Class V. may be deferred for the present.
As to Class VI., it seems, from the observations of physiologists as to the formation of cellular matter, and the production in living organisms' of compounds which have net yet been made by ordinary chemical processes, that, the vital force, if there be such, is not a force which does work, in the mechanical sense of the term, but merely directs, as it were, the other natural forces how to apply their energies. Were a railway train running on a smooth horizontal line of rails, it would retain forever its original veloc ity; but in turning a curve, it would be acted on by deflecting forces, without which its path would be straight. These forces do no work, as is evident, since this would be shown in alteration of the vis-viva, and none takes place. They modify, however, the direction in which the train moves.
When gangs of laborers and masons are at work building an edifice, the former are employed raising stones, mortar, etc., the latter in laying them; but there is present an overseer with a plan, who, doing no (mechanical) work himself, guides and directs the proper application of force by the working body. In this view of the case, the laborers are the physical forces, and the overseer the vital force. It is quite certain that the so-called crystalline force cannot properly be put in this category, as presenting even an analogy, however slight; it is probably an effect, not a cause, and due to the different forms of simple or compound particles of matter, and the consequent variations in their molecular forces in different directions.
So far, then, for the possible nature of the forces, which, with the probable excep tion of VI., certainly depend oti various fOrins of energy. Can these be transformed one into another, as the different kinds of mechanical energy can? Take the poten tial energy of gravitation to begin with. We can employ it to drive a water-wheel. This turns a shaft, to which, if a tight brake be applied, heat will be produced by fric tion, and light also, if a rough wheel on the shaft be made to rotate against a piece of flint or pyrites; or electricity may be produced by employing the moving power to turn an ordinary electrical machine, or a magneto-electric one; and from the elec tricity so produced, electrical charges and currents may be derived; from them heat and light again. Or the currents may be employed to magnetize a needle or a piece of soft iron, or to produce chemical decomposition.
Again, heat may be employed by means of a steam-engine as a substitute for the water-power or potential energy of gravitation, and the above effects be produced. It may also be employed in raising weights, and therefore in producing the potential energy in question; or it may be employed to produce thermoelectric currents, and thence all the ordinary effects of electricity, including the motion of a magnetic needle.
Light may be employed to produce chemical combination. or decomposition, as we see. in photography; it may also by the same means be made to produce electric currents, and consequent motion of a needle. It is not yet proved that light can pro duce magnetism directly, though there can be little doubt that, if properly applied, it is capable of doing so.
Chemical action in a voltaic battery can be made to produce motion, heat, light, electricity, electrical charges and magnetism, and to overcome other chemical affinity.
Capillary action has been employed to produce electricity, and mechanical effects, etc., but we need not go through the whole category.
In these experimental results, then, consist what is called the correlation of the physical forces—i.e., the transmutability of one of the latter into another or others. The idea is old, but the proofs of its truth have only become numerous within the last half-century. Grove has published an excellent treatise with the above title; to this we refer the curious reader for further detail on this interesting subject.
Conservation of a far more important principle, being, in fact, the pre. else statement of the preceding—which is somewhat vague—is that of the conservation of energy. It is simply the extension (to all physics) of the principle which we have given iu full, and proved in a particular case, at the beginning of this article—i.e., that the sum of the potential and kinetic energies of any set of moving bodies cannot be altered by their mutual action. Let us now heat, light, etc., to consist in the energy of vibratory movements of particles, and in their relative states of distortion, etc., and make the supposition that these particles act on each other—no matter by what means—in the line joining each two, and with forces which depend on their distance, and we have at once the theorem, that the sum of the potential and kinetic energies is a quantity unalterable in any system, save by external influences. Hence, when mechani cal power is said to be lost, as it is by the unavoidable friction in machinery, etc., it is really only changed to a new form of energy—in general, heat. Thus, when a savage lights his fire, he expends animal energy in rubbing two pieces of dry wood together. If these pieces of wood were not in contact, no force would be required to move them past each other—more and more is required as they are more strongly pressed together. The equivalent of the energy so expended is found in the heat produced. Davy showed that two pieces of ice might be melted by rubbing them together. A skillful smith can heat a mass of iron to redness by mere hammering. Here the energy actually employed is partly given out in the shape of heat, and partly stored up in the iron as potential energy due to the compression of the mass, or the forcible approximation of its parti cles. Amongst the earliest, and certainly the best experiments on this subject, are those of Joule (q.v.). Ile determined the relation between the units of heat and potential energy of gravitation, by various methods, which gave very nearly coincident results. One of these we may mention. A paddle-wheel is so fixed as to revolve in a closed vessel full'of water. The wheel is driven by the descent of a known weight through a measured space, and precautions are taken against losses of energy of all kinds. The water agitated by the paddle-wheel comes soon to rest, as we know; but this is due to friction between its particles; and the final result is the heating of the water. The quantity of water, and also the number of degrees by which its temperature is raised, being measured, a simple proportion enables us to find how many foot-pounds (see FOOT-POUND) of mechanical energy correspond to the raising by one degree the temper ature of a pound of water. The result is, that the heating a pound of water one degree Fahrenheit is effected by 772 foot-pounds—and this number is called Joule's equivalent. 'In other words, if 'a pound of water fall to the ground through 772 ft., and lie then sud denly arrested, its temperature will be raised one degree; and, conversely, the heat that would raise the temperatrure of a pound of water one degree would, if applied by a steam-engine or otherwise, raise 772 pounds one ft. high. Now (see the article BEAT), we know the amount of heat which is produced by the burning (in air) of any material whose composition is known. It follows, then, that from the mere quantity and compo sition of a substance, we can tell the amount of mechanical work due to its combustion; that is, supposing it all to be effective. As we have been led to the mention of heat of combustion, hit us consider what this is due to. Combustion (in air) is merely a chenii cal combination of the constituents of the burning body with oxygen—the heat and light which are developed are therefore, by the conservation of energy, equivalent to the excess of potential energy of the uncombined, over the combined, oxygen and combus tible.