128 SCIENCE PROGRESS. 



was found by Schloesing and Richard 1 to contain 85 per 

 cent, of oxygen and 12 per cent, of nitrogen and argon. 

 The pressure at this depth is about 1 50 atmospheres, hence 

 the tension of the oxygen in the bladder was about 127 

 atmospheres, and of the nitrogen about 18 atmospheres, as 

 compared with tensions of a fifth and four-fifths of an atmo- 

 sphere respectively in the sea-water. At such a pressure 

 complete absorption of the gas ought, on the diffusion 

 theory, to occur with the utmost rapidity, whereas nothing 

 of the sort happens, and these fishes must be able to secrete 

 oxygen and nitrogen against these enormous pressures. 

 The fact is the more remarkable since oxygen at anything 

 more than 4 or 5 atmospheres' pressure is exceedingly 

 poisonous to both animals and plants. Yet the wall of the 

 air-bladder is not susceptible to the poison. In this respect 

 it may be compared with the wall of the stomach, which, so 

 long as it is alive, resists the action of the exceedingly 

 poisonous hydrochloric acid which it secretes. In Dolium 

 Galea (or water snail) a digestive juice containing 4 per 

 cent, of sulphuric acid is secreted, yet this corrosive liquid, 

 which would instantly kill almost any other living tissue, 

 does no harm to the glands which secrete it. 



o 



The gas secreted by the walls of the air-bladder seems 

 usually to be a mixture of oxygen and nitrogen. The fact 

 that with increasing depth the oxygen percentage as a rule 

 increases would seem at first sight to suggest that oxygen 

 alone is actually secreted, the nitrogen simply diffusing in 

 until its partial pressure equals that of the nitrogen in the 

 water. This hypothesis is however not tenable. The 

 example quoted above shows that not only the oxygen 

 tension, but also the nitrogen tension in the air-bladder, 

 may far exceed that of the water. In some fishes, more- 

 over, the gas contained in the air-bladder may be pure, or 

 almost pure nitrogen. Thus in Coregonus Acronius, a fresh- 

 water fish living at a depth of 200 to 250 feet in the Lake 



1 Cbmptes Rendus, vol. cxxii. (1896), p. 615. The writers suggest 

 that the oxygen found by them may have been given off from the haemo- 

 globin of the blood while the animal was being drawn to the surface. 

 Evidently, however, this cannot be the case. 



