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a change takes place in the temper of glass under the influence of powerful magnetism.
Even from the limited view of the powers of nature which precedes, it is evident that the progress of science based upon experiment tends to show that the various forces of light, heat, motion, chemical affinity, electricity, and magnetism will ultimately be traced to one common origin; that they are so directly related, and mutually dependent, that they are convertible, motion producing heat, and heat motion; chemical affinity producing electricity, and electricity chemical action, &c., each mediately or immediately producing the other. These forces are transmitted through substances; they act upon matter, causing changes in the molecular structure of bodies either momentary or permanent, and reciprocally the changes indicate the action of these forces. Matter and force are only known to us as manifestations of Almighty power: we are assured that we can neither create nor destroy them that their amount is the same now as in the beginning. In chemical attraction the powers with which a molecule of matter is endowed, and which give rise to various qualities, never change; even when passing through a thousand combinations, the molecule and its power are ever the same.
Machinery does not create force; it only enables us to turn the forces of nature to the best advantage; it is by the force of wind or falling water that our corn is ground, and the steam engine owes its power to the force of heat and chemical action. As force cannot be created, neither can it be annihilated. It may be dispersed in various directions, and subdivided so as to become evanescent to our perceptions; it may be balanced so as to be in abeyance, or become potential as in static electricity; but the instant the impediment is removed the force is manifested by motion; it may also be turned into heat by friction, but it is never lost. Every motion we make, every breath, every word we utter, is a force that produces pulsations which are communicated to continually increasing particles of air, and conveyed through countless channels so as to become indeed imperceptible to our senses, yet they are demonstrated to exist as witnesses of the words we have spoken or the actions we have performed, by analysis, that all-powerful instrument of human reason.'
A body acquires heat in the exact proportion that the adjacent substances become cold, and when heat is absorbed by a body it becomes an expansive force at the expense of those around that contract, but it is not lost. In chemical action at a distance the principle of the conservation of force is maintained, for a chemical action may be produced miles away from an electro-magnet, perfectly equivalent to the dominant chemical action in the battery. The two electricities are developed in equal proportions, which may be combined so as to produce many changes in their respective relations, yet the sum of the force of one kind can never be made in the smallest degree either to exceed or to come short of the sum of the other. Experimental research proves that the conservation of force is an unalterable law of nature-" a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity. No hypothesis should be admitted, nor any assertion of a fact credited, that denies this principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences, but none should oppose or contradict it.”
Having thus expressed his conviction of the truth of this great principle, Dr. Faraday considers the case of gravity, and concludes that "the definition of gravity as an attractive force between the particles of matter varying inversely as the square of the distance, while it stands as a full definition of the power, is inconsistent with the principle of the conservation of force." For while in this definition the principle is maintained of the constancy of the force at the same distance, it implies a creation of force to an enormous amount when the distance is diminished, and an equal amount annihilated when the distance is increased,
66 an effect," he says, "which is equal in its infinity and its consequences with creation, and only within the power of Him who creates." He continues, "It will not be imagined for a moment that I am opposed to what may be called the law of gravitating action, that is, the law by which all the known effects of gravity are governed; what I am considering is the definition of the force of gravitation. That the result of one exercise of a power may be inversely as the square of the distance, I believe and admit; and I know that it is so in the case of gravity, and has been verified to an extent that could hardly
have been within the conception of Newton himself when he gave utterance to the law; but that the totality of a force can be employed according to that law I do not believe either in relation to gravitation, or electricity, or magnetism, or any other supposed form of power. That there should be a power of gravitation existing by itself, having no relation to the other natural powers, and no respect to the law of the conservation of force, is as little likely as that there should be a principle of levity as well as gravity. Gravity may be only the residual part of the other forces of nature, as Mossotti has tried to show; but that it should fall out from the law of all other forces, and should be outside the reach either of farther experiment or philosophical conclusions, is not probable. So we must strive to learn more of this outstanding power, and endeavour to avoid any definition of it which is incompatible with the principles of force generally, for all the phenomena of nature lead us to believe that the great and governing law is one. Thus gravitation can only be considered as part of a more general force whose law has yet to be discovered.
