RESPIRATION 325 



cated that quartz dust becomes relatively harmless when it is 

 mixed with other dust of the harmless variety. The lung cells 

 appear to clear out the quartz when they are clearing out the other 

 dust. 



It is evident that much further investigation is needed in order 

 to elucidate completely the physiology of dust excretion from the 

 lungs. It is equally evident, however, that this process is under 

 physiological control, just as much as other physiological activi- 

 ties are. 



Air of Wells. The case of the air of wells and other unventilated 

 underground spaces differs from that of mines owing to the fact 

 that no artificial ventilation is provided for. It might be supposed 

 that the air in a well, with only rock or brickwork round it, pure 

 water at the bottom, and the top more or less open, would never 

 be more than slightly contaminated. Experience shows, however, 

 that this is not the case, and that the air in even a shallow well, 

 only a few feet deep, is sometimes dangerously contaminated. 

 In 1896 I investigated this subject, visiting various wells where 

 men had been asphyxiated, in order to see what had happened. 20 

 I found plenty of foul air, and that its composition was similar 

 to that of black damp, and not simply CO 2 , as was then believed. 

 The composition of the gas varied from about 80 per cent nitro- 

 gen and 20 per cent CO 2 to almost pure nitrogen ; and it was quite 

 evident that this black damp or choke damp was simply the 

 residual gas from oxidation processes occurring in the strata 

 round the well. 



Another point which emerged quite clearly was that the state 

 of the air in any well liable to foul air depended entirely on 

 changes in barometric pressure. With a rising barometer the air 

 was quite clear, and with a falling barometer it was foul. Thus 

 any fall in barometric pressure might make a well very dangerous, 

 though an hour before the air was quite pure. Moreover with a 

 falling barometer the well might be brimful and rapidly over- 

 flowing with dangerous gas. The danger to which well sinkers 

 are exposed is thus evident At one well an engine house which 

 covered the top of the well had been built, and sometimes it was 

 unsafe to enter this building owing to the gas, unless doors and 

 windows were wide open. The engine man was much comforted 

 when I lent him an aneroid barometer and thus convinced him that 

 the outbursts of gas were due to natural and not supernatural 



20 Haldane, Trans. Inst. of Mining Engineers, 1896. 



