SECTION 2. THE CORRELATION OF FORCES.
[Footnote: For fuller information on this subject, Grove's
"Correlation of the Physical Forces," or Tyndall's "Lectures on
Heat considered as a Mode of Motion," may be consulted.]
It has long been known that heat and light are closely connected together. The accumulation of a certain amount of heat is always accompanied by the appearance of light. But when it was found that the light could be separated from the heat by various means, it seemed possible that the two phenomena were simply associated. It is now, however, ascertained that light and heat are identical in their nature, and that a vast number of other phenomena— electricity, galvanism, magnetism, chemical action, and gravitation, as well as light and heat, are different manifestations of one and the same thing, which is called force or energy. In a great number of cases it is possible for us, by the use of appropriate means and apparatus, to transform these manifestations, so as to make the same force assume a variety of forms. Thus motion suddenly arrested becomes heat. A rifle-ball when it strikes the target becomes very hot. The heat produced by the concussion against an iron shield is found sufficient to ignite the powder in some of the newly invented projectiles. The best illustration, however, is to be obtained from galvanism. By means of the Voltaic battery we set free a certain amount of force, and we can employ it at pleasure to produce an intense light in the electric lamp, or to melt metals which resist the greatest heat of our furnaces; it will convert a bar of iron into a magnet, or decompose water into its constituents, oxygen and hydrogen, or separate a metal from its combination with oxygen. But in all these processes no new force is produced—the force set free is unchangeable in itself, and we cannot increase its amount. Owing to the imperfection of our instruments and our skill a part of it will always escape from our control, and be lost to us, but not destroyed. When, however, due allowance is made for this loss, the results produced are always in exact proportion to the amount of force originally set free. Thus, if we employ it to decompose water, the amount of water decomposed always bears an exact proportion to the amount of metal which has been oxidized in the cells of the battery.
This force pervades everything which comes within the cognizance of our senses. It exists in what are termed the elementary substances of which the crust of the earth is composed. A certain amount of it seems to be required to maintain them in the forms in which we know them; for in many cases, when two of them are made to combine, a certain amount of force is set free, which commonly makes its appearance as heat. This seems to indicate that a less amount of force suffices to maintain the compound body than was requisite for its separate elements. Thus, when oxygen and hydrogen are combined to form water intense heat is produced. If we wish to dissolve the union, and restore the oxygen and hydrogen to a gaseous state, we must restore the force which has been lost. This, however, must be done by means of electricity, as heat produces a different change—converting the water into vapour, but not dissolving the union between its elements.
Force, in the shape of heat, determines the condition in which all inorganic bodies exist. In most cases we can make any given element assume the form of a solid, a fluid, or a vapour, by the addition or subtraction of heat. Thus if a pound of ice at 32 degrees be exposed to heat, it will gradually melt—but the water produced will remain unchanged in temperature till the last particle of ice is melted—then it will begin to rise in temperature; and, if the supply of heat be uniform, it will reach a temperature of 172 degrees in exactly the same time as was occupied in melting the ice. Thus then the force which was applied to the ice as heat passes into some other form so long as the ice is being melted—it is no longer perceptible by the senses—we only see its effect in the change from the solid to the fluid form. And this result is brought about by a definite quantity of force. Each of the inorganic materials of which the crust of the earth is composed seems thus to require in its composition a definite amount of force.
The life of vegetables is developed in the formation of fresh compounds of inorganic matter and force. No vegetable can thrive without sunlight, either direct or diffused. This supplies the force which the plant combines with carbon, hydrogen, and other elements to form woody fibre, starch, oils, and other vegetable products. When we kindle a fire, we dissolve the union which has thus been formed—the carbon and hydrogen enter into simpler combinations which require less force to maintain them, and the superfluous force supplies us with light and heat.
The life of animals is developed by a process exactly the reverse of vegetable life. It is maintained by the destruction of the compounds which the vegetable had formed. These compounds are taken into the body as food, and after undergoing certain modifications and arrangements are finally decomposed. Of the force thus set free a part makes its appearance as heat, maintaining an even temperature in the body, and another part supplies the power by virtue of which the muscles, &c., act. No manifestation of animal life is possible except by force thus set free. It seems all but certain that we cannot think a single thought without the decomposition of an equivalent amount of the brain. It must not, however, be concluded that force and life are identical. Force seems to be only the instrument of which the higher principle of life makes use in its manifestations.
Force then pervades the whole universe so far as it is cognizable by our senses. But we cannot conceive of force as acting, without at the same time conceiving of something on which that force acts. That something, whatever it may be, we designate "matter." We have not the slightest idea of what matter really is—no man has ever yet succeeded in separating it from its combination with force. Even if success were possible, which seems very improbable, it is not likely that matter by itself would be discernible by any of our senses. We know that two of them, sight and hearing, enable us to perceive certain kinds of motion, i. e. manifestations of force, and this is in all probability the case with the rest of them. The existence of matter then is not known by scientific proof but by inference. Our belief in it arises from something in the constitution of our minds which makes it a necessary inference.
There is one more point in reference to force which must be noticed. It is indestructible, but it is capable of what is termed "degradation." It may exist in various intensities and quantities, and a small quantity of force of a higher intensity may be changed into a larger quantity of force at a lower intensity. In the instance above given of the union of oxygen and hydrogen, heat is given out, but heat does not suffice to dissolve that union. The force must be supplied in the more intense form of Voltaic Electricity. But to reverse this process seems impossible for us. As, however, this is clearly explained in a previous volume of this series, [Footnote: Can we Believe in Miracles? p. 152.] it is not necessary to dwell upon it at length.
We may conclude then that the whole material universe is built up of matter and force in various combinations, but we can form no conception of what these two things are in themselves; they are only known to us by the effects produced by their union in various proportions.