The atomic theory of Dalton implied that there were more than 30 different kinds of matter, the chemical elements. William Prout (1815) was the first of a long line of distinguished speculators who sought to show, by argument and experiment, that this diversity overlay a more fundamental unity. Contrary to a common opinion, this was not an eccentric and unpopular movement, but involved many of the great names of nineteenth-century chemistry; and some of their speculations have proved to be very near the mark. It may be that this strong current of well-informed speculation enabled the discoveries of cathode rays and of radioactivity (which provided the first real evidence of the complexity of the atom) to be integrated into the body of science with such remarkable rapidity.
* A shortened version of this paper was read at the Summer meeting of the Society on 26 June 1964.
Quotations in the text have all been translated into English for uniformity; the responsibility for translation is in most cases the author's own.