The definition of the gravitating force immediately suggests the question of how it is transmitted; the full force of that question was felt by Newton himself when, in his third letter to Bentley, he wrote, "That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of anything else by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophic matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent, acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my readers."
Since Newton's time the continual decrease in the periodic times of the comets belonging to our system, and the undulatory theory of light and heat, have proved the existence of an extremely rare elastic medium filling space even to the most distant regions of which we are cognizant. But, rare as it may be, it has inertia enough to resist the motion of comets, and therefore must be material, whether considered to be ether or, according to Mr. Grove, the highly attenuated atmospheres of the celestial bodies.
Professor William Thomson of Glasgow has computed that in the space traversed by the earth in its annual revolution, a cube whose side is 1000 miles would contain not less than a pound weight of the ethereal medium, and that the earth, in moving through it, would not displace the 250th part of that pound of matter. Yet that is enormously more dense than the continuation of the earth's atmosphere would be in interplanetary space, if rarefied according to Bayle's law. But whatever be the density or nature of the ether, there is every reason to believe that it is the medium which transmits the gravitating force from one celestial object to another, or possibly it may possess a higher attribute with regard to gravity than its mere transmission.
Dr. Faraday, who discovered the magnetism of the atmosphere, is led to believe that the ethereal medium too is magnetic by the following experiment. Three solutions of the protosulphate of iron, l, m, n, the first of which contained 4 grains of the salt dissolved in a cubic inch of water, the second 8 grains, and the third 16 grains-these were respectively enclosed in three glass globules, all of which were attracted by the pole of a magnet. A quantity of the mean solution m was then put into a vessel, and the globule containing the strongest solution n was immersed in it, which was attracted as before, but the globule l, containing the weakest solution, was repelled when plunged into the same liquid. Here there was a diamagnetic phenomenon, although the glass globules and the liquid in which they were immersed contained iron. The effect was evidently differential, for when the liquid was less attracted than the globule, the globule approached the pole, and when the liquid was more attracted than the globule, the latter appeared to recede from the pole. In fact, the effect is the same as that of gravity on a body immersed in water; if it be more forcibly attracted than the water, it sinks; if less forcibly attracted, it rises, the effect being the same as if it were repelled by the earth. Hence the question, are all magnetic phenomena the result of a differential action of this kind, and is the ethereal medium less strongly attracted than soft iron, and more strongly attracted than bismuth, thus permitting the approach of the iron, but causing the bismuth to recede from the pole of a magnet? If such a medium exist, that is, if the ethereal medium be magnetic, then diamagnetism is
the same with paramagnetism, and the polarity of the magnetic force in iron and bismuth is one and the same.
The ethereal medium may be presumed to transmit the gravitating force; it transmits the magnetism of the solar spots, its undulations constitute light, heat, and all the influences bound up in the solar beam; and the most perfect vacuum we can make is capable of transmitting mechanical energy in enormous quantities, some of which differ but little from that of air or oxygen at an ordinary barometric pressure; and why not thus admit, says Mr. Thomson, the magnetic property, of which we know so little that we have no right to pronounce a negative?
Mr. Waterstone is also of opinion that it would be taking too narrow a view if we limited the function of the luminiferous ether to the conveying of physical pulses only. The atmosphere also conveys physical pulses, but that is the least important of its functions in the economy of nature. There is nothing that should hinder us attributing to the media concerned in the radiation of light and heat the higher functions of electrical polarity and gravitation. The special dynamic arrangements by which this is effected may ever elude our research; but as there is no limit to the vis viva (N. 222) which such media may conserve in their minutest parts, so there is no physical impossibility in that vis viva being suddenly transferred to the molecules of ordinary matter in the proportion and sequence required to carry out the order and system of nature.
The fundamental principle of action in such media must be in accordance with elastic impact, for upon that the dynamic theory of heat and conservation of force rests as a foundation. The statical and dynamical characteristics of gravitation and transfusion of force conform to it, so that all the forces that hold the molecules of bodies together must also be in subjection to it.*
* Phil. Mag. for May 1858.