Читать книгу The Poetry of Science; or, Studies of the Physical Phenomena of Nature - Robert Taylor Hunt - Страница 9

Table of Contents

MOLECULAR FORCES.

Conditions of Matter—Variety of organized Forms—Inorganic Forms—All matter reducible to the most simple conditions—Transmutation, a natural operation—Chemical Elementary Principles—Divisibility of Matter—Atoms—Molecules—Particles—Molecular Force includes several Agencies—Instanced in the Action of Heat on Bodies—All Bodies porous—Solution—Mixture—Combination—Centres of Force—Different States of Matter (Allotropic Conditions)—Theories of Franklin, Æpinus, and Coulomb—Electrical and Magnetic Agencies—Ancient Notions—Cohesive Attraction, &c.

In contemplating the works of nature, we cannot but regard, with feelings of religious admiration, the infinite variety of forms under which matter is presented to our senses. On every hand the utmost diversity is exhibited; through all things we trace the most perfect order; and over all is diffused the charm of beauty. It is the uneducated or depraved alone who find deformities in the creations by which we are surrounded.

The three conditions of matter are—the solid, the fluid, and the aëriform; and these belong equally to the organic and the inorganic world.

In organic nature we have an almost infinite variety of animal form, presenting developments widely different from each other, yet in every case suited to the circumstances required by the position which the creature, occupies in the scale of being. Through the entire series, from the Polype to the higher order of animals, even to man, we find a uniformity in the progress towards perfection, and a continuity in the series, which betrays the great secret, that the mystery of life is the same in all—a pervading spiritual essence associated with matter, and modifying it by the master-mechanism of an Infinite mind.

In the vegetable clothing of the surface of the earth, which fits it for the abode of man and animals—from the confervæ of a stagnant pool, or the lichen of the wind-beaten rocks, to the lordly oak or towering palm—a singularly beautiful chain of being presents itself to the contemplative mind, and we cannot but trace the gradual elevation in the scale of organization.

In the inorganic world, where the great phenomena of life are wanting, we have constantly exhibited the working of powers of a strangely complicated kind. The symmetrical arrangement of crystals—the diversified characters of mineral formations—the systematic aggregations of particles to form masses possessing properties of a peculiar and striking nature—all prove, that agencies, which science, with all its refinements, has not yet detected, are unceasingly at work. Heat, electricity, chemical power—whatever that may be—and the forces of cohesion, are known to be involved in the production of the forms we see; but contemplation soon leads to the conviction that these powers are subordinate to others which we know not of. We know only the things belonging to the surface of our planet, and these but superficially. The geologist traces rock-formations succeeding each other (from the primary strata holding no traces of organized forms, through the Paleozoic series, in which, step by step, the history of animal life is recorded,) to the more recent formations, teeming with relics, which, though allied to some animal types still existing, are generally such as have passed away. The naturalist searches the earth, the waters, and the air, for their living things; and the diversity of form, the variety of condition, and the perfection of organization which he discovers as belonging to this our epoch—differing from, indeed bearing but a slight relation to, those which mark the earth’s mutations—exhibit, in a most striking view, the endless variety of characters which matter can assume.

We are so accustomed to all these phenomena of matter, that it is with some difficulty we can bend ourselves to the study of the more simple conditions in which it exists.

The solid crusts of this telluric sphere—the waters and the atmosphere—the diversified fabrics of the vegetable kingdom—and the still more complicated structures of men and animals—are, altogether, but the aggregation of minute particles in accordance with certain fixed laws. By mechanical means all kinds of matter may be reduced to powder, the fine particles of which would not appear very different from each other, but each atom has been impressed with properties peculiar to itself, which man has no power to change.

To nature alone belongs the mysterious property of transmutation. The enthusiastic alchemist, by the agency of physical forces, dissipates a metal in vapour; but it remains a metal, and the same metal still. By the Hermetic art he breaks up the combination of masses; but he cannot alter the principles of any one of the elements which form the mass upon which his skill is tried.

Every atom is invested with properties peculiar to all of its class; and each one possesses powers, to which in mute obedience it is compelled, by which these properties are modified, and the character of matter varied. What are those properties? Do we know anything of those powers?

