RESPIRATION 317 



It will thus be seen that pure afterdamp, free from air, may 

 contain as much as 8.6 per cent of CO. Fresh afterdamp also con- 

 tains an appreciable percentage of HgS (not shown in the analy- 

 sis). This is a very poisonous gas, and o.i per cent will knock a 

 man over unconscious in a very short time. The most immediate 

 effect of fresh afterdamp may be due to HgS ; but on this point 

 there is no definite knowledge as yet. 



Considering the deadly composition of pure afterdamp it is at 

 first sight somewhat surprising that in actual colliery explosions 

 the men are not killed at once by the afterdamp, and that the CO 

 is so dilute in the atmosphere that kills them. It must, however, 

 be borne in mind that along the roads of collieries the coal dust 

 is never pure, and often contains so much shale dust that an ex- 

 plosion is not possible. The combustion is probably, therefore, far 

 from complete, so that much air is left, apart from what is drawn 

 in as soon as the air cools. Possibly, also, the percentage of CO in 

 the pure afterdamp is lower. 



Afterdamp is, of course, extremely dangerous to rescuers, and 

 many lives of rescuers have been lost owing to poisoning by CO. 

 They have gone too far into the poisonous air before becoming 

 aware of any danger, and the first symptom noticed is usually 

 faintness and failure of the legs, so that return is impossible. 

 Moreover the mental condition of men beginning to be affected 

 by CO is usually such, as already explained in Chapter VT, that 

 they will not turn back, and are reckless of danger. A lamp is of 

 course useless for indicating the danger. 



In order to give miners a practical means of detecting danger- 

 ous percentages of CO, I introduced the plan of making use of 

 a small warm-blooded animal such as a mouse or small bird.^^ 

 Owing to their very rapid general metabolism and respiration and 

 circulation small animals absorb CO far more rapidly than men. 

 Hence they show the effects of CO far more quickly, and can thus 

 be used as indicators of danger, although in the long run they are 

 possibly rather less sensitive to CO than men are. Thus a danger- 

 ous percentage which would require nearly an hour to affect a man 

 at rest will affect the bird or mouse within about five minutes. 

 This test has now come into very general use, and was, for in- 

 stance, largely used during the war by the tunneling companies. 

 It is easier to see the signs of CO poisoning in a bird in a small 

 cage, as it becomes unsteady on its perch, and finally drops, while 



"Haldane, Journ. of Physiol., XVIII, p. 448, 1895. 



