Works of Grant Allen

by Grant Allen

  A qualification must be added to prevent misconception. The cohering Molecules need not be supposed to be in actual physical contact with one another. It is sufficient that they should be within the sphere of one another’s attraction; just as the moon is kept in its place by the earth, and the planets by the sun, in spite of the intervening space. Theoretically, of course, every body in the universe attracts every other; but as the attraction decreases as the squares of the distance, at practically infinite distances it becomes practically infinitesimal and can be overcome by an infinitesimal Energy. This is the case ordinarily with Cohesion: at very slight distances its Force is so diminished that only an imperceptible amount of Energy is required to counteract it. But there is no reason to doubt that when the two rough pieces of iron are laid upon one another, the supporting points, so to speak, come within the sphere of mutual attraction, though their number and area are so small that we cannot perceive the resistance resulting from their Cohesion when we separate the pieces. In short, Cohesion always tends to act between all Molecules, but its effects may be disguised either by distance or by counteracting Energies. Other cases will be treated in the chapter on Mutual Interference of Forces. Adhesion and Capillarity are only forms of Cohesion.

  The Force which aggregates Atoms and resists the separation of Atoms is known as Chemical Affinity. As here employed it will be understood to mean not merely the Force which unites the Atoms of two or more elements into a compound molecule, but also the identical Force which unites two or more Atoms of the same element into a molecule such as that of ozone. When any two or more Atoms (or equivalents in combining proportions) are left free to act upon one another without the counteracting influence of an Energy, they aggregate in obedience to this Power. As in the case of cohesion, however, the Atoms must be brought into close contact with one another. When phosphorus is exposed to oxygen the aggregation is immediate. But in other cases a certain amount of molecular or Atomic motion is needed in order to bring the Atoms within the sphere of their mutual attractions. Thus heat is necessary to make carbon combine with oxygen, as in the ordinary phenomenon of combustion: while the more subtle motion of light suffices to effect a union between hydrogen and chlorine. But we may broadly assert that whenever free Atoms find themselves in the presence of a free Atom for which they have affinities (the proper proportions being of course supposed), and are brought within the sphere of their mutual attraction, the two Atoms or sets of Atoms aggregate under the influence of Chemical Attraction. Here, again, a qualification is needed. The above rule holds only for free Atoms. Just as a ball suspended by a rope from the ceiling does not fall to the ground, because the Force of cohesion outbalances the Force of gravitation, so, when two or more Atoms, united in stable combination, are brought into contact with other Atoms for which they have affinities less strong than those of their existing combination, they will not yield up their stronger to their weaker affinity. (See the subsequent chapter on Mutual Interference of Forces.) And again, just as the ball will break the rope, if gravitation outbalances cohesion; so, if the new affinities are stronger than the old ones, the Atoms will yield up their previous combination and enter into that to which they are most powerfully attracted. The second mode in which Chemical Affinity acts is in resisting the attempt to separate the component Atoms of a compound body. Setting aside for the present certain very abnormal cases in which ‘unstable’ bodies spontaneously decompose — cases which can only be explained at a very late stage of our exposition — all ordinary ‘stable’ compounds require an Energy to separate their Atoms. Thus heat is needed to divide the Atoms of oxygen from those of iron in ferric oxide: while electricity is necessary to sever the Atoms of hydrogen from those of oxygen in water. This statement must be understood as applying only to the separation of free elements, not the formation of new compounds. Mere juxtaposition is sufficient to make certain compound bodies yield up their weaker affinities in the presence of stronger ones: but (with the special exception noted above, chiefly referring to organic compounds) an Energy is required to separate any compound into its component Atoms in a free state, without the aid of stronger antagonistic affinities.

  The Force which aggregates Electrical Units and resists the separation of Electrical Units is known as Electrical Affinity. This Force is little understood, and can only be treated in a very symbolical manner. What few points can be formulated are briefly these. When Positive and Negative Electricities are left free to act within the sphere of their mutual attractions, they are aggregated by this Force, as in the discharge of a Leyden jar. In saying this, no implication of materiality is meant to be conveyed. In our present ignorance on the subject, Electrical Affinity must be placed in the same category as other Forces; though further researches will doubtless enable us to give a better account of its real nature. Similarly, an Energy is necessary to separate the Positive and Negative Electricities which subsist in combination in every material body. In the case of a glass rod or an electrical machine this Energy is that of mechanical motion: in certain other cases it is of thermal or chemical origin. These points will receive further consideration in the chapter on Electrical Phenomena.

  A table will put in a clearer light the classification here adopted.

  Forces or Aggregative Powers.

  Molar

  Molecular

  Atomic

  Electric

  Gravitation

  Cohesion

  Chemical Affinity

  Electrical Affinity

  ]

  CHAPTER V.

  THE SPECIES OF ENERGY.

