IV DEPTHS AND DEPOSITS OF THE OCEAN 179 



solve such calcium carbonate as it comes in contact with, 

 especially dead shells and skeletons. Three reasons for this 

 may be adduced : — 



(i) There may be local accessions of CO2, the dissolving 

 power of which has already been referred to. The sarcode of 

 molluscs and the albuminous binding material of their shells are 

 decomposed, on the death of the animal, to CO2 and ammonia, 

 the former being much in excess. The solvent thus provided, 

 in the case of any given shell-forming- organism, can only, how- 

 ever, be small relatively to the calcareous matter present. 



(2) The carbonate may be in a less stable, and therefore 

 more soluble, form than calcite. This is eminently true of 

 corals, which are mainly aragonitic. Some shells also are 

 wholly or partially aragonitic, and marine aragonitic algae 

 occur, such as Halimeda. Sea-water saturated for calcite 

 would, needless to say, be unsaturated for aragonite. 



(3) It is a familiar fact that freshly precipitated calcium 

 carbonate is much more soluble than the stable macrocrystalline 

 modification. The older theory, which supposed the former to 

 be basic or hydrated CaCOs, seems open to doubt, since there is 

 no sort of evidence that such compounds exist. More probably 

 the abnormal solubility is due to the exceedingly small size of 

 the particles. Above a certain limit of size, the concentration 

 of saturated solutions of a solid is constant, whether the 

 particles be large or small ; below this limit the concentration 

 becomes greater the smaller the particles, these stronger 

 solutions being in perfectly stable equilibrium with solid 

 particles of a definite magnitude. Experimental observations 

 of this phenomenon, which may be an effect of surface-tension 

 between solid and liquid, have in recent times been made on a 

 variety of substances.^ The limiting size for abnormal solubility 

 is about 2/u, diameter for gypsum, and will hardly be very 

 different for calcium carbonate. It may be that what is called 

 amorphous calcium carbonate is often merely calcite or aragonite 

 in a state of extremely fine subdivision, whence the higher 

 solubility. Abnormal solutions thus produced are of course 

 supersaturated for larger particles, but there is evidence that 

 they part with their surplus solute with extreme reluctance. 



In all probability, then, the particles of calcium carbonate of 

 organic origin in the sea, which are protected, during life, by 

 albuminoid matter, go into solution, in the course of their post- 

 mortem descent, by virtue of their minute size, and leave trails 



^ See Hulett, Zeiischr. Phys. Cketn., vol. xxxvii. p. 385, 1901. 



