This essay concerns an aspect of the speculative contributions of J. J. Thomson to a field of physics somewhat removed from that upon which his popular fame and scientific eminence were alike founded. He published a number of statements in the period 1903–1910 advocating a discontinuous structure of the electromagnetic field. His unorthodox conception of the field was based upon the presumed discreteness of Faraday's physical lines of electric force. While his ideas led to significant experimental work, they were not brought together in the form of a completed theory. It was at this same time that the quantum theory was independently evolving notions of a structure of the field, and Thomson's efforts at developing a theory of light were diverted into a protracted criticism of the hypothesis of quanta. In 1924–1936 he returned to the subject of the structure of light, but these latter speculations no longer had much relevance to contemporary physical thought.

In most of the more lively fields of physical enquiry in the first three decades of the seventeenth century, a striking contrast may be observed between the antiquity of the problems attacked, and the innovatory procedures applied to solve them. None of these questions, inherited from a past now remote, seemed more pressing than the time-honoured controversy of the plenum versus the vacuum, especially as the concept of the atomic structure of matter was so closely associated with the existence of the void. The more one studies the arguments produced at that time, the more one will be impressed with the value and importance, for the vacuists and the atomists, of the Pneumatics of Hero of Alexandria. When we read Galileo's explanation of combustion, “the extremely fine particles of fire, penetrating the narrow pores of the metal (too small to admit even the minutest particles of air or of many other fluids) would fill the minute intervening vacuum, and would set free these minute particles from the forced attraction which these same vacua exert’, we can hear clear echoes of Hero's account of disseminated micro-vacua. Now Hero's work is clearly divided into two sections: a theoretical preface and a collection of 78 “pneumatic devices”. One might be tempted to claim that he, rather than his fellow-atomist Lucretius, inspired the first tentative syntheses of seventeenth-century atomism but it would still be necessary to agree that his chief contributions to the achievements of that generation lie in the experimental techniques which they derived from his instrumentation. Were it not for the popularity of his pneumatic demonstrations, he could never have won such a great reputation as a natural philosopher. Without that reputation, his opinions on this topic would not have been valued as almost equal to those of the mighty Aristotle, especially since he was so decidedly in the minority.

Extreme variation in the meaning of the term “species” throughout the history of biology has often frustrated attempts of historians, philosophers and biologists to communicate with one another about the transition in biological thinking from the static species concept to the modern notion of evolving species. The most important change which has underlain all the other fluctuations in the meaning of the word “species” is the change from it denoting such metaphysical entities as essences, Forms or Natures to denoting classes of individual organisms. Several authors have taken notice of the role of metaphysics in the work of particular biologists. An attempt will be made in this paper to present a systematic investigation of the role which metaphysics has played in the work of representative biologists throughout the history of biology, especially as it relates to their species concepts.