  Energies may be most conveniently divided on the same principle as Forces, according to the nature of the particles of bodies in which they initiate or accelerate separative motion, and resist or retard aggregative motion. But owing to the existence of two modes of Energy, the Potential and the Kinetic, whose peculiarities will form the subject of our next chapter, it will not be possible to assign a single definite name to each species, as was the case with the various Forces. It must suffice for the present to quote a few well-known instances of each.

  The energies which separate Masses and resist the aggregation of Masses may be summed up under the title of Molar Energies. Of Molar Energies employed in actual separation, a familiar instance is given in our own persons, when we lift a weight from the ground or carry ourselves to the top of a hill, thereby counteracting the Molar Force of gravitation by raising a body to a greater distance than before from the centre of the earth’s attraction. Another instance is seen in a cannon ball fired vertically, or a stone lifted by a crane. On a larger scale, any fresh Energy employed in removing the moon further from the earth or a planet from the sun would be a Molar Energy. Any Mass thus separated from another attracting Mass is said in the current language of physics to possess Visible Energy of Position, a term which we shall examine and endeavour to amend hereafter. Of Molar Energies employed in resistance to aggregation the most familiar instance is that of orbital movement. The moon is prevented by this Energy from aggregating with the earth, and the planets with the sun, as they would otherwise do under the influence of Molar Force or gravitation. On a smaller scale, the Energy of a bird in flying or a cannon ball fired horizontally is largely employed in counteracting gravitation. It is seldom, however, that we see Energy thus employed, except in the case of the heavenly bodies, because the Molar Force exerted by the earth in its immediate vicinity is so strong as to overcome ordinary Energies after a very short period of dissipation. Masses of the sort here described are said in the current phraseology to possess Energy of Visible Motion, which expression, like the former one, will receive attention at a later point.

  The Energies which separate Molecules and resist the aggregation of Molecules may be summed up under the title of Molecular Energies. Of Molecular Energies employed in separation we have a common instance in heat, which draws apart the Molecules of a red-hot poker or a mass of boiling water, in opposition to the Molecular Force of Cohesion. The conta
ined Energy of water acts in the same manner on a lump of sugar or a mass of dry dough. Of Molecular Energies employed in resistance to aggregation, heat under its converse aspect affords us an example. The Molecules of all bodies are prevented from aggregating into their most compressed form by the presence of heat. Thus the red-hot poker only contracts so fast as it loses its Energy by radiation. The contained Energy (or ‘latent heat’) of water similarly prevents its aggregation into ice. Large masses of water before freezing part with their Energy in the visible form of heated mist.

  The Energies which separate Atoms and resist the aggregation of Atoms may be summed up under the title of Chemical Energies. A caution as to the sense in which this term must be here accepted is appended below. Of Chemical Energies employed in separation we have an instance in the electrolysis of water. The Energy disengaged by the union of elements in the battery is used up in producing chemical separation between the atoms of the electrolyte. Light produces a similar effect upon carbonic anhydride and water in the leaves of plants. Any Energy which separates a compound body into simpler or elementary bodies may be regarded as a Chemical Energy in the sense here intended. Of Chemical Energies employed in resistance to aggregation, no unequivocal instance can be cited at our present stage, though this apparent anomaly will be cleared up as we proceed. For the time the reader must be content to accept as an instance the fact that many Atoms will not combine with one another at a certain high temperature: the same temperature, in fact, at which they are driven off from their combination when actual. It will be noticed that, for the sake of uniformity, a somewhat new sense has here been given to the term ‘Chemical Energy.’ As ordinarily used at present, that term refers to the strength of the tendency which a body shows to unite with other bodies. It will be seen in the sequel that this is really a property depending upon separation and chemical nature: just as a body in proportion to its height and mass shows a tendency to aggregate with the earth: but, meanwhile, it is necessary to impose a new sense upon the term, in keeping with the analogous term ‘Chemical Affinity,’ which we have applied to the Force that aggregates Atoms.

  The Energies which separate Electrical Units and resist the aggregation of Electrical Units may be summed up under the title of Electrical Energies. As in the case of Electrical Forces, our treatment of this department must be considered purely temporary and symbolical. Of Electrical Energies employed in separation we have an instance in the electrical machine, where friction produces a disunion of the Positive and Negative Units. Similarly in the torpedo and gymnotus. Of Electrical Energies employed in resisting aggregation there is again no unequivocal instance. The illustration of this deficiency must be left to later chapters.

  Throughout, both in the case of Forces and Energies, it will be noticed that the same Power which initiates and accelerates one kind of motion equally resists and retards the other kind of motion. Thus, Gravitation both initiates movements of masses towards centres of attraction, and resists movements of masses away from centres of attraction. Cohesion both draws molecules together, and resists the separation of molecules: while heat draws molecules apart and resists the aggregation of molecules. So that these two Powers, the aggregative and the separative, are incessantly opposing and antagonising one another in all bodies, great or small. The amount of aggregation reached by any system of bodies at any point of time depends upon the relative proportions of its Forces and its Energies at that moment.

