1843] WRITINGS OF JOSEPH HENRY. 207 



To determine whether the emanation obeys the laws of the 

 reflection of light, a piece of common looking-glass was so 

 arranged with the path of the spark, the slits in the screens, 

 and the pan of lime, that the angle of reflection could be 

 compared with the angle of incidence: but with this ar- 

 rangement no impression on the lime could be obtained ; the 

 want of permeability in the glass apparently preventing any 

 reflection from the silvered side of the mirror. A plate 

 of polished black glass was next used, so as to get the reflec- 

 tion from the anterior surface: the result however was of 

 the same negative character as before. It would therefore 

 appear, that glass neither reflects nor transmits the phos- 

 phorogenic emanation, except in a ver}' small degree. 

 When a metallic mirror was employed, a well defined line 

 of light was impressed on the lime from the reflected ema- 

 nation, and from the position of this it was found that the 

 two angles were equal. 



The refraction and dispersion of the emanation were read- 

 ily obtained, by employing for the purpose a prism of rock 

 salt, instead of one of glass. The dispersion was shown by 

 the conversion of the narrow line of light, by means of the 

 prism, into a broad band. 



The next question was in reference to the polarization of 

 the phosphorogenic emanation ; and in obtaining a satisfac- 

 tory answer to this, several difficulties were encountered. 

 Attempts were first made to polarize the beam by passing it 

 through tourmaline; but it was found that this substance 

 is less permeable to the emanation than even glass or mica. 

 Nicol's polarizing prisms were next employed, but no im- 

 pression could be made on the lime through two of them ; 

 and since the emanation is not reflected by glass, and the 

 polarization from polished metal is very feeble, these sub- 

 stances could not be employed in the process. At length 

 an indirect method was adopted which gave positive results. 

 This was founded on an experiment of Melloni, in his in- 

 teresting researches on radiant heat. A pile of exceedingly 

 thin plates of mica, prepared according to the method of 

 Professor Forbes, of Edinburgh, was placed between the spark 



