The Several Forms of Calcium Carbonate. 491 



is so great that one may very plausibly imagine aggregates of 

 them when dried to be made up of minute nuclei, each sur- 

 rounded by a layer of material which is porous to some slight 

 and variable extent ; for which reason the aggregates as they 

 are observed may appear to be amorphous, and may show 

 properties differing from those of undoubted crystalline mate- 

 rial. The centers of many of our crystalline aggregates of 

 calcium carbonate had a lower refractive index than the sur- 

 rounding material and were nearly or quite isotropic ; these 

 centers were, therefore, probably " amorphous " in the sense 

 referred to above. The washed and dried gelatinous precipi- 

 tates obtained by mixing concentrated solutions of CaCl 2 and 

 Na 2 C0 3 were isotropic and had an apparent refractive index 

 when immersed in oil of 1*51— 1*53. 



It would lead too far to go fully into the question of the 

 nature of these finest grains, a question which is discussed in all 

 up-to-date books on colloids ; but we shall give the merest out- 

 line of the reasoning in favor of the view at present most gen- 

 erally adopted. It has been shown by von Weimarn that, by 

 appropriate variation of the conditions of precipitation, barium 

 sulphate can be obtained in any size ranging continuously 

 from well-formed crystals through microscopic and ultramicro- 

 scopic particles to particles which are too fine to be detected 

 by the ultramicroscope ; in other words, there is for barium 

 sulphate — and therefore, in all probability, for other solid sub- 

 stances — a continuous gradation of many properties as we pass 

 from undoubted crystals to " amorphous " material. Now the 

 smaller the particle, the greater the surface force acting upon 

 it, and this surface force increases in intensity more and more 

 rapidly as the size of particle is reduced,* thereby masking the 

 vectorial crystal forces, until it balances, and finally transcends 

 them in importance. Aggregates of the smallest particles, 

 therefore, have " amorphous " properties, though they may 

 well be virtually crystalline since, when placed in a supersatu- 

 rated solution of the substance, they can serve as nuclei about 

 which crystallization takes place. 



However this may ultimately prove to be, it is at the present 

 time simplest to consider " amorphous " CaCO s , not as a sepa- 

 rate and distinct form, but merely as a state of the substance 

 whose properties in this state depend upon the size of the ulti- 

 mate particles ; and, since an actual sample of such material is 

 not made up of particles of a single size, but contains a range 

 of sizes in proportions depending upon its mode of production, 



* A direct manifestation of this force is the fact that the solubility (as 

 distinct from the rate of dissolution) of fine particles of a substance is 

 greater than that of coarser particles — a phenomenon made use of when one 

 allows a very fine precipitate to stand in order to obtain it in filterable form. 



