judged as a connected argument. Single departments of electric theory can be, and usually are, more concisely formulated when the nature of their connexion with other departments is not a question of immediate interest. Here the foundation is intended to be definite and universal, thus precluding the adoption of an independent standpoint on each branch of the subject, to be developed as far as it will go, with the help if need be of empirical modification irrespective of the requirements of other branches: and the main question to be determined is not whether the presentation is entirely free from flaw, but whether and in what respects it is an advance sufficient to merit further attention with a view to its improvement. The time has fully arrived when, if theoretical physics is not to remain content with being merely a systematic record of phenomena, some definite idea of the connexion between aether and matter is essential to progress; the discussions which follow are largely concerned with the development of the consequences of one such conception. It seems reasonable to hold that the utility of such a discussion will not be entirely removed should this conception turn out to be practically incomplete, or even incoherent: for it is concerned with a body of ideas relating to the ultimate activity of molecules which is on a different plane from the indefinite notion of mutual forces to which dynamical molecular science has mostly been restricted.
It is recognized of course that every attempt at improvement in scientific exposition must have a limited range, and that the chief critical interest will soon be transferred from what can be explained by any new formulation to what it has not shown itself competent to include. Yet it has turned out, to take a famous instance, to be no insuperable objection to Dalton's chemical development of the atomic theory, that he could form no idea as to how his atoms held each other in combination: although in fact Wollaston, and to some extent Davy, hesitated about accepting the atoms while holding to the laws of combination in definite and multiple proportions that
