Increased Pressure 1033 



depths, including the celebrated Bathybius, which, after playing 

 such an important part in the new philosophies of nature, seems to 

 have been relegated to mineral matter, 13 it is clear that they 

 undergo no immediate and mechanical effect from the enormous 

 pressure to which they are constantly subjected and with which 

 they are perfectly in equilibrium. Circumstances would be dif- 

 ferent if an animal accustomed to live at 2000 meters were sub- 

 merged to 4000 meters, for example; the excess of pressure would 

 cause a lessening of the volume of its body, which very probably 

 would have a harmful effect on its organism. Conversely, an ani- 

 mal brought from a depth of 4000 meters to the surface will ex- 

 pand considerably (about 15 thousands of its original volume), 

 and this sort of distention of the tissues is largely responsible for 

 the death of animals caught in deep-sea dragging. 14 



The mechanical influence of compression or decompression acts 

 in a very effective and very energetic way upon animals equipped 

 with air bladders, especially when they are closed, as in sea fish. 

 In this case, as M. A. Moreau 1 ' has satisfactorily demonstrated, any 

 sudden variation of pressure which, acting on the volume of their 

 bladder, can modify their average density enough to bring them 

 a few meters above or below their place of equilibrium, will, in 

 the first case, lift them to the surface, their bladder dilating to 

 the bursting point; in the second case, will make them sink in- 

 definitely in the depths of the ocean, their bladder contracting and 

 the density of their own body increasing in the same ratio as that 

 of the water. Let us note that since the natural variations of 

 barometric pressure do not exceed two centimeters of mercury 

 (26 cm. of water) per day, and the extreme variations are only 5 

 centimeters (65 cm. of water) at the most, the fish are not seriously 

 affected. Furthermore, as the remarkable experiments of M. Moreau 

 have shown, they can in time compensate for this influence, either 

 by secreting oxygen in their swimming bladder, or on the contrary 

 by absorbing the oxygen which it contains, and thus vary at the 

 same time its volume and their density. 



We have seen that aquatic animals are killed by oxygen when 

 compression introduces a sufficient quantity of it into the water 

 (p. 777). But this dangerous effect can evidently take place only 

 if the compression acts first on the air and then forces into the 

 water oxygen in growing proportion, following Dalton's Law; but 

 the pressure exerted by the column of water itself upon its deep 

 parts does not at all modify the real tension of the oxygen. Fur- 

 thermore, direct analyses of the ocean water taken from great 



