SOME ILLUSTRATIVE PROBLEMS 421 



stable or unstable phases appear.- Both may be forming at the same 

 time, but the relative rates of growth may be overwhelmingly in favor of 

 the less stable form. As soon as active growth of the deposit has ceased 

 the supersaturation which allowed the growth of the unstable form may 

 be reduced so as to permit the more stable form to continue its slow 

 growth at the expense of the less stable form. The factors controlling 

 the rates of formation of substances (besides the well known temperature 

 effect) are little known. Numerous reactions involving liquids and gases 

 are known to be greatly speeded up by the presence of certain substances, 

 called catalysts, which furnish no material to the substances which are 

 formed. Almost nothing is known about that action of catalysts which 

 finally results in increasing the rate of growth of crystals from solutions. 



Some illustrative Problems 

 dolomite 



Knowledge of the chemistry of dolomite goes scarcely farther than in- 

 ferences from the relations of this mineral as revealed from geological 

 studies. It is apparent that in some localities calcium carbonate reacts 

 with sea-water to produce dolomite ; furthermore, the sea is, and probably 

 has been, the only adequate source of magnesia for the great dolomite 

 formations. Many of the dolomites do not bear evidence of being replace- 

 ments of limestones. Thus arises the question whether or not they formed 

 as primary deposits through direct precipitation, either chemical precipi- 

 tation or precipitation through the intervention of minute dolomite-pro- 

 ducing organisms of unknown characteristics. Is dolomite and not cal- 

 cium carbonate the stable phase in presence of sea-water under the various 

 conditions of temperature of the oceans? Would a slightly higher ratio 

 of magnesia to lime in sea-water cause either a chemical precipitation of 

 dolomite or a rapid alteration of calcium carbonate to dolomite? Or is 

 dolomite one of tliose substances which at ordinary temperatures forms 

 very slowly, in solutions in which it is stable, unless a catalyst is present? 

 Is the general similarity but greater complexity of tlie crystal structure 

 of dolomite as compared with calcite a dominating cause of the slow re- 

 action of calcite to form dolomite ? 



Inasmuch as rates of reaction arc alxjut doubhMl by iiici-eases of tem- 



* The terms "stable" and "unstable" are used hf.'ve in the ordinary sense, which dis- 

 regards the effects of the adsorl)ed layer of foreign material on tlie surface of the 

 nuclei. In reality this laynr is not in general a permanent thing, for it is subject to 

 the jostlings of the swiftly moving ions of the solution. \\iii(;ii jwe tending tlius to 

 break It down and allow additions to be made to the nuclei. 



