AGENCIES IN DEPOSITION OF CALCIUM CARBONATE 741 



contact with the solution, for there is a definite and quickly attained 

 equilibrium between the proportion of C0 2 in the adjacent atmos- 

 phere and the concentration of free C0 2 in the water, the factor of 

 proportionality being the absorption coefficient (the solubility) of 

 C0 2 in the solution at the particular temperature. It is true that 

 water at depth can hold more C0 2 in solution, if it gets hold of it, 

 for in that case bubbles of C0 2 gas cannot form until its virtual 

 pressure just exceeds the hydrostatic pressure; but slow diffusion 

 upward would tend to equalize the concentration at various depths. 

 In the ocean, on the other hand, the content of C0 2 is only what it 

 was able to absorb when at the surface, supplemented by that which 

 has been produced by organic processes — the latter being in all 

 probability but a small fraction of the whole in deep water. 1 How- 

 ever this may be, it is manifest why the water at depth should con- 

 tain more C0 2 , for its present low temperature, retained from its 

 polar days, establishes the fact that when at the surface in contact 

 with the atmosphere it was cold, and lowering of temperature 

 increases very markedly the amount of C0 2 which water can absorb 

 through contact with an atmosphere containing a constant propor- 

 tion of C0 2 . 2 This fact, combined with its present low temperature 

 — for, as we have seen, lowering of temperature of itself increases the 

 solubility of CaC0 3 — suffices to account for the well-known fact that 

 all shells and tests disappear in the depths of the ocean. 3 



Now let us revert to the consequence of abstraction of C0 2 , and 

 consider what will happen when, in the course of the oceanic circu- 

 lation, this cold water, which carries more C0 2 and more CaC0 3 than 

 the warmer surface waters, 4 reaches the surface and is slowly warmed. 



1 Buchanan (Proc. Roy. Soc, XXII [1874], 483) writes: "Down to nearly 2,000 

 fathoms life is still abundant; below this depth, however, the amount rapidly decreases 

 till, at about 2,800 fathoms, it is, for carbonic acid producing purposes, practically 

 extinct." 



2 Thus in contact with any atmosphere, water (or a dilute salt solution) absorbs 

 about twice as much CO2 at o° as at 20 . 



3 See the report of the Challenger expedition or the work of Sir John Murray. 

 In the present connection it is immaterial whether these shells consist of calcite or 

 aragonite, although assertions to the contrary may be found. 



4 This appears a necessary prerequisite, no matter what be the mechanism of 

 precipitation. Dittmar, in his article in the Encyclopaedia Britannica, states that there 

 is a slight but indubitable increase in concentration of calcium with depth. Moreover, 



