CHAPTER XX 

 RADIANT ENERGY 



Man has long been aware of his radiation environment. He 

 has known that he is sensitive to the rays of the sun, that they 

 warm him, tan his skin, and improve his health. He has also 

 suspected that moonlight, which differs from sunlight, has its 

 influence, though here his imagination has had the best of him 

 at times. Within recent years, science has substantiated many 

 of these beliefs and added others of which primitive man never 

 dreamed, for our radiation environment is by no means Hmited 

 to light as we ordinarily think of it. No one with the slightest 

 scientific knowledge would today question the possibiUty of 

 organisms, and therefore protoplasm, being sensitive to such 

 forms of radiation as ultraviolet light, electric rays, X rays, and 

 cosmic radiation. It becomes our task to determine how far 

 living matter responds to its radiation environment. 



We have not only the problem of the effect of external radiation 

 on protoplasm but also the possibility of protoplasm being 

 itself a source of radiation. This question takes us into a much 

 controverted field; but if we consider the problem subjectively, 

 we are forced to come to the conclusion stated by F. Daniels, 

 viz., that there is no fundamental reason why protoplasm should 

 not give off photons, or units of light energy. 



The Electromagnetic Spectrum. — Forms of radiant energy 

 now known include electric (hertzian or radio) rays, heat, infra- 

 red, visible light, ultraviolet, X rays, gamma rays, and cosmic 

 rays. All can be placed in a scale of wave lengths extending 

 from waves several miles in length to waves so small that we 

 cannot appreciate their minuteness (Fig. 166). It is extraor- 

 dinary that these numerous and diverse forms of energy are all 

 waves, apparently just aUke except for length, and yet how differ- 

 ent they seem to us when we consider them separately. 



The discovery of the essential unity in such seemingly different 

 forms of energy as occur in the spectrum of electromagnetic radi- 



390 



