320 RAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII. 



MEMOIR XXIII. 



ON A REMARKABLE DIFFERENCE BETWEEN THE RAYS OF 



INCANDESCENT LIME AND THOSE EMITTED BY AN 



ELECTRIC SPARK. 



From the Philosophical Magazine, December, 1845. 



CONTENTS : Non-permeability of glass to spark radiations. Permeabil- 

 ity to calcium-light radiations. Different refrangibility of the spark 

 and the calcium -light radiations. Shadows imbedded in Canton's 

 phosphorus. Evolution of old shadows in an order of succession. 



SOME years ago M. Becquerel discovered that the rays 

 of an electric spark, if transmitted through a screen of 

 glass, could not excite the phosphorescence of lime sul- 

 phide. 



To make this experiment, wash a metallic plate over 

 with gum-water. Dust upon it from a fine sieve a quan- 

 tity of Canton's phosphorus (oyster-shells calcined with 

 sulphur), and allow the plate to dry. A uniform surface 

 is thus obtained suitable for these purposes. Place be- 

 fore that surface a piece of glass and a piece of polished 

 quartz, and discharge a Leyden-jar a few inches off, so 

 that the rays of its spark may fall on the plate. It will 

 be found that under the quartz the phosphorus will 

 shine as much as on the spaces that have not been cov- 

 ered, but under the glass it will remain almost entirely 

 dark. 



Last winter I observed the curious fact that when this 

 experiment is made with a piece of lime incandescing in 

 a stream of oxygen directed through the flame of a spirit- 

 lamp, the glass, so far from being unable to transmit the 



