CALCIUM PHOSPHATE AND CALCIUM CARBONATE. 287 



ment travel about in the solution, colliding with and working past 

 others. Suddenly two of these, instead of passing, will be drawn 

 together, as though pulled by a strong attractive force, and will 

 merge completely in one large spherule, which in its turn may 

 absorb small ones. At these moments of coalescence the spherules 

 give the observer the impression of being fluid, flowing together 

 like two drops of water, and lead one to believe that Link was 

 correct in interpreting these small spherules as liquid crystals. As 

 these bodies become larger they lose their movement, settle down, 

 and become of glassy hardness. There is also a later growth by 

 addition of material from the surrounding clear solution. 



Richards and Archibald have shown that the particles of pre- 

 cipitates at their first appearance seem to have a rounded outline 

 to the eye even under high magnification, but by photographs, how- 

 ever, they have demonstrated that these particles are real crystals 

 of typical shape. This condition is true for some particles of 

 calcium carbonate formed in aqueous solutions, but in colloids a 

 great deal of the deposit is in the form of particles which are 

 round, and persist in this shape as they grow. There are other 

 particles also, which clearly are not round, and give the crystalline 

 form of deposit which occurs simultaneously with the spherules. 



The spherules formed in gelatin (Figs. 13 and 31) are very 

 perfect, with a smooth surface, and clear, transparent, glassy ap- 

 pearance. Those formed in albumen (Figs. 26 and 29) often show 

 a slightly irregular periphery and give indications of their internal 

 structure. They appear to be built up of concentric layers, and 

 often exhibit an amorphous center. Extending from this center to 

 the periphery is also a faint, but quite distinct, radial striation in 

 certain cases. These appearances have already been well described 

 and illustrated by Rainey, Harting, and Biedermann. 



It ought to be noted here that Harting has been erroneously 

 credited by many writers with the discovery, in 1872, of the calco- 

 spherites formed in colloidal solutions. Rainey described and gave 

 accurate illustrations of these bodies in 1857, and had observed 

 them as early as 1849, and so deserves the credit of this discovery. 



Combined precipitates of calcium carbonate and phosphate (Fig. 

 36) show each salt acting quite independently, the phosphate occur- 

 19 



