Prof. Draper on the Phosphorescence of Bodies. 83 



absorbing it, and retaining it by so feeble a power, that a very 

 trivial cause suffices for its extrication. This was the view of 

 Lenieiy, and was published in 1709. 



2nd. That phosphorescence ai-ises from an actual combustion, 

 or burning taking place in the sulphureous parts of the glowing 

 body. It is to be remembered that sulphm- figured largely in 

 the chemistry of those days. The diamond was believed to con- 

 tain it. This was the hypothesis of Dufay. 



To this celebrated electrician we owe a veiy able investigation 

 of the phosphorescence of various bodies, and especially of the 

 diamond. He recognised the fact which Boyle had overlooked, 

 that the gem must first be exposed to the light ; and then when 

 taken into a dark place it shines for a time, and gradually fades 

 away. But the glow can be re-established by raising the tem- 

 perature, and an exposure of a single second to the sun is quite 

 enough to commence the process. 



To recognise feeble degrees of luminosity, it is necessaiy for 

 the observer to remain in the dark until the pupil of the eye is 

 quite dilated, and the impression of light to which the retina has 

 been exposed worn off. Dufay gives a singular but very conve- 

 nient practical process. He recommends the experimenter to 

 keep one eye bound up or closed, for the pui-pose of observing in 

 the dark, and to use the other in conducting his processes in the 

 light. He remarks the curious fact, that the eye which has been 

 shut will not have the delicacy of its indications afi'ected by that 

 which has been exposed to the light. 



In this manner Dufay found, that of four hundi'ed yellow dia- 

 monds all were phosphorescent; but some that were white, or rose- 

 coloured, or blue, or green, were not ; nor was there any external 

 indication by which it could be told whether any given one of 

 these kinds would shine. He cbscovered, too, that the glow 

 took place under various coloured media ; as stained glass, under 

 water, milk, but not under ink. He also made attempts to 

 compel the gem to preserve its light by enveloping it in opake 

 media, such as ink, black wax, &c., under the idea that the light 

 could not get out ; and concluded that he had partially succeeded, 

 because in some instances the diamonds would shine after being 

 shut up for six or twelve hours. He verified Boyle's fact on the 

 eifects of hot water and heating generally, l)ut carried his tem- 

 peratures to far higher degrees, even above a white heat, and 

 found that the stone had lost none of its qualities, for it would 

 take bglit again when it was cold on a momentary exposure to 

 the «un. He also investigated how far the glow was connected 

 with electrical rchitions, and sliowed its perfect independence. 

 He also greatly increased the list of phosphori, asserting, that, 

 80 far from the quality being a peculiarity of the Bolognian 



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