The earth, so far as we are acquainted with it, is composed of about sixty principles, which we call elementary. These are the most simple states to which we can reduce matter, and from them all the forms of creation yet examined by the chemist are produced. These elementary principles are, some of them, permanently gaseous under the ordinary temperature, and others exist as solid masses; the difference between the two conditions being regulated, as it appears, by the opposing forces of heat and cohesive attraction.

Matter has been regarded by some as infinitely divisible; but the known conditions of chemical combinations lead to the conclusion that there are limits beyond which matter cannot be divided.[25] The theory of atoms having determinate characters, and possessing symmetric forms, certainly has the advantage of presenting to the human mind a starting point—a sort of standing ground—from which it can direct the survey of cosmical phenomena. The metaphysical hypothesis, which resolves all matter into properties, and refers all things to ideas, leaves the mind in a state of uncertainty and bewilderment.

Adapting the views of Dumas, with some modifications,[26] it will be found more satisfactory to regard the ultimate atoms of matter as points beyond the reach of our examination; which, according to a law, determined by the influences of the so-called imponderable forces, unite to form molecules. Again, these molecules combine to form the particles of the mass which we may regard as the limit of mechanical division. The particles of solid bodies are solid, those of fluids fluid, and those of gaseous bodies are themselves aëriform; but it does not follow that the molecules of any body should be necessarily solid, fluid, or aëriform, from the circumstance of their having formed the particles of a body in one of these states.

As this planet—a molecule in space—is formed of aggregated atoms, and enveloped by its own physical agencies—and as it is involved in the infinitely extending influences of other planetary molecules, and thus forms part of a system—so the molecules of any mass are grouped into a system or particle, which possesses the great characteristic features of the whole.

In an aëriform body the particles are in a state of extreme tenuity, the molecules being themselves, by the influence of some repulsive force, just on the verge where cohesion exerts its decaying power. In fluid bodies the attenuation of the particles is less—the particles and also the molecules are nearer together—whereas, in the solid body, the forces of cohesion are most strongly exerted, and all the molecular conditions brought more powerfully into action.

Under the term molecular force, we include several agencies—not alike in the phenomena which they exhibit, but which are all-powerful in producing the general characteristics of bodies. These require a somewhat close examination. All the particles of even a solid mass may be brought under conditions on which they are free to move. By heat we can increase the length and thickness of a bar of iron, or any other metal, and at length produce the fluid state—a melted metal flows as freely as water in a stream. Fluids, and gases in like manner obey the dispersive influence of caloric. From these and other analogous results we learn that all bodies have a greater or less degree of porosity. The distance at which the particles of fluid bodies are maintained is strikingly proved by the fact, that hydrated salts dissolved in water occupy no more space than that which is equal to the water contained in the crystalline body; while anhydrous salts dissolve without at all increasing the bulk of the fluid. All the solid matter of the salt must, in these cases, it would appear, go to fill up the interstitial spaces which we suppose to exist in the liquid.[27]

The conditions which regulate the solubility of bodies, and the power of solution, regarded either as a mechanical or a chemical process, are very obscure. We might be led to suppose, that those bodies possessing the largest amount of unoccupied space were capable of holding the greatest quantity of soluble matter dissolved. This, however, is far from being the case, the denser fluids generally having the greatest solvent power.

The peculiar manner in which hydrogen gas appears to dissolve solid substances—as iron, potassium, sodium, sulphur, phosphorus, selenium, and arsenic, may be explained by regarding the results as a manifestation of the powers of chemical affinity over the forms of bodies. In like manner, the solution of salt in water, or the mixture of alcohol in that fluid, may be viewed as chemical phenomena, although usually considered as simple cases of solution or mixture: alterations of temperature and other physical changes taking place in either. If two masses of metal—either tin and copper, for example—are melted and combined, the united mass will not equal the bulk of the two masses. If a pint measure of oil of vitriol and an equal quantity of water are mixed together, the combined fluids will not fill a two pint measure.[28]

In these instances a large quantity of heat is rendered sensible, as if it had been squeezed out by the force with which the particles combined, from interstices, which were filled with, what we may be allowed to call, an atmosphere of heat. Hence we conclude that, amongst the influences determining the molecular constitution of a body, heat performs an important part. All these facts go to prove that the atoms which form the compound body, whatever may be its character, are disposed of as so many centres of force, which act by influences of a peculiar character upon each other. That these influences are dependent upon known physical forces is certain; but the laws by which the powers of the ultimate atom are altered remain still unknown.

