MEMOIR XXIIL] KAYS OF INCANDESCENT LIME, ETC. 321 



rays, appears to be as transparent to them as quartz or 

 atmospheric air. 



A screen of glass is opaque to the phosphorogenic 

 rays of an electric spark, but it is quite transparent to 

 those of incandescent lime. 



It might be supposed that the very brief duration of 

 an electric spark has something to do with this phenom- 

 enon, but the voltaic arc passing between charcoal points 

 gives the same results. I caused its rays to impinge on 

 a plate for thirty seconds, and observed the obstructing 

 effect of glass in a very satisfactory manner. That a 

 certain portion of the rays passes through may be 

 shown by continuing the light for a few minutes, when 

 the phosphorus will begin to shine under the glass. 



I came to the conclusion, also, that the transient dura- 

 tion of the light had nothing to do with the phenomenon, 

 because the lime light occasions phosphorescence through 

 glass in the space of a single second, but in that time 

 the rays from a voltaic arc could not traverse a piece of 

 glass so as to produce a sensible effect ; the phosphorus 

 beneath it appearing quite dark, and yet this light is in- 

 comparably brighter than the lime light. 



The blue light emitted when a platinum wire in con- 

 nection with one pole of the voltaic battery is brought 

 down upon some mercury in connection with the other, 

 and the green light obtained when the copper wires are 

 the medium of discharge, appear to produce the same 

 effect as charcoal points. 



It is therefore neither the color nor the duration of 

 the light that determines this result. It seems to depend 

 on a peculiarity of the electric discharge. 



Some time ago I determined the refrangibility of the 

 rays of an electric spark which excite phosphorescence 

 in lime sulphide ; they are found at the violet extremity 

 of the spectrum. I have made attempts to ascertain the 



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