  A table is scarcely needed for the contents of this chapter; yet for the sake of symmetry one is here appended.

  Forces or Aggregative Powers.

  Molar

  Molecular

  Atomic

  Electric

  Molar Energy

  Molecular Energy

  Chemical Energy

  Electrical Energy

  CHAPTER VI.

  THE MODES OF ENERGY.

  Energy has two Modes, ordinarily known as the Potential and the Kinetic: but the terms Statical and Dynamical are much preferable. Nevertheless, in order not to disturb unnecessarily the received terminology, the former expressions will be generally preserved in this treatise.

  The two Modes of Energy are interchangeable with one another: the Potential can pass into the Kinetic, and the Kinetic into the Potential. Each species of Energy, Molar, Molecular, Atomic, and Electrical, is represented in both modes.

  Potential Energy (a very bad name) is equivalent to actual or statical separation. Any mass, molecule, atom, or electrical unit, in state of separation from other masses, molecules, atoms, or electrical units, possesses Potential Energy. The subject may conveniently be considered under the four heads hence arising.

  Molar Potential Energy is equivalent to the statical separation of Masses. The moon possesses this Energy relatively to the earth, and the planets to the sun. The cannon ball, shot vertically, has Molar Potential Energy at the instantaneous neutral point when it has reached its greatest height and has not yet begun to fall. A stone on a mountain top or a head of water on its side has also the same Energy. In short, Molar Potential Energy is possessed by all discrete Masses in virtue of their separation. It is commonly known as Visible Energy of Position.

  Molecular Potential Energy is equivalent to the statical separation of Molecules. Two planed surfaces of iron possess this Energy, until by apposition they are made to unite. The molecules of water, dispersed as steam, similarly possess it, in the form commonly known as ‘latent heat.’ When steam condenses or water freezes, the Energy is yielded up in the Kinetic form.

  Atomic Potential Energy is equivalent to the statical separation of Atoms. It is possessed by every free Atom of an element, and by every compound Atom whose affinities are not fully saturated. Thus an Atom of carbon has Potential Energy in relation to two separate Atoms of oxygen, with which it may unite to form carbonic anhydride. Similarly, chlorine has Potential Energy relatively to sodium, with which it may unite to form common salt. Such cases, however, must be carefully distinguished from those of preferential attraction where a body leaves its union with one element to combine with another for which it has stronger affinities: as when the Cl of HCl leaves the H to unite with Na in NaCl. This last instance is really analogous to that of the cannon ball which breaks the rope that ties it because the Force of Gravitation has outbalanced that of Cohesion.

  Electrical Potential Energy is equivalent to the statical separation of Electrical Units. In a Leyden jar, the opposite electricities of the inner and outer coats exhibit this relation. In a thunder cloud and the earth beneath it we have a substantially similar division of the Positive and Negative Units. The statement of these facts must be accepted with the usual caution as to the purely symbolical nature of our electrical conceptions.

  From the potential we pass on to the Kinetic Mode. It will not be immediately apparent in what sense Kinesis is an Energy in accordance with our definition: but, here again, the reader must courteously waive his objections for the present, and accept the statement provisionally, so far as he finds possible. Many difficulties of this sort necessarily beset the explanation of every new point of view, especially where previous misconceptions have clouded and embarrassed the mental vision.

  Kinetic Energy is equivalent to motion. Any mass, molecule, atom, or electrical unit, in a state of motion, possesses Kinetic Energy. The subject may be conveniently considered under the four heads hence arising. But, just as before, when dealing with Energy generally, we found that we could not divide it into species so definite in their likeness as those of Force, because Energy was manifested in two Modes, the Potential and the Kinetic: so, here, when we are dealing with Kinetic Energy specially, we shall find that it cannot be divided into species so definite as those of the Potential Mode, because Kinesis itself is divisible into several Kinds, whose nature will form the subject-matter of the succeeding chapter.

  Molar Kinetic Energy is equivalent to the relative motion of Masses. It is seen in the fall of an unsupported weight or a spent cannon ball to
the earth. It is also seen in the rising of the ball, the flying of a bird, or the walk of a man. Again, it is seen in the orbital motion of the planets, and in the spinning of a top. These various Kinds of Kinesis will be fully discussed in the next chapter.

  Molecular Kinetic Energy is equivalent to the relative motion of Molecules. It is found in the falling together of Molecules of steam into water. It also occurs in the disruption of a cohering mass. And it is more conspicuous in the phenomenon of heat.

  Atomic Kinetic Energy is equivalent to the relative motion of Atoms. It is seen in that rushing together of Atoms which results in chemical combination. It also occurs in the severing of Atoms from the combined state. But it is not known to have any continuous form analogous to the orbital motion of a planet, the spinning of a top, or the regular vibration of heat.