In the great operations of nature, changes are produced which we cannot understand, and variations of condition do certainly occur, which may be regarded as instances of transmutation.

Amongst others, we may adduce the different states in which we know carbon to exist. We have the diamond with its beautiful light-refracting property, its hardness and high specific gravity, capable of being converted into graphite and coke.[29] Charcoal, graphite, and the diamond, are totally unlike each other, yet we know they are each composed of the same atoms. Charcoal is a black irregular substance, light, and readily inflammable; graphite is crystallizable; but the forms of its crystals cannot be referred to those of the diamond, and it burns with difficulty. The diamond occurs in the most regular and beautifully transparent forms; and it can be burned only at the highest artificial temperatures. We are, however, convinced by experiment that the brilliant and transparent gem is made up of the same atoms as those which go to form the dull black mass of charcoal. From diamonds, as is above stated, coke has been formed by the heat of the voltaic battery, and recent experiments have proved that the volatilized carbon constantly passing off from one of the poles of a sufficiently powerful battery, is deposited in a crystalline powder, possessing most of the properties, as it regards hardness, &c. of true diamond dust. What is the mystery of this? We know not. The peculiar conditions have been the subjects of anxious study; but science has not yet let in a ray of light upon the mystery. That a different state—it has been called an allotropic condition—is often induced in the same class of atoms is certain; and hence the variety of the resulting compounds. To continue our illustrations with carbon—may not its combinations, in uniform proportions with oxygen and hydrogen,[30] owe their differences to some allotropic change in the ultimate atoms of this element.

We know that silicon—the metallic base of flint—is capable of assuming two or more different states; and that sulphur, selenium, phosphorus, and arsenic, are susceptible of these remarkable changes in which, without the slightest variation in the chemical character, a complete change in the physical condition is produced. Copper, iron, tin, and manganese, are known to exist in at least two states of physical dissimilarity, and many of the rarer metals exhibit the same peculiarity.[31] Hence, may we not infer that some of those substances, which we now term elementary, are but altered conditions of the same element? The resemblance between many of those bodies strengthens the supposition. Iridium and platinum—iron and nickel—chlorine, bromine, iodine, and probably fluorine—are good examples of these similarities, although these bodies are all distinguished by physical and chemical differences.

The light-refracting gem, which glistens on the neck of beauty, and is valued for its transparency, differs only from the rude lump of coke in its molecular arrangement. Chemistry teaches us that we may, without producing any disarrangement of the affinities, but by merely setting up molecular disturbance, effect decided changes, as is strikingly shown in the colour of iodide of mercury changing from red to yellow under slight influences of heat, and back again to red by a gentle mechanical disturbance. By a slight change, merely molecular, iron may be made to resemble platinum in its physical properties.[32] An iron wire plunged into nitric acid is attacked by the acid with violence; but if one extremity of the wire is heated in the flame of a spirit lamp, such a change of state is produced throughout the entire length of the wire, that if it be now plunged into nitric acid no effect is produced upon it. On studying this question, we find good reason for supposing that bodies which, though physically different, resemble each other in some of their properties, iodine, bromine, &c., are the results of different allotropic conditions which have been impressed upon the ultimate atoms, similar to those observed in the substances named. This hypothesis appears to be more in accordance with the great principles which we must conceive guided the labours of an Infinite Mind, than that which supposes a vast number of individual creations. It will be seen in the sequel that light, heat, electricity, and chemical action, have the power of producing yet more striking changes in the forms of bodies. Is it not probable that, according to the operations of these agents, either combined or separate, acting over different spaces of time, and under varying circumstances, in relation to the molecular forces, all those allotropic states may be produced? Hence bodies may be discovered, which—from the imperfections of science—resisting our means of analysis, must, for a time, be regarded as new elements, whereas they are possibly only altered states of the same substance.

