Thus, to select an example from among physical effects, the vibratory phenomena that occur in telephone transmissions, under the influence of a very feeble electric current, show us that the molecular constitution of a solid body is extremely variable, although within slight limits.  The feeblest modification in the electric current may be shown by molecular motions capable of propagating themselves to considerable distances in the conducting wire.  Conversely, it is logical to suppose that a modification in the molecular state of a body must bring electricity into play.  If, in the phenomena of solidification, and particularly of crystallization, we collect but small quantities of electricity, that may be due to the fact that, under the experimental conditions involved, the electricity is more or less completely absorbed by the work of crystal building.

On another hand, the behavior of electricity shows in advance the multiple role that this agent may play in the various physical, chemical, and mechanical phenomena.

There is no doubt that electricity exists immovable or in circulation everywhere, latent or imperceptible, around us, and within ourselves, and that it enters as a cause into the majority of the chemical, physical, and mechanical phenomena that are constantly taking place before our eyes.  A body cannot change state, nature, temperature, form, or place, even, without electricity being brought into play, and without its accompanying such modifications, if it presides therein.  Like heat, it is the natural agent par excellence; it is the invisible and ever present force which, in the ultimate particles of matter, causes those motions, vibrations, and rotations that have the effect of changing the properties of bodies.  Upon entering their intimate structure, it orients or groups their atoms, and separates their molecules or brings them together.  From this, would it not be surprising if it did not intervene in the wonderful phenomenon of crystallization?  Crystallization, in fact, depends upon cohesion, and, in the thermic theory, this force is not distinct from affinity, just as solution and dissociation are not distinct from combination.

On this occasion, it is necessary to say that, between affinity, heat, and electricity there is such a correlation, such a dependency, that physicists have endeavored to reduce to one single principle all the causes that are now distinct.  The mechanical theory of heat has made a great stride in this direction.

The equivalence of the thermic, mechanical and chemical forces has been demonstrated; the only question hereafter will be to select from among such forces the one that must be adopted as the sole principle, in order to account for all the phenomena that depend upon these causes of various orders.  But in the present state of science, it is not yet possible to explain completely by heat or electricity, taken isolatedly, all the effects dependent upon the causes just mentioned.  We must confine ourselves for the present to a study of the relations that exist between the principal natural forces—­affinity, molecular forces, heat, electricity, and light.  But from the mutual dependence of such forces, it is admitted that, in every natural phenomenon, there is a more or less apparent simultaneous concurrence of these causes.