214 PHYSICAL INDEPENDENCE OF THE PHOSPHOROGENIC RAYS. 



ical Society in May, 1843, in which he discussed all the leading mechanical properties 

 of the phosphorogenic rays, and among other important experiments, made some with 

 a view of determining this particular question. A Daguerreotype plate and some sul- 

 phuret of lime were simultaneously exposed to the sky; the plate was stained, but.no 

 effect was produced on the lime. A Daguerreotype plate and some sulphuret of lime 

 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 succession 

 of sparks for ten minutes, with a sheet of mica interposed, an impression was made. 

 Lirne exposed to the moon did not phosphoresce, but a sensitive plate under the cir- 

 cumstances is said to be stained. In view of these different facts, Professor HENRY ob- 

 serves, " 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 radi- 

 ation." 



987. From WILSON'S experiments, it appears that he was aware that when the phos- 

 phorescent surface is warmed, so as to hasten the disengagement of light, the moon- 

 beams may be found to have left traces of action upon it ; feeble, it is true, but never- 

 theless very apparent. We have seen, also, that the peculiarity of an electric spark is 

 due to its transient duration. Before, therefore, a final decision can be obtained on 

 this point, we are required to examine the effect of the tithonic rays and phosphoro- 

 genic emanation, under circumstances which are precisely similar as to intensity and 

 time. 



988. For the transient rays of an electric spark, quartz is transparent, and glass is 

 nearly opaque. Having prepared a bromo-iodized silver plate so as to be exceedingly 

 sensitive, I set in front of it, at the distance of about one third of an inch, a disc of 

 quartz and one of crown glass, of equal thickness ; and between a pair of copper wires, 

 the interval of which was three eighths of an inch, I passed the spark of a Leyden vial 

 fifteen times ; the distance between this spark and the sensitive plate was about two 

 inches. On mercurializing, the plate was deeply whitened all over, equally so through 

 the glass, through the quartz, and on the uncovered spaces ; but a spot of sealing-wax 

 which I had put on the glass, left its shadow on the plate beautifully depicted, as 

 were also the edges of the glass and the quartz. The two discs overlapped one an- 

 other to a certain extent, but the corresponding portion of the silver plate was as deeply 

 stained there as anywhere else. 



989. Next I put a surface of sulphuret of lime in the place of the Daguerreotype 

 plate, everything remaining as before. On passing fifteen sparks, the lime phospho- 

 resced powerfully under the quartz, but not under the glass, so that the difference be- 

 tween its shadow and that of the spot of wax could not be distinctly seen. 



990. For these reasons, therefore, I adopt the view expressed by Professor HENRY, 

 that the phosphorogenic emanation and the tithonic rays are distinct. Under the same 

 circumstances, glass to the one is transparent, to the other it is opaque. 



991. Now upon what sort of evidence is it that M. MELLONI is universally admitted 

 to have established the physical independence of light and heat 1 Was it not by show- 



