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THE POPULAR SCIENCE MONTHLY. 



Fig. 1. 



We find the transmitted light to be colored differently by the different 

 fluids, but that these latter themselves begin to shine in different col- 

 ors for instance, eosine, green ; quinine sulphate, blue. By the former, 



the transmitted light is red ; by the 

 latter, almost white. In both cases, 

 consequently, the rays emanating 

 from the red-hot gas were screened 

 as it were, and the retained part con- 

 verted into rays of another number 

 of undulations, to wit, into green 

 with eosine, and blue with quinine. 

 Far more intense and admirable 

 in color become the appearances, 

 when we make the powerful rays 

 of the electric light parallel by 

 means of a condensing lens, pass them through a square glass vessel 

 filled with pure water, and to this add the substances by drops. We 

 will begin with eosine. We pour a little of the solution into the water, 

 and an admirable, vividly green-colored cloud at once spreads within 

 the vessel. If we place a white screen behind the vessel, we find that 

 the transmitted light appears red upon it. Eosine, consequently, 

 possesses the property of only permitting the red rays to pass, and of 

 altering them into green light, while absorbing all the others. We 

 take fresh water, and repeat the experiment with fluoresceine. The 

 green of the generated cloud now is far more vivid, while the trans- 

 mitted light is yellow. We close the experiments with quinine sul- 

 phate. The cloud is colored delicately blue, but the transmitted light 

 is pure white. Which rays were absorbed in this case ? A later ex- 

 periment will answer the question. 



There is an occurrence very generally found in nature which is 

 dissimilar in form, but analogous in essence, with the fluorescence. 

 With fluorescent substances, the emission of light ceases as soon as 

 illumination is interrupted. If light is thrown upon calcium prepara- 

 tions, a part of the rays is also absorbed, and altered into rays of 

 another number of waves. But these preparations emit the absorbed 

 rays partly only after the cessation of illumination. Owing to the 

 weakness of the light emitted, it becomes visible only ofter the prep- 

 arations have been placed in darkness. Since this peculiarity of sub- 

 sequent illumination is analogous to the development of light occur- 

 ring when a piece of phosphorus is rubbed in darkness, it has been 

 called phosphorescence. Both the duration and intensity of this sub- 

 sequent light depend upon the nature of the substances employed. 

 There are those known, the light of which disappears very quickly 

 after its emission, and again those by which the illumination lasts as 

 long as eighteen hours of course, while growing constantly feebler. 

 For the study of this phosphorescence we again make use of a 



