he defines a force as a power which tends to bring together portions of matter (and possibly of ether), while an energy has the opposite effect. He divides forces into four species: gravitation, which aggregates masses of matter; cohesion, which aggregates molecules; chemical affinity, which aggregates atoms; and "electrical affinity," which aggregates "electrical units." An instance of the operation of this last force is the discharge of a Leyden jar, by which positive and negative electricities are brought together. "In our present ignorance of the subject," he says, "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." Of the unit on which this force acts, the "electrical unit," he says that its nature "is very inadequately known to us," but that it "must be considered for our present purpose as in some way the analogue of the others, though we have no sufficient warrant for giving it any material properties." For further particulars he refers to a chapter on "Electrical Phenomena," but diligent search fails to discover such a chapter in the book. The author divides energies on the same principle as forces. "But owing," he says, "to the existence of two modes of energy, the potential and the kinetic, it will not be possible to assign a single definite name to each species." An instance of the action of a "molar energy" is afforded when we lift a weight from the ground. Heat, which separates molecules, is a "molecular energy." As an instance of a "chemical energy" employed in separation, he gives the power which effects the electrolysis of water. Hence, if we understand Mr. Allen rightly, the action of a current of electricity in electrolysis is the action of two powers of opposite kinds. For his aggregative power, or force,"electrical affinity" must be acting between the poles of the battery immersed in the water, while his separative chemical power, energy, is tearing apart the atoms of hydrogen and oxygen. He mentions light and intense heat as other examples of chemical energy. His instance of (separative) "electrical energy" is the friction which produces a disunion of the positive and negative electrical units in the electrical machine. But he adds that "as in the case of electrical forces, our treatment of this department must be considered purely temporary and symbolical." There are two modes of energy, the potential and the kinetic, and each of the four species of energy may exist in either mode. Motion has three kinds: separative, aggregative, and continuous or neutral. Each species of kinetic energy has a form of each kind. The principle commonly called the conservation of energy Mr. Allen names "the indestructibility of power," applying the former term in accordance with his use of the word energy, while he uses "the persistence of force" to denote the indestructibility of "aggregative power." In stating these principles the author gives us another distinction between force and energy, the former being inherent in the particles of matter, never passing from one unit to another, while energy may be transferred from one particle or set of particles to another. Not only do energies oppose forces, but one force may "interfere" with another: thus, when a weight is suspended by a cord, the cohesion of the cord counteracts the force of gravitation. So also energies may be "suppressed" by forces or by other energies. "Liberating energies" are those which release bodies from the control of one force and bring them under that of another.
With two short chapters on the nature of energy and the nature of motion the author closes the "abstract or analytic" part of his book. In the "concrete or synthetic" part, which follows, he describes the operations of force and energy in the evolution of the sidereal system, the solar system, the earth, and organic life, closing with a general view of the energies which the earth possesses. In an "apology" prefixed to the volume the author states that he has kept his theory in manuscript for a number of years, and explains why it is now published. "It pretends to be," he says, "no more than a suggestion, an aperçu, an attempt at a theory: I ask for it nothing better than honest consideration."
Hume's Treatise of Human Nature. Edited by L. A. Selby-Bigge, M. A., Fellow and Lecturer of University College. Macmillan & Co., Clarendon Press. 1 vol., pp. 709. Price, $2.25.
This celebrated treatise is now reproduced in admirable style, containing, in addition to the text, the original title-pages of