318 RESPIRATION 



easier to handle, and less apt to die suddenly and thus leave the 

 miner without any test. The animals recover very quickly as soon 

 as purer air is reached and this greatly increases their value as a 

 test. 



After an explosion it is very necessary to have some test for 

 CO. The ventilation system is thrown out of action owing to doors 

 and air crossings being blown in. On the other hand it is very im- 

 portant to get in as soon as possible in case men are still alive, and 

 in order to deal with any smoldering fires left by the explosion. 



When air in a mine is for any reason not safe to breathe, self- 

 contained breathing apparatus are now frequently employed. It 

 is beyond the scope of this book to describe these apparatus in 

 detail ; 13 but it may be mentioned that the usual principle employed 

 is that the wearer breathes through a mouthpiece into and out of 

 a bag, the nose being closed by a noseclip. Into the bag there is 

 directed a stream of oxygen from a steel cylinder carried behind ; 

 and by means of a reducing valve and properly adjusted opening 

 beyond it the stream is kept steady at not less than 2 liters 

 per minute. This is as much as a man uses during pretty hard 

 exertion. If he uses less, the excess is allowed to blow off. 

 If he uses more, the oxygen percentage in the bag may fall 

 rather low, or the bag may become flat before the end of a full 

 inspiration. In the former case he will begin to pant more than 

 usual, but will not fall over so long as the 2 liters are coming in. 

 If less than about 2 liters are coming in he will be liable to fall 

 over, owing to a rapid fall in the oxygen percentage. If the bag 

 begins to go flat he will notice this, and either turn on more oxy- 

 gen through a by-pass, or exert himself less. The carbon dioxide 

 in the expired air is absorbed by a purifier containing caustic 

 alkali. 



In another form of apparatus the delivery of oxygen is gov- 

 erned by the state of fullness of the bag; but in applying this 

 principle there is the difficulty that the oxygen may not be quite 

 pure, and the contained nitrogen may thus accumulate in the bag, 

 or a little nitrogen may leak in from the air at the mouthpiece. 



In still another form use is made of liquid air, of which a large 

 amount can be carried, so that most of the expired CO 2 can be 

 allowed to pass out and only a small purifier is needed. 



13 A thorough discussion of the apparatus in use in America and the principles 

 and practice applicable to it is given in U . S. Bureau of Mines Technical Paper, 

 No. 82, 1917, by Yandell Henderson and J. W. Paul. Numerous investigations, 

 including two full reports by myself, have appeared in Great Britain. 



