

VC it8J." ] PROCEEDINGS OF THE NATIONAL MUSEUM. 221 



ties of the materials of which the oceanic floor is composed; and, lastly, 

 in the food supply. 



As determined by physicists and chemists, the water of the deep sea 

 varies in the proportions of mineral salts, carbonic acid, and air con- 

 tained in it very much as does the surface water. In general, at the 

 surface the warmer water of the tropics has the more salt and the less 

 nitrogen. When carried by currents to the Polar regions and cooled 

 this tropical water sinks to the bottom carrying its excess of salt along 

 with it. The Polar waters are less saline and contain more nitrogen. 

 The proportion of atmospheric air in the water is found strictly related 

 to the temperature, the pressure at great depths being regarded as hav- 

 ing no bearing on the question. The amount of oxygen in the sea water 

 diminishes gradually as we descend from the surface until about 850 

 fathoms is reached, when it ceases to change, or, at most, increases 

 slightly until the bottom is attained. 



Carbonic acid, according to Tornoe, does not exist in a free state in 

 sea water, but only in the form of carbonates, or, to a less degree, of 

 bicarbonates. Unless the decomposition of animal matter in some 

 manner sets free the carbonic acid, this conclusion is one which can not 

 be adopted without question, especially when we consider the great dif- 

 ficulties which are encountered in any attempt to obtain, or when ob- 

 tained to analyze, abyssal water. The effect of erosion on the shells 

 dredged from the deeps, even when they contain the living animal, is 

 so strongly marked, the devices for protection against erosion are so 

 recognizable in various species, that the biologist may well call the phys- 

 icist to a halt, while-the latter reexamines his data. It is certain that 

 erosive agencies, of which the effects are indistinguishable from those 

 known to be due to carbonic acid in other instances, arc extremely act- 

 ive in the deeps. 



In general, it seems as if we might safely assume that the composi- 

 tion of abyssal sea water shows no very important differences from that 

 of other sea water, and that the animals existing in it are not exposed 

 to any peculiar influences arising from this source alone. 



This can not be said of the physical conditions. Every one knows 

 how oppressive to the bather is the weight of the sea water at only a 

 few feet below the surface, and how difficult it is to dive, still more to 

 remain on the bottom, if only for a few seconds. 



But it is difficult to convey any adequate idea of the pressure at such 

 a depth as 2,000 fathoms, or about 2 miles below the surface. Pope 

 made impervious by tarring is said to have become reduced one-third 

 in its diameter by a descent into these depths. Any hollow object not 

 pervious or elastic is at once crushed. There is no doubt that at some 

 points on the ocean floor the pressure may amount to several tons to 

 the square inch. If we recall that the average pressure in steam boil- 

 ers is probably much less than 100 pounds to the square inch, it may 

 help towards an appreciation of the abyssal conditions, 



