264 SCIENCE FROM AN EASY CHAIR 



cylinder of compressed oxygen gas which is carried by 

 the diver. The advantage of such an arrangement is that 

 the diver is free from pumping apparatus and can go 

 where an ordinary diver could not. Mr. Fleuss was able, 

 by diving with this apparatus, to prevent an immense loss 

 of property by arresting the flooding of the Severn tunnel 

 which was imminent during its construction. A difficulty 

 in regard to the Fleuss apparatus has been that oxygen 

 gas is a poison, causing inflammation of the lungs and 

 convulsions when under a pressure of from two to three 

 atmospheres that is to say, at from 30 ft. to 60 ft. depth 

 in water. The pressure exercised by the air of the atmo- 

 sphere at sea-level is equal to that exercised by a column 

 of water 30 ft. high, and hence at 30 ft. depth in the sea 

 the oxygen gas would be under the pressure both of the 

 atmosphere itself and of water to the same amount 

 which is expressed by saying that it is under two atmo- 

 spheres' pressure, or twice the atmosphere's pressure. The 

 pressure of the atmosphere is, in plain figures, 15 Ib. on 

 every square inch of surface. Of course, the oxygen is 

 compressed far beyond this point in the cylinders in 

 which it is carried. In using it, it is allowed to escape by 

 opening a valve leading into an elastic sac, and is then 

 and there subject to the pressure depending on the depth 

 of water to which the diver has descended. It is found to 

 be dangerous for a diver with this apparatus to descend 

 to a depth of more than 30 ft. having pure oxygen in his 

 apparatus, because the oxygen is then compressed under 

 a pressure of two atmospheres. Accordingly, Dr. Haldane, 

 of Oxford, has proposed that the oxygen should be 

 diluted with atmospheric air, so as to give a mixture 

 of equal volumes of oxygen and nitrogen. With this 

 mixture the diver can safely descend to a depth of 60 ft. 

 The apparatus is provided with a partition containing 

 caustic soda, which absorbs the carbonic gas thrown out 



