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THE POPULAR SCIENCE MONTHLY. 



he defines a force as a power which tends to 

 bring together portions of matter (and pos- 

 sibly of ether), while an energy has the op- 

 posite effect. He divides forces into four 

 species : gravitation, which aggregates mass- 

 es of matter; cohesion, which aggregates 

 molecules ; chemical affinity, which aggre- 

 gates atoms ; and " electrical affinity," which 

 aggregates "electrical units." An instance 

 of the operation of this last force is the dis- 

 charge of a Leyden jar, by which positive 

 and negative electricities are brought to- 

 gether. "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 doubt- 

 less 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 proper- 

 ties." For further particulars he refers to a 

 chapter on " Electrical Phenomena," but dili- 

 gent 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 en- 

 ergy " 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 elec- 

 trolysis is the action of two powers of oppo- 

 site 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 chem- 

 ical energy. His instance of (separative) 

 "electrical energy" is the friction which 

 produces a disunion of the positive and nega- 

 tive 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 sym- 

 bolical." 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 for- 

 mer 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 prin- 

 ciples 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 en- 

 ergy 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 gravita- 

 tion. 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 sidere- 

 al system, the solar system, the earth, and or- 

 ganic 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 manu- 

 script for a number of years, and explains why 

 it is now published. " It pretends to be," he 

 says, " no more than a suggestion, an apergu, 

 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. Macmil- 

 lan & Co., Clarendon Press. 1 vol., pp. 

 709. Price, $2.25. 



This celebrated treatise is now repro- 

 duced in admirable style, containing, in ad- 

 dition to the text, the original title-pages of 



