LAWS OF PRECIPITATION. 121 



nature of the precipitation. This statement is applicable both to com- 

 pounds present in solutions before precipitation begins and to compounds 

 formed by precipitation itself. Once any precipitate begins to form, par- 

 ticles of that precipitate are present and influence further precipitation, 

 precisely as do other solids which were present before the precipitation 

 began. The proof of the influence of the solids present is furnished by 

 the very well-known tendency to the enlargement of mineral particles 

 already existing in preference to the formation of new individuals. 



The growth of mineral particles already present is probably con- 

 nected with the phenomenon of adsorption, described on pages 64-65. It is 

 there noted that the contact film of solutions with solids contains more than 

 an average amount of material in solution. It may be suggested that this 

 is due to the molecular attraction of the crystal for the molecules in solu- 

 tion, just as the adherent film of the liquid itself is due to the molecular 

 attraction between the solids and liquids. As the particles in solution move 

 about they continually impinge against the solids in the solutions. These 

 particles thus come within the limits of the molecular attraction of the solids 

 and are to a certain extent held, and hence the concentration. It would 

 follow that the adherent films of liquid are likely to become supersaturated 

 in advance of the remainder of the solutions. Under these circumstances the 

 moment supersaturation is reached with reference to the compounds forming 

 a given particle, these materials will be deposited upon the particle, and 

 will grow. Precipitation immediately follows supersaturation of the con- 

 centrated film because of the orienting and selecting power of the mineral 

 particle already existing. It is probable in the case of a given mineral that 

 for compounds other than those which can unite to produce the mineral 

 supersaturation can take place to some extent, and that from this slightly 

 supersaturated adherent film this material may escape into the free solution. 

 However, when such solutions become supersaturated in the presence of a 

 mineral which could use them they would be thrown down. By this process 

 is explained the selective power by which each mineral particle is able to 

 take from solution material like itself and add it to itself; and also the fact 

 that particles once formed abstract materials like themselves from solutions 

 in preference to the formation of new particles. The presence of any 



«For explanation of adsorption see Ostwald, W., Grundlinien der anorganischen Chemie, 

 Engelmann, Leipzig, 1900, pp. 387-389. 



