In order to explain electric or magnetic phenomena, and also those of crystallization, it is admitted that the atoms of which bodies are composed are surrounded, each of them, with a sort of atmosphere formed of electric currents, owing to which these atoms are attracted or repelled on certain sides, and produce those varied effects that we observe under different circumstances.  According to this theory, then, atoms would be small electro-magnets behaving like genuine magnets.  Entirely free in gases, but less so in liquids and still less so in solids, they are nevertheless capable of arranging themselves and of becoming polarized in a regular order, special to each kind of atom, in order to produce crystals of geometrical form characteristic of each species.  Thus, as Mr. Saigey remarks in “Physique Moderne” (p. 181):  “So long as the atmospheres of the molecules do not touch each other, no trace of cohesion manifests itself; but as soon as they come together force is born.  We understand why the temperatures of fusion and solidification are fixed for the same body.  Such effects occur at the precise moment at which these atmospheres, which are variable with the temperature, have reached the desired diameter.”

[Illustration:  Figs. 1., 2., and 3.]

Although the phenomenon of crystallization does not essentially depend upon temperature, but rather upon the relative quantity of liquid that holds the substance in solution, it will be conceived that a moment will arrive when, the liquid having evaporated, the atmospheres will be close enough to each other to attract each other and become polarized and symmetrically juxtaposed, and, in a word, to crystallize.

Before giving examples of the production of electricity in the phenomenon of crystallization, it will be well to examine, beforehand, the different circumstances under which electricity acts as the determining cause of crystallization or intervenes among the causes that bring about the phenomenon.  In the first place, two words concerning crystallization itself:  We know that crystallization is the passage, or rather the result of the passage, of a body from a liquid or gaseous state to a solid one.  It occurs when the substance has lost its cohesion through any cause whatever, and when, such cause ceasing to act, the body slowly returns to a solid state.

Under such circumstances, it may take on regular, geometrical forms called crystalline.  Such conditions are brought about by different processes—­fusion, volatilization, solution, the dry way, wet way, and electric way.  Further along, we shall give some examples of the last named means.

Let us add that crystallization may be regarded as a general property of bodies, for the majority of substances are capable of crystallizing.  Although certain bodies seem to be amorphous at first sight, it is only necessary to examine their fracture with a lens or microscope to see that they are formed of a large number of small juxtaposed crystals.  Many amorphous precipitates become crystalline in the long run.