274 THE POPULAR SCIENCE MONTHLY 



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 ex- 

 pression. 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 ele- 

 ments 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 prop- 

 erties 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 frame- 

 work 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 