The experiments of Faraday and of Plücker prove that all matter exists in certain polar conditions, having powers of mutual attraction and repulsion.[33] Are the molecular forces, so called, to be referred to any of those powers which are involved in the general term magnetic-polarity? Are they not probably the result of some ultimate principle of which these properties are but the modified manifestations? These questions will now be generally answered in favour of magnetism; but in our ignorance we should pause; the next generation will without doubt find another solution for the problem.

Franklin supposed the ultimate atoms of bodies to be surrounded by a subtile fluid or ether, which they have the power of condensing upon their surfaces with great force—and we have experiments showing that this is probable[34]—whilst he regarded the atoms of the ether itself as mutually repellent, thus establishing an equilibrium of forces. Æpinus reduced the hypothesis of Franklin to a mathematical theory; and Coulomb proved that the force with which the repulsion of the ethereal atoms and the attraction of the material molecules are produced, is, like universal attraction—to whatever power that may be due—regulated by the law of the inverse ratio of the square of the distance. These views are found, upon minute examination, to hold true to the phenomena with which inductive science has made us acquainted; and the striking manner in which, when submitted to the rigorous investigations of geometers, they agree with known conditions of electricity, appears certainly to favour the opinion that this power may be materially connected with these molecular arrangements.

Many of the phenomena which are connected with the magnetic influences also bear in a remarkable manner upon this inquiry. But, without the necessary proof of direct experimental evidence, it were as unphilosophical to refer the binding together of the molecules of matter to the agency of electricity, as it would be to adopt the theory of the hooked atoms of Epicurus, or the astrological dream of the sympathies of matter.[35]

Science, however, enables us to infer with safety that the mechanical powers which regulate the constitution of a cube of marble, or a granite mountain, are of a similar order to those which determine the earth’s relation to the other planets in the solar system, and that solar system itself a unit, in the immensity of space, to the myriads of suns which spangle the stellar vault.

In fine, cohesion, or the attraction of aggregation, is a power employed in binding particle to particle. To cohesion, we find we have heat opposed as a repellent force; and the mysterious operations of those electrical phenomena, generally referred to as polar forces, are constantly, it is certain, interfering with its powers. In addition, we have seen that in nature there exists an agency which is capable of changing the constitution of the ultimate atoms, and of thus giving variety to each resulting mass. What this power may be, our science cannot tell; but our reason leads us, with firm conviction, to the belief that it is a principle which is, beyond all others in its subtile influences which equally universal with, appears to rise superior to gravitation; and which, like a spirituality, shadows forth to our dwarf conceptions the immensity of the divine power of the omniscient Creator.

The molecular forces involve a consideration of all the known physical powers, the study of which, in their operations on matter, will engage our attention. But it is pleasant to learn, as we advance step by step in our examination of the phenomena of creation, that we may study the grand in what externally appears the simple, and learn, in the mysteries of a particle, the high truths which science has to tell of a planet.

It may appear that the forces of gravitation and cohesion are regarded as identical. Many phenomena, which we are enabled to reach by the refinements of inductive inquiry, certainly present to us a striking similarity in the laws which regulate the operations of these powers; but it must be remembered that their identity is not established. So far from this, we know the law of gravitating force. Newton determined with surprising accuracy, that the action of this power diminishes with the distance as the universe square, but cohesive force is exerted only at such distances that it is impossible to determine whether or not it is subjected to the same law. To quote the words of Young: “The whole of our inquiries respecting the intimate nature of forces of any kind must be considered merely as speculative amusements, which are of no further utility than as they make our views more general, and assist our experimental investigations.”[36]

FOOTNOTES:

