2 BIOLOGICAL EFFECTS OF RADIATION 



little known as yet), are between light and that subtlest variety of matter 

 to which I have already referred under the name of "negative electricity" 

 or "electrons." To the best of our knowledge and belief, all of what we 

 normally call matter consists of atoms, each of which consists of a certain 

 number of "orbital" electrons (varying from one element to another) 

 and a relatively massive "nucleus" bearing a positive charge equal in 

 magnitude and opposite in sign to the sum of the negative charges of the 

 electrons. With the nuclei, light has no transactions, save in the rare 

 exceptional cases to which I have just alluded. Absorption of light — 

 refraction, dispersion, scattering of light — excitation of one kind of light 

 by another kind — the electrical, the thermal, the beneficent, and the 

 injurious effects of light, on inorganic and on organic matter — are almost 

 exclusively due to transactions between hght and orbital electrons. 

 It would, of course, be grossly extravagant to pretend that all of these 

 phenomena are fully understood; but so much of each of them is under- 

 stood, that the foregoing statement may be made as confidently as any 

 statement in physics which is not merely a description of an actual 

 observation. 



It would doubtless be suitable at this point to define the word "light," 

 but to do this properly would be a formidable task. I might say "light 

 is electromagnetic radiation," but this is rather a description than a 

 definition — a description of a theory of light which is verified by the 

 phenomena of which I have to speak, in addition to a number of others. 

 To say that "light is that which is perceived by the eye" would be to 

 commit a very bad anachronism, since this definition has been out of 

 date for physicists for 134 years, and even for the general public since 

 the term "ultra-violet light" became of common usage. I shall evade 

 the issue by listing the various special terms for various kinds of light 

 which the general name includes: visible light, ultra-violet light, infra- 

 red light, thermal radiation (now a rare term, superfluous in view of the 

 present use of "infra-red"). X-rays, gamma rays. To these should be 

 added, for completeness, Hertzian or "radio" waves and (probably) 

 some cosmic rays; but these last fall outside of the scope of this treatise. 

 These diverse forms of light are best delimited from one another (though 

 the delimitation has little importance) by citing the ranges of wave-length 

 which they have; but inasmuch as the corpuscular picture of hght is to 

 be by far the more prominent in this essay, it is desirable to introduce the 

 brief allusion to wave-lengths by saying something of the relations 

 between waves and corpuscles. 



Wave theory and corpuscular theory, as they are nowadays imagined 

 in deahng with light (and, incidentally, with electricity and matter), 

 are not antagonistic. It must be confessed that the union of the two 

 conceptions is still far from perfected, but at least much progress has 



