WAVES AND COKPUSCLES IN MODERN PHYSICS ' 



By Louts de Broolik 



[With 2 plates] 



A study of the properties of common bodies leads us to put them 

 into two classes: Composite bodies, which may be chanjied into more 

 simple ones by appropriate operations, and simple bodies, or ele- 

 ments, which resist all attempts at reduction into simpler ones. An 

 examination of the quantitative laws, following which these sim- 

 ple bodies, or elements, unite to make the composite ones, has led 

 chemists for a century to adopt the following theory : a simple body 

 is formed of small particles, each the same and called the atoms of 

 this simple body; composite bodies are formed of molecules made by 

 the union of several of these atoms. According to this hypothesis, 

 the dissociation of a compound body into the elements of which 

 it is composed means the breaking up of its molecules and the set- 

 ting free of the atoms which the latter contain. The number of 

 these simple bodies or elements actually known is 89. It is believed 

 that in all there will finally be found 92. Therefore out of 92 differ- 

 ent species of atoms all material bodies are constructed. 



The atomic theory has been successful not only in chemistry but 

 also in physics. If the substances of bodies are made of molecules 

 and atoms, then their physical properties should be interpretable 

 from their atomic constitution. The pro})erties of a gas, for exam- 

 ple, should be capable of explanation, supposing it to be formed of a 

 very great number of atoms or of molecules in rapid movements. 

 The pressure of a gas upon the walls of its container will be due to 

 the blows of the molecules upon these walls; the temperature of the 

 gas should be related to the mean agitation of these molecules, this 

 agitation increasing as the temperature rises. This conception of a 

 gas has been developed as the " Kinetic theory of gases " and has 

 led to an explanation of the gas laws revealed by experiment. If 

 the atomic theory is a proper representation of nature, the proper- 



* Lecture dellverod by M. de BroKlle, who had been awarded the Nobel prize, at a 

 conference at the Burenii of Arts and Measurcn, presided over by M. I'aliilev^, Jan. 28, 

 lO.'iO. Translated by pernjisslon from Kevue gCnCrale des Sciences, vol. 41, No. 4. Feb. 28, 

 1030. 



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