MODERN SUN CULT STURMER 197 



light blue. Tlie explanation of such extraordinary ellccts is that 

 these substances, and many others which show color under ultra- 

 violet have the property of reducing the latter radiation to longer 

 wave lengths ellecting a step-down, as we might call it, and thus 

 bringing the rays within the range of the visible spectrum. 



FLUORESCENCE 



This phenomenon is called lluorescence, because it was observed 

 first in the mineral fluorite. 



Not only tiie color, but also the intensity of the fluorescence, varies 

 greatly. Thus, for example, both sodium and potassium salicylate 

 show a violet-blue; but tlie fluorescence of the sodium salt is much 

 more pronounced. Again some solids which show little or no 

 fluorescence, may develop it to a marked degree when they are dis- 

 solved in an appropriate solvent. The examination of substances 

 under ultra-violet is a new development in science, and one which 

 gives })romise of many applications in solving problems in practical 

 chemistry. 



Certain substances fluoresce with a red light, that is to say, they 

 step down the invisible rays to about 7,000 A. If the waves coming 

 from the chemical when it is subjected to ultra-violet are mostly 

 waves of about 5,500 A, these will appear to our eyes as green ; and 

 so on through the range of colors which are seen in the rainbow. 

 But we must remember that just as visible light represents not a 

 single wave length but a range from about 7,700 A to about 3,G00 A, 

 so does ultra-violet radiation represent a range of wave lengths, 

 beginning where the shortest waves of the visible spectrum end, at 

 about 3,G00 A, and diminishing in length to about 1,000 A. Im- 

 mediately beyond the ultra-violet there lies an unexplored field; and 

 then, farther on, in the neighborhood of 100 A there are the X rays 

 concerning which we have some definite knowledge. And, lastly, 

 beyond the X rays, come the shortest waves known to science, the 

 gamma vays. from radium, and beyond these tlie cosmic rays about 

 which we now are receiving rather startling information. 



ABSORPTION OF ULTRA-VIOLET 



We are all familiar with the fact that some substances, like soot, 

 absorb all the rays of common light, and hence appear black, while 

 other substances reflect indiscriminately all the wave lengths of 

 light, as does this paper, and hence appear white. We know also 

 that many substances exhibit selective absorption, failing to appro- 

 priate the red, the yellow, the green, or the blue, and tlius possess 

 the colors of the wave lengths which are not absorbed. 

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