1889-90.] Dr J. Murray & Mr R. Irvine on Coral Beefs. 97 
excess of carbonic acid in the sea water over that necessary to form 
a normal carbonate, but especially over that required to form a 
sesqui-carbonate with the surplus base. It also appears that this 
is a much more effective agent in the removal of carbonate of lime 
shells than the solvent power of sea water itself, although, as we have 
already shown, sea water from which carbonic acid is absent can 
dissolve carbonate of lime in addition to what it normally contains, 
still the quantity that can be thus taken up is relatively small. 
Mr Buchanan’s observations have also shown that carbonic acid is, 
as a rule, more abundant in bottom and intermediate waters than 
in surface waters, although in some places surface waters contain a 
very large quantity, as, for instance, among the coral islands of the 
Fijis. A series of experiments have recently been conducted which 
show that carbonated waters under high pressure will take up more 
carbonate of lime than when at the normal of atmospheric pressure.* 
These results may now be applied to the explanation of the 
removal of carbonate of lime shells and structures from the deep 
sea deposits, and from the lagoons of coral islands. The fact that 
carbonic acid is more abundant in the deeper waters of the ocean is 
evidently connected with the respiration and decay of the organisms 
that live on the sea bottom, and with the decay of those that fall 
down to the bottom from the surface of the sea. The water that 
fills the deeper hollows has also in its passage to the equator passed 
over thousands of square miles of the sea floor covered with living 
animals. As this deep water has a very slow motion and is but 
slowly renewed, we would expect an accumulation of carbonic 
acid, and a deficiency of oxygen in these abysmal depths. 
When, therefore, a carbonate of lime-secreting organism dies in 
the surface waters and the dead body commences to fall towards 
the bottom, its shell is at once exposed to solution from the action 
of the sea water and the carbonic acid, produced, it may be, by 
the decomposition of its own body. If the shell be an exceedingly 
thin one, as in the case of Heteropods and some Pteropods, it may 
be wholly removed before reaching the bottom, at the depth of a 
few hundred fathoms. The great majority of shells, however, are 
only partially removed during the first few hundred fathoms, and 
on reaching the bottom at these lesser depths they accumulate 
* Reid, Proc. Roy. Soe. Edin vol. xv. pp. 151-157, 1888; Appendix, p. 108. 
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