4:36 Prof. Draper on the Rays of Incandescent. Lime. 



•i; A screen of glass is opake to the phosphoric rays of an 

 electric spark, bttfeisrquile tai^i^^aeeqtto those of incandescent 



It might be supposed that the very brief duration of an 

 electric spark has something to do with the phaenomenon; but 

 the voltaic arc passing between charcoal points gives the 

 same results. I caused its rays to impinge on a plate fon 

 thirty seconds, and observed the obstructing effect of glass ij^j 

 a very satisfactory manner. That a certain portion of the rays 

 does pass through, may be shown by continuing the light for 

 a few minutes, when the phosphorus will begin to shine und^i 

 the glass. 



I came to the conclusion, also, that the transient duration 

 of the light had nothing to do with the phaenomenon, because 

 the lime light occasions phosphorescence in 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 sen- 

 sible effect, the phosphorus beneath it appearing quite dark ; 

 and yet this light is incomparably brighter than the lime 

 light. I ,j'>jj: ' -rn 



The blue light which is emitted when a platina wire, in 

 connexion with one pole of the voltaic battery, is brought 

 down upon some mercury in connexion with the other, and 

 the green light which is obtained when two copper wires are 

 the medium of discharge, appear to produce the same effect 

 as charcoal points. 



It is therefore neither the colour nor the duration of the 

 light that determines this result. It seems to depend on a, 

 peculiarity of the electric discharge. j ; 



Some time ago I determined the refrangibility of the rays 

 of an electric spark which excite phosphorescence in sulphuret 

 of lime; they are found at the violet extremity of the spec- 

 trum. I have made attempts to ascertain the position of the 

 active rays of incandescent lime. They cannot pass through 

 the blue solution of ammonio-sulphate of copper ; but through 

 the red solution of sulphocyanide of iron, and also a strong 

 solution of bichromate of potash, they pass; in the latter case 

 almost as copiously as through atmospheric air. 



The phosphoric rays of an electric spark are in the violet 

 space, but those of incandescent lime are at the other extre- 

 mity of the spectrum. 



An Argand lamp, when made to burn very brightly, emits 

 phosphoric 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 



