RESPIRATION 315 



with different percentages of methane when an ordinary oil flame 

 is lowered to the extent required in testing. 



To obviate the danger arising from ignition of fire damp mix- 

 tures by lamps, some sort of safety lamp is now always used in 

 fiery mines. A safety lamp may be either an oil lamp constructed 

 on the general principle introduced by Davy, or an electric lamp; 

 but the latter has of course the disadvantage that it does not indi- 

 cate the presence of fire damp and black damp. 



As regards its physiological properties, fire damp behaves as 

 an indifferent gas like nitrogen or hydrogen. A mixture of 79 per 

 cent of methane and 21 of oxygen has the same physiological 

 properties as air, except that the voice is altered ; and the physio- 

 logical action of methane is simply due to the reduction which it 

 causes in the oxygen percentage. Its action can thus be deduced 

 from the data in Chapters VI and VII. In actual practice the 

 danger from asphyxiation by fire damp is considerably greater 

 than from black damp, since a man going with an electric lamp or 

 no lamp into air progressively vitiated by fire damp has little 

 physiological warning of impending danger. He is in a similar 

 position to an airman at a very high altitude, and if he suddenly 

 falls from want of oxygen he is very likely to die from failure of 

 the respiratory center. 



Afterdamp. Afterdamp is the gas produced as the result of an 

 explosion, and has been known for long to be specially dangerous. 

 In 1895 I made an inquiry into the causes of death in colliery 

 explosions,^ and found that nearly all (about 95 per cent) of the 

 men who died underground were killed by CO, although a con- 

 siderable number had received such serious skin burns that they 

 could hardly have survived in any case. Death was never due to 

 deficiency in the oxygen percentage of the air, nor to excess of 

 CO2, nor, apart from exceptional cases, to more than 2 per cent 

 of carbon monoxide. It was clear that the men had died in air 

 containing plenty of oxygen, and not much carbon monoxide. 

 That carbon monoxide was the actual cause of death was clear 

 from the fact that the venous blood was usually about 80 per cent 

 saturated with CO ; and that death was slow, and therefore due to 

 a low percentage of CO, follows from the fact that about the same 

 saturation was found all over the body. With more than about 

 2 per cent of CO the venous blood has not time to become evenly 

 saturated and the saturation is usually a good deal lower. 



" Haldane, Report on the Causes of Death in Colliery Explosions and Fires, 

 Pari. Paper C, 81 12, 1896. 



