MOLECULAR ATTRACTION—MOLECULAR THEORY. In chemistry it is conceived that bodies are made up of indivisible atoms, each having a definite uniform weight and a general character. The service that this theory has rendered to science may be seen by con sulting ATOMIC THEORY. The ultimate particles of matter are termed atoms, the word molecules being applied to the constituent or hetero geneous whole. [MOLECULE.] If the molecular constitution of bodies were known,—that is, the specific nature of these molecules,—and the laws of the forces which retain them in connection, whether these forces be of attraction or repulsion, science would then be able to define the changes and sequences of the material universe. Various efforts have been made from time to time to construct theories on this ground sufficiently general to afford a basis for important and wide con clusions, and yet to avoid injurious restrictive conditions. According to the theory of Boscovich, matter floes not consist of solid particles, but of mere mathematical centres of force. Each body is supposed to be made up of a number of geometrical points, from which emanate forces following certain mathematical laws, in virtue of which the forces become at certain small distances attractive, at certain other dis tances repulsive, and at greater distances again attractive. [ATTRACTION.] " From these forces of the points arise the cohesion of the parts of the same body, the resistance which it exerts against the pressure of another body, and, finally, the attraction of gravitation which it exerts upon bodies at a distance." Dr. Whew ell, from whom we have just quoted (` History of Scientific Ideaa,' voL ii., 1858), regards this theory as homogeneous and consistent, and thinks it probable that it may be used for investigating and express ing true laws of nature, although the attempt to identify the forces by which the particles of bodies are bound together with mechanical attraction appears to be a confusion of two separate ideas. This theory, he remarks, " may be so conceived as possibly to involve an explanation of all the powers which their parts exert (such powers, namely, as those which produce optical, thermotical, and chemical phenomena) ; but this theory cannot supply an explanation of the mechanical properties of a body as a whole, especially of its inertia. A collection of mere centres of force can have no inertia. If two bodies are considered as two collections of centres of force, the one attracting the other, there is in this view nothing to limit or determine the velocity with which the one body will approach the other. A world composed of such bodies is not a material world ; for matter implies not only force, but something which resists the action of force." Representing matter as a collection of molecules or centres of force, modern chemists have used the molecular hypothesis as a basis for calculations respecting the elementary forces of bodies, supposing the properties of bodies to depend upon forces emanating from immovable points of their mass. The mechanical philosopher, also, thus treats the properties of bodies on a small scale as Newton had already done with central forces on a large scale. Newton himself remarks, in the
preface to his Principia,' "Many things induce me to believe that the rest of the phenomena of nature, as well as those of astronomy, may depend upon certain forces by which the particles of bodies, in virtue of causes not yet known, are urged towards each other and cohere in regular figures, or are mutually repelled and recede ; and philosophers knowing nothing of these forces have hitherto failed in their exami nation of nature." Laplace, following up this hypothesis, has shown its value, if we suppose the forces exerted by the particles to decrease so rapidly with the increasing distance from them that the force is finite only at distances imperceptible to the senses, and vanishes at all refnoter points. " He has taught the method of expressing and calcu lating such forces ; and he and other mathematicians of his school have applied this method to many of the most important questions of physics, as capillary action, the elasticity of solids, the conduction and radiation of beat. The explanation of many apparently unconnected and curious observed facts by these mathematical theories, gives a strong assurance that its essential principles aro true. But it must be observed that the actual constitution of bodies, as composed of distinct and separate particles, is by no means proved by these coincidences. The assumption, in the reasoning, of certain centres of force acting at a distance, is to be considered nothing more than a method of reducing to calculation that view of the constitution of bodies which supposes that they exert force at every point. It is a mathematical artifice of the same kind as the hypothetical division of a body into infinitesimal party in order to find its centre of gravity, and no more implies a physical reality than that hypothesis does." Dr. Whewell, to whom we owe this quotation, refers also to Poisson's views of capillary action, and also to Wollaston's attempt to bring the doctrine of ultimate atoms to the test of observation in the case of the ATMOSPHERE, under which head some notice of Wollaston's argument will be found. But we may here remind the reader of Wollaston's conclusion that the observed phenomena accord with the supposition that the earth's atmosphere is of finite extent, limited by the weight of ultimate atoms of definite magnitude, no longer divisible by repulsion of their parts. We must, however, refer to Dr. Inewell'e work for his objections to this line of reasoning, and conclude by naming the most important works which have of late years assisted the molecular theory, such as Gauss on ` Terrestrial Magnetism ; ' Ohm's Contributions to Molecular Physics,' in which he supposes that the ultimate molecules of matter have both simple and polar powers, and by means of this hypothesis be attempts to form a complete system from which the phenomena of light, heat, and electricity necessarily arise; Mr. Grove's work, ` On the Correlation of the Physical Forces,' is also in the same direction.