120 



SCIENCE 



[N. a Vol. XXXVII. No. 943 



is little probability that the number of 

 molecules in a cubic centimeter of a gas 

 under standard conditions differs by more 

 than that amount from 27.09 billion billion. 

 We have learned, too, a great deal about 

 the insides of the atom. We have proved 

 that it has electrical constituents, and that 

 these also have an atomic structure. In 

 other words, we have superposed upon an 

 atomic theory of matter a much more fun- 

 damental and at the same time a much 

 more simple theory of electricity. And we 

 have found most convincing demonstra- 

 tions of the correctness of the view that 

 every electrical charge is built up out of an 

 exact number of electrical atoms, and that 

 every electrical current consists in some 

 kind of a transport of these electrical atoms 

 through the conducting body. In fact, we 

 can now count the number of free electrons 

 upon a small charged body as directly and as 

 infallibly as we can count our fingers and 

 toes." We have measured, too, the exact 

 value of this elementary electrical atom, 

 and found it to be 4.774 X lO"'" absolute 

 electrostatic units.^ 



Furthermore, we have added much to our 

 knowledge about how atoms and molecules 

 behave as aggregates. We have found the 

 most convincing demonstrations, both quan- 

 titative and qualitative,* of the correct- 

 ness of the fundamental assumptions of the 

 kinetic hypothesis, and have proved ex- 

 perimentally that every molecule in a gas, 

 whether of the size of the hydrogen unit or 

 ten billion times as big, is endowed at a 

 given temperature with exactly the same 

 average kinetic energy of agitation. And 

 we have measured with a fraction of a per 

 cent, of accuracy the value of this univer- 

 sal constant. 



Finally, we have tremendously extended 



'Physical Revieio, XXXII., p. 349, 1911. 



^Physical Eevierv, 1913. 



* Popular Science Monthly, LXXX., p. 417, 1912. 



our kinetic conceptions of matter through 

 the study of radioactive processes, and have 

 recently actually seen on photographic 

 plates" the tracks of alpha and beta cor- 

 puscles as they shoot out spontaneously 

 from radioactive atoms with speeds un- 

 dreamed of in connection with projectiles 

 of any kind twenty years ago— speeds 

 which are of the same order of magnitude 

 as the velocity of light. 



In a word the last fifteen years have 

 shown the atomic and kinetic conceptions to 

 be certainly the most fruitful, may we not 

 also say the most fundamental conceptions, 

 not excepting even the principle of the 

 conservation of energy, which have ever 

 been introduced into physical science. 

 Only in one domain have atomistic points 

 of view failed completely to possess the 

 field, and that, oddly enough, the only do- 

 main in which they were securely en- 

 trenched two hundred years ago, but from 

 which they were driven, apparently for- 

 ever, at the beginning of the last century, 

 by the epoch-making work of Fresnel and 

 Young. Upon this lost domain of radiant 

 energy they are now making renewed at- 

 tack. It is my purpose to-day to survey 

 this field of conflict and to endeavor to ap- 

 praise the successes and failures of each of 

 the opposing forces from the point of view 

 of experimental physics alone. 



My first observation is that in this at- 

 tack upon the domain of radiant energy, 

 atomistic conceptions do not at present 

 show a united front. In other words, there 

 is not one sharply defined atomistic theory, 

 but there are five distinct brands of 

 "quantum" theory of various degrees of 

 concentration. These are alike in that they 

 all have to do with certain assumptions as 

 to the nature of radiant energy, or as to the 

 conditions under which such energy is ab- 



'C. T. E. Wilson, Proc. Soy. Soc, Vol. 87, p. 

 277, 1912. 



