1843] WRITINGS OF JOSEPH HENRY. 209 



no impression was produced on the plate. When however 

 the plate was exposed very near to a succession of sparks 

 continued for ten minutes, with a plate of mica interposed, 

 an impression was made. 



The sulphuret of lime was also exposed for several min- 

 utes to the direct light of the full moon, without any 

 phosphorescent effect. A sensitive plate, similarly exposed, 

 according to the statement of Dr. Draper, receives a photo- 

 graphic impression. These experiments, although not suffi- 

 ciently extensive, appear to indicate that the phosphorogenic 

 emanation is distinct from the chemical, and that it exists 

 in a much greater quantity in the electrical spark than either 

 the luminous or the chemical emanation. 



Professor Henry remarked that in considering these ema- 

 nations as distinct, he had reference only to the classification 

 of the phenomena, for if they be viewed in accordance with 

 the undulatory hypothesis they may all be considered as the 

 results of waves, differing in length and amplitude, and 

 possibly also slightly differing in the direction of vibration. 



The phosphorescence of the lime may also be excited by 

 exposure to the light of a burning coal, and in this case the 

 emanation is also screened by a plate of mica. It was also 

 found that the magneto-electrical spark from a surface of 

 mercury excites the luminous condition of the sulphuret, 

 and it has long been known that heat, applied to the bottom 

 of the vessel containing the article, produces the same effect. 



To determine whether the phosphorescence could be excited 

 by electro-dynamic induction, a quantity of the sulphuret 

 was placed between two plates of quartz, and a covered cop- 

 per wire was wound around the whole, so that the lime 

 occupied the axis of a spiral. But when a discharge of elec- 

 tricity was passed through the wire the lime gave no indica- 

 tions of phosphorescence; the same negative result was also 

 obtained when the sparks were passed through the bottom of 

 the leaden pan. 



It has been supposed that the phosphorescence of the lime 

 is due to the disturbance of the electricity of the mass of the 

 substance, and the continuance of the light to the subse- 

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