Some Contemporary Advances in Physics — VII 

 Waves and Quanta 



By KARL K. DARROW 



THV. in\;ilual)lc agoiit of c)iir best knowledge of the ciniroiiiiisj 

 world, aiul \et itself unknown except by inference; the inter- 

 mediary between matter and the finest of our senses, and yet itself not 

 material ; intangible, and yet able to press, to strike blows, and to recoil ; 

 impalpable, and yet the \ehicle of the energies that fiow^ to the earth 

 from the sun — light in all times has been a recognized and conspicuous 

 feature of the ph\-sical world, a perpetual reminder that the material, 

 the tangible, the palpable substances are not the only real ones. \c{ 

 its apparent importance, to our forerunners who knew only the rays 

 to which the eye responds and suspected no others, was as nothing 

 beside its real importance, which was realized \-ery gradually during 

 the nineteenth century, as new families of rays were disco\cred 

 one after the other with new detecting instruments and with new 

 sources. Radiation is not absent from the places where there is 

 no eye-stimulating light; radiation is omnipresent; there is no region 

 of space enclosed or boundless, vacuous or occupied by matter, which 

 is not pervaded by rays; there is no substance which is not perpetu- 

 ally absorbing rays and giving others out, in a coniiiuial interchange 

 of energy, which either is an equilibrium of equal and opjiosite ex- 

 changes, or is striving towards such an equilibrium. Radiation 

 is one of the great general entities of the physical world; if we could 

 still use the word "element," not to mean one of the eight\- or ninei\ 

 kinds (if material atoms, but in a deeper sense and somewhat as liu' 

 ancient^ used it, we niiglil describe radiation and in, hut. or possiiiK- 

 radiation and eieclricily, as coec|ual elements. Also the problem 

 111 ilie nature and structure of radiation is of no lesser importance 

 than the jiroblem of the structuri' and nature of matter; and in fact 

 neither can be treated separateK-; tiu'\' are so inextricably inter- 

 twined that whoever sets out to expound the jire.sent condition of 

 one soon finds liimself outlining the <iilui-. One cannot write a 

 discourse on the nature of radiation alone nor on the structure of liie 

 atom alone, one can iuit \ary the relative emphasis laid u|)on these 

 two subjects, or rather upon these two aspects of a single subject; 

 and in this article I shall restate main- things about the atom which 

 were stated in former articles, but the emphasis will be laid tipon light . 

 Speaking very generall>' and rather \-aguel\-, light has been much 

 more tractable to the theorists than most of the other objects of 



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