322 KAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII. 



position of the active rays of incandescent lime. They 

 cannot pass through the blue solution of ammonio-sul- 

 phate of copper, but through the red solution of sulpho- 

 cyanide of iron, and also through a strong solution of 

 bichromate of potash, they pass ; in the latter case al- 

 most as copiously as through atmospheric air. 



The phosphorogenic rays of an electric spark are in 

 the violet space, but those of incandescent lime are at 

 the other extremity of the spectrum. 



An Argand lamp, when made to burn very brightly, 

 emits phosphorogenic rays which traverse glass. As 

 has been proved long ago, the sun-rays possess the same 

 property. 



Thus, therefore, the rays of incandescent lime, of an oil- 

 lamp, and of the sun can excite phosphorescence through 

 glass, and differ from those of an electric spark or voltaic 

 discharge, in which that peculiarity is deficient. 



I have also remarked some curious cases of spectral 

 appearances. They are analogous to those instances to 

 which I first drew public attention in 1840, and which 

 at a later date M. Moser brought before the British As- 

 sociation. They are interesting as affording an ocular 

 proof of secondary radiation. The following experiments 

 may serve as illustrations : 



Place a key or any other opaque object before a sensi- 

 tive phosphorescent surface, and having made that sur- 

 face glow intensely by a voltaic discharge between char- 

 coal points continued for two or three minutes, on re- 

 moving the key an image of it will of course be seen. 

 This image in a short time will disappear. Then shut 

 the plate in a dark place, where no light can have access 

 to it in the daytime. If in a day or two the surface be 

 carefully inspected in the dark, no trace of anything will 

 be visible upon it ; but if it be laid on a piece of warm 

 iron, a spectral image of the key is suddenly evolved. 



