tennis ball does to the earth; and the masses are in about the same proportion. If we emulate Archytas,
numero carentis arenae mensorem,
"the measurer of the innumerable sands," and estimate the number of molecules in even a drop of water, we obtain a result far beyond our powers of realization; a number requiring 22 figures for its expression. It is surely not to be reckoned among the least achievements of science that it has determined the order of this enormous quantity and has even made us reasonably certain of the first figure.
Such is the modern atom. It would seem impossible to penetrate farther into the details of so minute a structure, one too whose elements defied attack by physical and chemical agencies. It was felt, however, that a system based upon some eighty distinct kinds of primordial matter could hardly be an ultimate solution of the problem; and the suggestion was early made that the atoms are complex groups of a fundamental atom—possibly that of hydrogen, the smallest known. This hypothesis, suggested and supported by the fact that many atoms are very nearly exact multiples of the hydrogen atom in mass, has proved attractive to those who saw in the orderly succession of properties among the elements (known as the periodic law) indications that matter has reached its present state of multiplicity through some process of evolution. Similar indications were thought to be found by some in the study of the spectra of the stars. But these views were speculative, and direct evidence was lacking; and little light was thrown upon the subject until just before the close of the last century new lines of investigation were opened which greatly extended and modified our views as to the nature of the atom. This expansion was determined by the simultaneous development of the modern or what might be called the atomic theory of electricity, usually known as the electron theory.
That electricity, like matter, consists of indivisible units or atoms had long been suspected, since experiments of Faraday had shown that the quantities of electricity carried by atoms were always either equal to or exact multiples of a single charge—that carried by the hydrogen atom; and the term electron had been suggested as a name for the atom of electricity. As early as 1878 the great Dutch physicist Lorentz had based an explanation of the refraction and dispersion of light upon the presence in matter of equal discrete particles or atoms of electricity, and this hypothesis was afterwards developed into a complete framework of a theory of electrical and optical phenomena. But in the absence of experimental confirmation little attention was paid to these theories until the investigations to which I alluded brought the electrons themselves forcibly before the scientific world.
In 1897 J. J. Thomson was investigating the electrical discharge