[25] “The divisibility of matter is great beyond the power of imagination, but we have no reason for asserting that it is infinite; for the demonstrations which have sometimes been adduced in favour of this opinion are obviously applicable to space only. The infinite divisibility of space seems to be essential to the conception that we have of its nature, and it may be strictly demonstrated that it is mathematically possible to draw an infinite number of circles between any given circle and its tangent, none of which shall touch either of them except at the general point of contact; and that a ship following always the same oblique course with respect to the meridian—for example, sailing north-eastwards—would continue perpetually to approach the pole without ever completely reaching it. But when we inquire into the truth of the old maxim of the schools, that all matter is infinitely divisible, we are by no means able to decide so positively. Newton observes that it is doubtful whether any human means may be sufficient to separate the particles of matter beyond a certain limit; and it is not impossible that there may be some constitution of atoms, or single corpuscles, on which their properties, as matter, depend, and which would be destroyed if the units were further divided; but it appears to be more probable that there are no such atoms, and even if there are, it is almost certain that matter is never thus annihilated in the common course of matter.”—The Essential Properties of Matter: Young’s Natural Philosophy; ed. by Rev. P. Kelland.

[26] “Two very different hypotheses have been formed to explain the nature of matter, or the mode of its formation; the one known as the atomic theory, the other, the dynamic. The founder of the former and earlier was Leucippus: he considered the basis of all bodies to be extremely fine particles, differing in form and nature, which he supposed to be dispersed through space, and to which his follower Epicurus first gave the name of atoms. To these atoms he attributed a rectilinear motion, in consequence of which such as are homogeneous united, whilst the lighter were dispersed through space. The author of the second hypothesis was the famous Kant. He imagined all matter existed, or was originated, by two antagonist and mutually counteracting principles, which he called attraction and repulsion, all the predicates of which he referred to motion. Most modern philosophers, and foremost amongst them Ampère and Poisson, have adopted an hypothesis combining the features of both the preceding. They regarded the atoms as data, deriving their origin from the Deity as the first cause, and consider their innate attractive and repulsive force as a necessary condition to their combination in bodies. The main features of this hypothesis are borrowed from Aristotle, inasmuch as he supposed the basis of all bodies to be the four elements known to the ancients, the particles of which, endued with certain powers, constituted bodies. According to Ampère, all bodies consist of equal particles, and they again of molecules that, up to a certain distance, attract each other. Their distance from each other he supposed to be regulated by the intensity of the attractive and repulsive forces, the latter of which preponderates.”—Peschel’s Elements of Physics; translated by E. West, 1845.

[27] This was first proved by the researches of Dr. Dalton: the subject will be again alluded to under the consideration of atomic volumes.

[28] These peculiar phenomena may be studied advantageously in the works of most of the eminent European chemists. In our own language the reader is referred to Dr. Thompson’s Outline of the Sciences of Heat and Electricity, 2nd edition; Brande’s Manual of Chemistry—Art. Specific Heat; Graham’s Elements of Chemistry; and Daniell’s Introduction to the Study of Chemical Philosophy.

[29] The conversion of the diamond into graphite and coke was first effected by the agency of the galvanic arc of flame, by M. Jaquelini, and communicated to the Academy of Sciences in 1847, in a Memoir entitled, De l’action calorifique de la pile de Bunsen, du chalumeau à gaz oxygène et hydrogène sur le carbon pur, artificiel et naturel. See Comptes Rendus, 1847, vol. xxiv. p. 1050; also Report of the British Association, for 1847, (Transactions of Sections) p. 50.

[30] “In the annual report on the progress of chemistry, presented to the Royal Academy of Stockholm, in March 1840, I have proposed to designate by the term allotropic state, that dissimilar condition which is observed in certain elements, and long known examples of which are found in the different forms of carbon, as graphite and diamond.

“Although these dissimilar conditions, which I have here called allotropic, have long since attracted attention in one or two elements, still they have been regarded as exceptions to the general rule. It is at present my object to show that they are not so rare; that it is probably rather a general property of the elements to appear in different allotropic conditions; and that although we have hitherto been unable to obtain several of the elements when uncombined in their allotropic states, still their compounds indicate the same with tolerable distinctness.”—Berzelius on the Allotropy of the Elementary Bodies, &c.: Poggendorff’s Annalen, 1844. Scientific Memoirs, vol. iv. p. 240.

