114< Dr. Draper's Further considerations 



For the transient rays of an electric spark, quartz is trans- 

 parent and glass is nearly opake. 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 be- 

 tween a pair of copper wires, the interval of which was three- 

 eighths of an inch, I passed the spark of a Leyden phial fifteen 

 times; the distance between this spark and the sensitive plate 

 was about two inches. On mercurializing, the plale was 

 deeply whitened all over, equally so through the glass, through 

 the quartz, and on the uncovered spaces ; but a spot of seal- 

 ing-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 another to a 

 certain extent, but the corresponding portion of the silver plate 

 was as deeply stained there as anywhere else. 



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 phosphoresced powerfully 

 under the quartz, but not under the glass, so that the differ- 

 ence between its shadow and that of the spot of wax could 

 not be distinctly seen. 



For these reasons therefore I adopt the view expressed by 

 Prof. Henry, that the phosphorogenic emanation and the ti- 

 thonic rays are distinct. Under the same circumstances, glass 

 to the one is transparent, to the other it is opake. 



Now upon what sort of evidence is it that M. Melloni is 

 universally admitted to have established the physical inde- 

 pendence of light and heat? was it not by showing that rock- 

 salt is perfectly transparent to calorific rays, that glass is much 

 less so, that Rochelle salt, alum, and sulphate of copper are 

 almost opake ? It is surely impossible to confound the phos- 

 phorogenic emanation of an electric spark with its rays of 

 light; the latter pass perfectly through glass, the former do 

 not. So far as the eye can distinguish, an electric spark, the 

 rays of which have passed through glass, differs in no respect 

 from one the rays of which are received directly into the eye. 

 If we consider the constitution of such a ray, previous to and 

 after its passage through glass, the eye can discover no differ- 

 ence; but, as respects the phosphorogenic emanation, there 

 was something existing in that ray at the first of these epochs 

 which had ceased to exist in it at the second ; a something 

 not having the quality of communicating any impression to 

 the organ of vision ; and that which we cannot see, surely no 

 man will acknowledge to be light. 



But the reader may inquire, what has all this discussion of 



