326 RESPIRATION 



causes. By always carrying a lighted candle or lamp with him, a 

 well sinker can guard most effectually against the danger from 

 black damp ; but it is quite unsafe to trust to previous tests. 



It is thus evident that a well acts as a chimney communicating 

 with a large air space in the substance of the surrounding rock, 

 or in crevices within it. Air may either be going down this chim- 

 ney or returning; and if the rock contains any oxidizable material 

 such, for instance, as iron pyrites, the returning air or gas has 

 lost more or less of its oxygen, and possibly also gained some COo. 

 If, however, less than about 4 per cent of CO2 were present in 

 the black damp it would be lighter than air, and thus likely to 

 escape unnoticed. 



An interesting case which came under my notice later may be 

 mentioned in this connection. While a tunnel was being driven with 

 compressed air under the Thames it was found that in a large 

 cold storage on the river bank lamps or candles were extinguished. 

 The air was analyzed for CO2, but no noticeable excess was found. 

 On analysis I found the air very poor in oxygen. On further 

 investigation it turned out that air very poor in oxygen, but with 

 practically no excess of CO2, was coming up the shaft of a well 

 belonging to the building.^-"- The flow did not depend on baro- 

 metric pressure, and nothing of the sort had occurred before the 

 construction of the tunnel began. It was evident, therefore, that 

 the flow was due to compressed air escaping deep down through 

 the London clay from the advancing end of the tunnel, and 

 forcing a way to the well, but at the same time losing oxygen 

 owing to the presence in the clay of oxidizable material such as 

 iron pyrites. The pure black damp contained 99.6 per cent of 

 nitrogen and 0.4 per cent of CO2. 



Air of Railway Tunnels. Although the great difficulties form- 

 erly experienced in the ventilation of long railway tunnels have 

 been overcome by the substitution of electric traction for steam 

 locomotives, it may be worth while to record here some of these 

 difficulties. Probably the worst cases were those of single-line 

 tunnels on a stiff gradient in the Apennines. When the wind was 

 blowing in the same direction as a train was traveling on an up- 

 gradient the smoke from the engine or engines tended to travel 

 with the train. Thus the air rapidly became poisonous from the 

 presence of CO, and the oxygen percentage fell so low that some- 

 times lights were extinguished and steam began to fail, owing 



" Blount, Journ. of Hygiene, VI, p. 175, 1906. 



