the lamp is in direct contact with the 

 gauze, and if there is any defect in the wire-work, 



