RESPIRATION 313 



and can be deduced from the data already given as to the physio- 

 logical actions of CO2 and oxygen. It should be noted that the 

 CO2 diminishes greatly the risk that would otherwise exist from 

 diminution of the oxygen percentage. This risk is greatly di- 

 minished, owung to the fact that the CO2 firstly increases the oxy- 

 gen percentage in the alveolar air by stimulating the breathing, 

 and secondly raises the hydrogen ion concentration of the blood, 

 thus increasing the circulation rate and assisting the dissociation 

 of oxyhaemoglobin in the tissue capillaries. There is therefore 

 little or no danger from lack of oxygen till the oxygen percentage 

 in the air falls to 6 or 7 per cent; but if the oxygen falls much 

 lower death occurs from want of oxygen. The very evident effect 

 of the CO2 on the breathing gives good warning of the danger, 

 so that apart from the ample warning given by a lamp a man is 

 not likely to go into a dangerous percentage of black damp unless 

 he does so suddenly, as in descending a shaft or steep incline. 



In former times miners often worked in air containing so much 

 black damp as to put a great strain on their breathing while they 

 were at work. Air containing, say, 3 per cent of CO2 doubles the 

 breathing during rest; but this effect is scarcely noticeable sub- 

 jectively. During work, however, the breathing is also about 

 double what it would otherwise be, and the lungs are thus strained 

 to the utmost, Probat)ly a great deal of the emphysema from 

 which old miners used to suffer was due to this cause. ''^ 



The ordinary fire damp of coal mines is, practically speaking, 

 pure methane (CH4). In a very "fiery" seam as much as 1,500 

 cubic feet of methane may be given off per ton of coal extracted. 

 The methane is adsorbed in the coal,^ and may come off under a 

 pressure of 30 atmospheres or more. Of other higher hydro- 

 carbons a small amount is also adsorbed in the coal, but held more 

 firmly, so that only in the last fractions of gas coming off from 

 coal can their presence be clearly demonstrated by analysis. 

 No carbon monoxide comes off with the methane, but appreciable 

 quantities of CO2 and nitrogen are often given off. It occasionally 

 happens, however, that enormous quantities of CO2 are adsorbed 

 in coal and may come off in very dangerous outbursts. This is un- 

 known in British and American coal fields, but has been met with 

 in France. Sudden outbursts of adsorbed gas, whether methane 

 or CO2, can only occur, however, where coal has been locally dis- 

 integrated, as is apt to be the case near a fault. Ordinary solid coal 



' Haldane, Trans. Inst, of Mining Engineers, LI, p. 469, 1916. 

 'Graham, Trans. Inst, of Mining Engineers, LII, p. 338, 19 16. 



