ATOM. 449 



atoms, each of which is devoid of extension. According to Boscovich's theory, 

 all action between bodies is action at a distance. There is no such thing in 

 nature as actual contact between two bodies. When two bodies are said in 

 ordinary language to be in contact, all that is meant is that they are so near 

 together that the repulsion between the nearest pairs of atoms belonging to 

 the two bodies is very great. 



Thus, in Boscovich's theory, the atom has continuity of existence in time 

 and space. At any instant of time it is at some point of space, and it is never 

 in more than one place at a time. It passes from one place to another along 

 a continuous path. It has a definite mass which cannot be increased or di- 

 minished. Atoms are endowed with the power of acting on one another by 

 attraction or repulsion, the amount of the force depending on the distance 

 between them. On the other hand, the atom itself has no parts or dimensions. 

 In its geometrical aspect it is a mere geometrical point. It has no extension 

 in space. It has not the so-called property of Impenetrability, for two atoms 

 may exist in the same place. This we may regard as one extreme of the 

 various opinions about the constitution of bodies. 



The opposite extreme, that of Anaxagoras the theory that bodies apparently 

 homogeneous and continuous are so in reality is, in its extreme form, a theory 

 incapable of development. To explain the properties of any substance by this 

 theory is impossible. We can only admit the observed properties of such sub- 

 stance as ultimate facts. There is a certain stage, however, of scientific progress 

 in which a method corresponding to this theory is of service. In hydrostatics, 

 for instance, we define a fluid by means of one of its known properties, and 

 from this definition we make the system of deductions which constitutes the 

 science of hydrostatics. In this way the science of hydrostatics may be built 

 upon an experimental basis, without any consideration of the constitution of a 

 fluid as to whether it is molecular or continuous. In like manner, after the 

 French mathematicians had attempted, with more or less ingenuity, to construct 

 a theory of elastic solids from the hypothesis that they consist of atoms in 

 equilibrium under the action of their mutual forces, Stokes and others shewed 

 that all the results of this hypothesis, so far at least as they agreed with 

 facts, might be deduced from the postulate that elastic bodies exist, and from 

 the hypothesis that the smallest portions into which we can divide them are 

 sensibly homogeneous. In this way the principle of continuity, which is the 

 basis of the method of Fluxions and the whole of modern mathematics, may 



VOL. II. 5? 



