THE LIFP] OF MATTER. 421 



according- to efficient -^geometrical laws so as to produce the typical 

 form b}^ a constructive process that ma}^ be compared to the em))rvo- 

 genic process that l)uikls up the body of an animal. Now this opera- 

 tion may be disturbed ])y accidents in the surrounding medium or by 

 the predetermined intervention of the experimenter. The crystal is 

 then mutilated. Pasteur saw that these mutilations repaired them- 

 selves. "When/' said he, "a crystal from which a piec(> has been 

 broken otf is replaced in the mother liquor, we s(>e that while it 

 increases in every direction by a deposit of crystalline particles, an 

 excessive activity occurs at the place where it was broken or deformed; 

 and in a few hours this suffices not only to build up the regular amount 

 required for the increase of all parts of the crystal, but to reestab- 

 lish regularity of form in the mutilated part." In other words, the 

 woi"k of formation of the crystal is carried on much more actively at 

 the point of lesion than it would have been had there been no lesion. 

 The same thing would have occurred with a living being. 



Meclmnism of the reparution. — Gernez some 3^ears later made known 

 the mechanism of this reparation, or at least its immediate cause. He 

 showed that on the injured surface the crystal becomes less soluble 

 than on the other facets. This is not, however, an exceptional phe- 

 nomenon. It is, on the contrary, quite frequently observed that the 

 difl'erent faces of a crystal show marked differences in solubility. 

 This is what occurs in evei-y case for the nuitilated face in comparison 

 with the othei's; the matter is less soluble there. The consequence of 

 this is evident; the growth must preponderate on that face, since there 

 the mother liquor will become supersaturated })efore being so for the 

 others. We may make this result understood in another wa}. Each 

 face of the crystal in contact with the mother liipior is exposed to tw^o 

 antagonistic effects: The matter deposited upon a surface may betaken 

 away and redissolved if, for any reason whatever, such matter becomes 

 more soluble with reference to the liquid stratum in contact with it; 

 in the second place, the matter of that liquid stratum may under con- 

 trary conditions be deposited and thus increase the body of the crystal. 

 There is, then, for each point of the crystalline facet, a positive oper- 

 ation of deposit which results in a gain and a negative operation of 

 redissolution which results in a loss. One or the other effect predom- 

 inates according as the relative solubility is greater or less tor the 

 matter of the facet under consideration. On the nuitilated surface it 

 is diminished; deposition then prevails. But this is only the imme- 

 diate cause of the phenomenon, and if we wish to know why the solu- 

 bility has diminished on the mutilated surface M. Ostwald will explain 

 it to us by showing that crystallization tends to form a polyhedron in 

 which the .surface energy is a minimum relative. 



