208 WRITINGS OF JOSEPH HENRY. [1843 



and the pan containing the lime, with its plane at right 

 angles to the line joining the middle of the two. In this 

 position of the pile, no impression was made on the lime by 

 the electrical discharge; but when the plane of the pile was 

 inclined to the line just mentioned, so as to form with it the 

 polarizing angle, a luminous spot was excited. 



By this change of the position of the pile, the thickness of 

 the path to be traversed by the phosphorogenic beam was 

 considerably lengthened ; and yet the permeability was much 

 increased. This remarkable result could only be the effect 

 of the successive polarization of the several parts of the beam 

 as they passed the several films of mica, and were thus pre- 

 pared for a more ready transmission by the succeeding films. 



After the emanation was found to be polarizable, it was 

 important to determine if the intensity of the action on the 

 lime would be different, in case the beam were transmitted 

 through crystals in different directions in reference to their 

 optical axis, but no difference could be observed, when the 

 beam was passed through crystals of carbonate of lime, and 

 of quartz — parallel, and perpendicular to the axis. 



From the foregoing results it is evident that the exciting 

 cause of the phosphorescence of the sulphuret of lime is an 

 emanation possessing the mechanical properties of light, and 

 yet so different in other respects as to prove the want of 

 identity. That the same emanation also differs from heat is 

 manifest from the fact that the lime becomes as luminous 

 under a plate of alum as under a plate of rock salt, although 

 these substances are almost entirely opposite in their prop- 

 erty of transmitting heat. 



Some experiments were also made to compare the phos- 

 phorogenic emanation with the chemical radiation. For 

 tliis purpose a sensitive daguerreotype plate and a pan of 

 sulphuret of lime were exposed together to the light of the 

 sky for five seconds. The plate by this exposure was marked 

 with a photograpic impression, but little or no effect was 

 produced on the lime. Another Sensitive plate and the same 

 pan of lime were similarly exposed to the light of an elec- 

 trical discharge; the lime was now observed to glow, while 



