IV DEPTHS AND DEPOSITS OF THE OCEAN 193 



are, in the last resort, of biological origin. The phosphoric Phosphatk 

 acid in sea-water is derived chiefly from the skeletons and concretions. 

 tissues of the marine fauna. At certain spots great masses of 

 these skeletons are heaped up at the bottom, and here or here- 

 abouts phosphatic nodules are presently formed. In order to 

 explain why the phosphate of decaying bones goes into solution 

 it is not necessary to postulate exceptional conditions in the 

 surrounding sea- water. The solubility of tricalcium orthophos- 

 phate in water is a matter which bristles with complications, and 

 experimental difficulties have hitherto proved too great for its 

 exact measurement ; but it seems to be of the order of deci- 

 grammes per litre. The solubility is much enhanced by the 

 presence of H' ions, i.e. of acids. The solvent action of carbonic 

 acid which has been suggested seems, however, to be merely 

 hypothetical. Carbonic acid is so weak that at best it can 

 produce only a negligible concentration of H" ions ; moreover, 

 there is experimental evidence that so long as excess of lime 

 (as bicarbonate) is present, calcium phosphate is no more 

 soluble in carbonated than in pure water. In all probability the 

 rapid dissolution of the calcium phosphate and carbonate in fish- 

 bones is simply due to the fine state of division. This effect 

 has already been discussed with reference to sea-shells. The 

 extreme fineness of the inorganic particles disseminated in the 

 gelatinous matter of fish-bones is attested by the translucency 

 of the mass. Or it may even be that the carbonate and 

 phosphate are present in a colloidal form. In either case they 

 will readily yield supersaturated solutions when the enclosing 

 ossein rots away, and as soon as a nucleus presents itself the 

 formation of concretions is ready to begin. Since phosphatic 

 concretions usually occur, as already indicated, in positions where 

 organic remains accumulate on the bottom at a rapid rate, as in 

 areas having a great range of surface temperature, the trans- 

 ference of matter from bones to nodules must have taken place 

 without much delay. Consequently there has been little 

 opportunity for differentia] diffusion of carbonate and phosphate, 

 so that these calcium salts are invariably found to have been 

 deposited simultaneously. The "Challenger" analyses show 

 i^ to 3 parts of tricalcium orthophosphate to one of calcium 

 carbonate. Magnesium phosphates being considerably more 

 soluble than those of calcium, the phosphate of bones is re- 

 deposited unchanged after its passage through sea-water ; only 

 a trifling percentage of magnesium is shown by the analyses, 

 and this is probably present as carbonate. 



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