3 i2 RESPIRATION 



presence can often be detected only by analysis, on account of the 

 predominance of fire damp. Occasionally there is so little CO 2 

 present in black damp that it is lighter than air; or it may be 

 lighter than air owing to admixed fire damp. I found that the 

 black damp formed simply in the oxidation of coal at ordinary 

 temperatures contains small percentages of CO, 5 but black damp 

 as ordinarily found in considerable concentrations in mines is 

 practically free from CO. 



The action of black damp on lamps and candles is of much 

 practical importance, particularly as a miner trusts to his lamp 

 to warn him of the presence of black damp or fire damp. A flame 

 is extremely sensitive to any variation in the oxygen percentage 

 in air. If the oxygen percentage is increased the flame becomes 

 brighter and hotter, and substances which are not inflammable 

 in ordinary air may then become readily inflammable. If the 

 oxygen percentage is diminished the flame becomes dimmer and 

 less hot, unless the diminution is due to the addition of an inflam- 

 mable gas to the air. When the oxygen percentage is dimin- 

 ished by the addition of nitrogen or black damp to the air, the 

 light given by a candle or lamp diminishes by about 3.5 per cent 

 for a fall of o.i per cent in the oxygen percentage. 6 With a fall 

 of about 3 to 3.5 per cent in the oxygen an oil or candle flame is 

 extinguished. Aqueous vapor is even more effective than nitrogen 

 in causing extinction of flame. It should be noted that it is to the 

 percentage, and not the partial pressure, of oxygen that the flame 

 is so sensitive, whereas it is the partial pressure that is of physio- 

 logical importance. A fall in the oxygen percentage of 3 per cent 

 is of very little importance to a man, though it extinguishes a 

 flame. On the other hand a flame still burns well when the atmos- 

 pheric pressure is diminished to a third, while a man is soon as- 

 phyxiated. Gas flames may be much less readily extinguished by 

 fall in oxygen percentage than oil or candle flames. Thus a hydro- 

 gen flame may not be extinguished till the oxygen percentage 

 falls to half or even less, the extinction point depending to a con- 

 siderable extent on the velocity with which the gas is issuing from 

 the burner. An acetylene lamp will burn till the oxygen percentage 

 falls to about 12. 



The physiological action of black damp added to air depends 

 within wide limits on the percentage of CO 2 in the black damp, 



* Haldane and Meachem, Trans, Inst. of Mining Engineers, 1899. 

 8 Haldane and Llewellyn, Trans. Inst. of Mining Engineers, XLIV, p. 267, 

 1902. 



