318 THE ACTION OF GLASS AND QUARTZ. [MEMOIR XXII. 



The inability of an electric spark to cause phosphor- 

 escence is connected with its transient duration. The 

 voltaic light enables us at pleasure to imitate the effects 

 of an electric spark or those of the sun. The phosphor- 

 ogenic rays, whether they originate in an electric spark 

 or from the sun, occupying thus the same place in the 

 spectrum, and even exhibiting the same peculiarities as 

 the chemical rays on iodide of silver, we have next to de- 

 termine whether this is an apparent or a positive identity. 



Professor Henry, of Princeton, read a paper before the 

 American Philosophical Society, in May, 1843, in which 

 he discussed all the leading mechanical properties of the 

 phosphorogenic rays, and, among other important exper- 

 iments, made some with a view of determining this par- 

 ticular question. A daguerreotype plate and some lime 

 sulphide were simultaneously exposed to the sky: the 

 plate was stained, but no effect was produced on the 

 lime. *,.A daguerreotype plate and some lime sulphide 

 were exposed to the light of an electric spark : the lime 

 was observed to glow, but no impression was produced 

 on the plate. When the plate was exposed to a succes- 

 sion of sparks for ten minutes with a sheet of mica inter- 

 posed, an impression was made. Lime exposed to the 

 moon did not phosphoresce, but a sensitive plate under 

 the circumstances is said to be stained. In view of 

 these different facts, Professor Henry observes : " These 

 experiments, although not sufficiently extensive, appear 

 to indicate that the phosphorogenic emanation is distinct 

 from the chemical, and that it exists in a much greater 

 quantity in the electric spark than either the luminous 

 or chemical radiation." 



From Wilson's experiments it appears that he was 

 aware that when the phosphorescent surface is warmed 

 so as to hasten the disengagement of light, the moon- 

 beams may be found to have left traces of action upon 



