178 DEPTHS OF THE OCEAN 



which, according to the older chemical notions, was expressed 

 by the equation 



(NH,X,C03 + CaSO^=CaC03 + (NHJ,SO,, 



seems to have been first suggested in this connection by 

 Forchhammer, and was fully proved and worked out experi- 

 mentally, with respect to marine organisms, by Murray and 

 Irvine.^ It accounts for the enormous amount of calcium 

 carbonate at the bottom of the ocean, which once formed part 

 of the tests or skeletons of living organisms. A limited 

 amount of purely inorganic precipitation does, indeed, take place 

 in coral reefs and some shallow-water deposits and in the Black 

 Sea. In the Mediterranean, for instance, stone-like crusts are 

 plentiful, consisting of clay cemented by calcium carbonate, 

 which latter is produced by ammonium carbonate arising from 

 the decay of organic matter in the mud below bottom-level 

 meeting with fresh sea-water from above. We have further the 

 lime-concretions of the Pourtales, Argus, and Seine banks, the 

 "Challenger" casts of shells from the Great Barrier Reef,^ and 

 so on. But all these must be regarded as rarities. A great 

 many of the reactions here referred to are believed to be ruled 

 by enzymes and catalytic substances. 



Whilst a great deal of calcium is thus being taken out of 

 solution throughout the ocean, conversely the carbonate is 

 continually being redissolved. Calcium and magnesium carbon- 

 ates are held in solution mainly as bicarbonates ; but since 

 these compounds are incapable of existence in the solid state, 

 questions of precipitation and dissolution, so far as they can be 

 approached on theoretical grounds, must be decided by the 

 solubilities of the normal carbonates. The solubility of CaCOg 

 in water (foreign salts being absent), and the equilibrium of 

 the various molecules and ions concerned, have been fairly 

 thoroughly elucidated.^ When MgCOg is also present and 

 sea-water is the solvent, matters become so complicated that 

 we cannot calculate, from first principles, how near sea-water 

 is to saturation for calcium carbonate. . There are, however, 

 direct empirical data on this point. From the experiments of 

 Anderson with natural, and of Cohen and Raken with artificial, 

 sea-water, it would appear that with regard to CaCOg, in the 

 final stable modification of calcite, sea-water is saturated and 

 incapable of taking up more, under conditions of stable 

 equilibrium. Nevertheless the ocean does unquestionably dis- 



1 Proc. Roy. Soc. Edin., vol. xvii. p. 79, 1889. 

 - Deep-Sea Deposits Chall. Exp., pp. 170, 172, 1891. * Bodlander, loc. cit. 