[31] “Copper, when reduced by hydrogen at a heat below that of redness, on exposure to air soon becomes converted throughout its mass into protoxide; and when it is triturated for some time with an equivalent quantity of sulphur, it combines with it according to Böttcher’s experiments, producing flame, and forming sulphuret of copper. If, however, the copper be reduced by hydrogen at a red heat, still considerably below the temperature at which it softens and begins to melt, it remains for years unchanged by exposure to air, and cannot be made to combine with sulphur without the application of heat. Iron, cobalt, and nickel, when reduced by hydrogen below a red heat, inflame after they have cooled, if exposed to the air; and if they are immediately placed in water to avoid their taking fire, they inflame when they are again removed, and have become nearly dry. If we compare this behaviour with that of iron reduced by heat, and with iron in that state in which it forms the conductor of a galvanic current without becoming oxidized, it would appear that these peculiarities depended upon something more than a difference of mechanical condition.”—Berzelius on the Allotropy of Elementary Bodies. See On the Isomeric Conditions of the Peroxide of Tin: by Prof. H. Rose.—Chemical Gazette, Oct. 1848.

[32] On this curious subject, and its history, see Bergman’s Dissert. de Phlog. quantitate in Metallis, 1764. Kirwan, On the Attractive Powers of Mineral Acids: Philosophical Transactions. Kier’s Experiments and Observations on the Dissolution of Metals in Acids: Phil. Trans. 1790.

From these valuable papers it will be seen that the peculiar states of iron had already attracted attention, particularly those “inactive conditions” noticed in a “Note sur la Manière d’agir de l’Acide nitrique sur le Fer, par J. F. W. Herschel,” Aug. 1833; and previously indicated by M. H. Braconnot, Sur quelques Propriétés de l’Acide nitrique, Annales de Chimie, vol. lii. p. 54. Reference should also be made to the Memoirs of Sir John Herschel, On the Action of the Rays of the Solar Spectrum on Vegetable Colours, &c.: Phil. Trans. vol. cxxxiii. p. 221; and On the Separation of Iron from other Metals: Phil. Trans. vol. cxi. p. 293; and several papers by Schönbein, in the Philosophical Magazine, from 1837.

[33] Faraday, in his memoir On new Magnetic Actions, and on the Magnetic Conditions of all Matter, says:—“By the exertion of this new condition of force, the body moved may pass either along the magnetic lines or across them, and it may move along or across them in either or any direction, so that two portions of matter, simultaneously subject to this power, may be made to approach each other as if they were mutually attracted, or recede as if mutually repelled. All the phenomena resolve themselves into this, that a portion of such matter, when under magnetic action, tends to move from stronger to weaker places or points of force. When the substance is surrounded by lines of magnetic force of equal power on all sides, it does not tend to move, and is then in marked contradistinction with a linear current of electricity under the same circumstances.”—Phil. Trans. for 1846, vol. cxxxvii.

[34] New Experiments and Observations on Electricity made at Philadelphia, in America.—Addressed to Mr. Collinson, from 1747 to 1754. By Benjamin Franklin. Of these Priestley remarks:—“It is not easy to say whether we are most pleased with the simplicity and perspicuity with which the author proposes every hypothesis of his own, or the noble frankness with which he relates his mistakes, when they were corrected by subsequent experiments.”

[35] “The atomic philosophy of Epicurus, in its mere physical contemplation, allows of nothing but matter and space, which are equally infinite and unbounded, which have equally existed from all eternity, and from different combinations of which every visible form is created. These elementary principles have no common property with each other; for whatever matter is, that space is the reverse of; and whatever space is, matter is the contrary to. The actual solid part of all bodies, therefore, are matter, their actual pores space, and the parts which are not altogether solid, but an intermixture of solidity and pore, are space and matter combined.

“The infinite groups of atoms, flying through all time and space in different directions and under different laws, have interchangeably tried and exhibited every possible mode of rencounter: sometimes repelled from each other by concussion, and sometimes adhering to each other from their own jagged or pointed construction, or from the casual interstices which two or more connected atoms must produce, and which may be just adapted to those of other figures—as globular, oval, or square. Hence the origin of compound and visible bodies; hence the origin of large masses of matter; hence, eventually, the origin of the world itself.”—Dr. Good’s Book of Nature.

[36] Young’s Lectures on Natural Philosophy and the Mechanical Arts. Lecture 49, On the Essential Properties of Matter.