104 WRITINGS OF JOSEPH HENRY. [1855- 



Again; it is well known that if we expose a diamond for 

 an instant to the rays of the sun, and then convey it to 

 a dark place, we shall see it glow with a pale phosphorescent 

 light ; but this effect, long familiar as it has been to the 

 natural philosopher, is now known to be the result of an 

 emanation differing in some essential particulars from all 

 the other emanations which we have mentioned. To prove 

 this, it is sufficient to place the diamond under a plate of 

 transparent mica, a substance which transmits freely light, 

 heat, and the chemical emanation. This will screen the 

 diamond ; and the glowing, which was before very striking, 

 will not now be produced. That this effect is not the re- 

 sult of the absorption of a ray of light will be evident when 

 we mention the fact that a diamond will glow when placed 

 under a thick plate of smoky quartz, which intercepts both 

 light and chemical emanation, but freely transmits what 

 is denominated the phosphorogenic ray. These results are 

 all in accordance, in a general way, with the constitution of 

 the aetherial medium which we have presented. 



Light and heat appear to differ only in the lengths of the 

 waves, which become shorter and more intense as the tem- 

 perature of the source of emanation increases ; though in 

 some cases, as in that of luminous phosphorus and the light 

 of the glow worm, it is emitted freely from bodies of low tem- 

 perature. It is possible that light from these different sources 

 may possess different physical properties. 



Electricity. — The phenomena of light, of heat, of the chem- 

 ical and phosphorogenic emanations have all been referred 

 to vibrations of the setherial medium, and all the facts which 

 have thus far been observed are in accordance with this gen- 

 eralization. The question however naturally arises as to 

 what explanation we can give of the multiplied and various 

 phenomena constantly presenting themselves to us in con- 

 nection with the changes which are taking place around 

 us in nature, or which exhibit themselves to the chemist 

 and phj'^sicist in their investigations of the minuter re-ac- 

 tions which are brought about by their agency, and which 

 are classed under the general name of electricity. It is a 



