Page:Popular Science Monthly Volume 23.djvu/100

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We find the transmitted light to be colored differently by the different fluids, but that these latter themselves begin to shine in different colors—for instance, eosine, green; quinine sulphate, blue. By the former, PSM V23 D100 Geissler tube of four compartments.jpgFig. 1. 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 converted 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 transmitted light is yellow. We close the experiments with quinine sulphate. The cloud is colored delicately blue, but the transmitted light is pure white. Which rays were absorbed in this case? A later experiment 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 preparations, 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 after the preparations have been placed in darkness. Since this peculiarity of subsequent illumination is analogous to the development of light occurring when a piece of phosphorus is rubbed in darkness, it has been called phosphorescence. Both the duration and intensity of this subsequent 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