The system which reduces every thing in matter to movements is as simple as it is curious. It exhibits at the same time a character of grandeur and of unity which is seductive. Matter in the universe, it says,[198] remains the same in quantity; it is neither created nor destroyed; its phenomena are merely transformations.
According to this system, the abstract notion of force does not exist. Force is a cause of motion; and the cause of motion is a motion itself. Physical phenomena, as heat, light, electricity, and magnetism, are certain kinds of motion, which beget each other. Heat is transformed into electricity and electricity into light. Transformations take place according to fixed rules, and are reduced to rigorously determined unities.
In another order of facts, cohesion, chemical affinity, and gravity, are equally the effects of communication of motion, since the phenomena which derive from them exist only by attraction—that is to say, by the movement of molecules and bodies toward each other.
The same rule holds good in the system of the universe; the heavenly as well as the terrestrial bodies have not in themselves that which attracts them to each other. Universal gravitation is only the expression of a result; it merely means that every thing happens as if bodies had the intrinsic property of attracting each other in the direct ratio of their quantity and the inverse ratio of their distance.
It is not this force or property; it is the ether which is the cause of attraction. The ether is composed of atoms which collide with each other and with neighboring bodies. It is everywhere diffused, forming a universal medium, and exercising a continual pressure on all the molecules in nature. The gravity of bodies is owing to the pressure of this medium. Their movement is, as it were, a transformation of the motions of ether. Thus, the ether, moving in every direction, and obeying no fixed pressure, produces material attraction without being subject to it; it gives to bodies their gravity, while it remains itself imponderable.[199]
It had been already physically demonstrated that sound and light were the result of undulations—that is, of motions; sonorous and luminous movements which have been measured and verified in all their modes. The nature of caloric movement has not yet been so completely understood; but the mechanical effects of heat have been established in the most precise manner. The identity of heat and of mechanical labor has become a commonly received idea for several years past. Heat, which was formerly regarded as a material substance, is now considered as a mere mode of motion; it is by their repercussion that the molecules of bodies cause us to experience the sensation of heat; and the intensity of these repercussions determines the degrees of temperature. This heat, manifesting itself by different effects, produces now light or sounds, again mechanical labor.
The energy or the living force which molecules or bodies in motion possess, in a degree exactly known, is partially lost if these molecules produce a work, that is to say, if they displace a quantity of matter; but in that case the living force which they lose is stored up in the labor produced, and is reborn when the latter ceases to exist.
Just as the calorific and luminous fluids are no longer regarded as possessing a special substance and existence, so also the electric fluid, positive as well as negative, and the magnetic fluid, which is only one of its derivatives, are but opposite movements of matter. The electrical movement of imponderable matter, or ether, is not even a vibratory motion; it is a real current, a real transport which takes place in the conducting body; and it is so far of the same nature as the luminous motion that it has approximately the same velocity—that is to say, it travels seventy-five thousand leagues a second.
Now, all these motions of heat, all these motions of light and electricity, which correlative phenomena offer, are all reducible to the idea of mechanical labor. Produced by one labor, they reproduce another. Thus disappear from chemistry, as from the natural sciences, the forces called repulsive as well as those called attractive. The molecules no longer act at a distance; actions take place by contact, by the communication of movements. In the same way the pressure exercised by the ethereal atoms on the molecules of the sidereal bodies takes the place of the initial force or acquired velocity which astronomy regarded as the cause of their movements.
According to this sovereign unity, the physical world is composed of one single element. There are no simple bodies. Oxygen and hydrogen, like gold or platina, are composed only of atoms. There is no difference in material quality; properties vary according to the diversity of movements. Becoming grouped and interwoven, the atoms form the molecules, and the changes of these movements constitute for us the different phenomena, the mode of which depends on the masses and the velocities which are in play.