284 JAMES CRAWFORD WATT. 



lently agitated, and the agitation becomes quickly denned as being 

 caused by very minute particles, which are in very rapid motion, 

 both oscillatory and translatory. These particles, appearing only 

 as small dots even under the highest magnification, grow in size, 

 losing their motion as they increase in bulk, until finally they form 

 the fine amorphous granules which gather in clumps, and masses, 

 and fine films, to form the precipitate. 



Precipitation of Calcium Carbonate. 



Calcium carbonate was precipitated by the interaction of a M/10 

 solution of calcium chloride with a M/4 solution of sodium car- 

 bonate. The process here resembles that described above for cal- 

 cium phosphate in the form of myriads of fine particles in rapid 

 Brownian movement. All movement finally ceases as these parti- 

 cles agglutinate into large clumps, and then a remarkable change 

 occurs, for the granular masses just formed fade from the sight, 

 dissolving again into the solution, while scattered here and there 

 larger, rapidly growing particles occur, which become defined as 

 crystals, angular in outline, or as small spherical bodies, known as 

 spherules, which are later converted into crystals as they increase 

 in size (Figs. 1, 2, 3, and 25). 



Two or three hours after the formation the precipitate is all 

 crystalline. It is very rare to see a spherule persisting in pure 

 aqueous solutions. 



The appearance of precipitates of calcium carbonate in the form 

 of spherules has been frequently noted before. As long ago as 

 1839 it was described by Link, who, in accordance with this phe- 

 nomenon, thought that crystals at their first origin were fluid and 

 only later became hard and angular, as solidification occurred. 



Precipitation in Colloidal Solutions. 



The colloids employed were gelatin and egg albumen. 



Various concentrations of gelatin were experimented with, rang- 

 ing from an 8 per cent, solution, which solidified quickly, to a 1 per 

 cent, solution, which remained fluid for some time and then formed 

 a soft jelly. As the various concentrations gave similar results the 

 weaker ones were used as a matter of routine, being easier to 

 handle. 





