MEMOIR XXIL] THE ACTION OF GLASS AND QUARTZ. 



it, feeble, it is true, but nevertheless 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 re- 

 quired to examine the effects of the chemical rays and 

 phosphorogeuic emanations under circumstances which 

 are precisely similar as to intensity and time. 



For the transient rays of an electric spark, quartz is 

 transparent and glass is nearly opaque. Having pre- 

 pared a bromo-iodized silver plate so as to be exceeding- 

 ly sensitive, I set in front of it at the distance of about one 

 third of an inch a disk 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 Ley den-jar fifteen times ; the dis- 

 tance between this spark and the sensitive plate was 

 about two inches. On mercurializing this plate, it 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, so also were 

 the edges of the glass and the quartz. The two disks 

 overlapped one another to a certain extent, but the cor- 

 responding portion of the silver plate was as deeply 

 stained there as anywhere else. 



Next I put a surface of lime sulphide in the place of 

 the daguerreotype plate, everything else remaining as be- 

 fore. On passing fifteen sparks the lime phosphoresced 

 powerfully under the quartz, but not under the glass, so 

 that the difference between its shadow and that of the 

 spot of wax could not be distinctly seen. 



For these reasons, therefore, I adopt the view ex- 

 pressed by Professor Henry, that the phosphorogenic 

 emanation and the chemical rays are distinct. Under 

 the same circumstances glass is transparent to the one, 

 to the other it is opaque. 



