DARK TITHONIC RAYS. 163 



period of time differing for different bodies. Guided, therefore, by the analogy of heat, 

 I perceive that bodies have a relation to this imponderable agent corresponding to that 

 of specific heat. It follows, therefore, with certainty, that, 



G87. The specific tithonicity of bodies is the prime function on which their sensi- 

 tiveness depends. Under this point of view the sensitiveness is inversely as the spe- 

 cific tithonicity. 



G88. The circumstances under which this experiment is made serve also to show 

 that metallic bodies are non-conductors of tithonicity. 



G89. This contrasts remarkably with their action towards heat. 



690. Having exposed a sensitive plate, a b, to light until it would whiten if mercuri- 

 alized, as before, and having prepared a second (c d,Jig. 102) in total darkness, with- 

 out allowing any light to have access to it, suspend this latter over the former at the 

 distance of one eighth of an inch, so as to cover it about half. Keep the two plates in 

 darkness for several hours, and then mercurialize both. That portion, a c, of the first 

 not covered by the second, will not whiten ; that portion of the second, b d, not cov^ 

 ered by the first, will also remain unchanged ; but both on those parts that have looked 

 towards each other will whiten. 



691. From this I infer, that the portion of the first not overshadowed by the second 

 does not whiten, because its tithonicity escapes away under the form of dark tithonic 

 rays. 



692. I also infer, that as both plates are nearly equally whitened on those portions 

 of their surfaces that have looked towards each other, the dark tithonic rays that have 

 escaped from the first plate, notwithstanding their invisibility, have retained their pecu- 

 liar chemical force, and have affected the second plate. 



693. The analogy with heat is here perfectly observed. A hot, non-conducting plate, 

 set partially opposite a cold one, would warm that plate on the portion looking towards 

 it, and, through the consequent retardation of radiation, would retain its own heat to a 

 certain extent. But all those portions unopposed by the cold plate would cool down, 

 by radiation rapidly. 



694. This experiment proves, in a clear and undoubted manner, the total physical 

 independence of tithonicity and light. 



695. Hence the absolute necessity of some such nomenclature as that proposed ; the 

 chemical rays of light is a misnomer. 



696. On the surface of a sensitive plate that has been suitably exposed, as hereto 

 fore, place a fragment of perfectly clean and colourless glass. Allow it to remain there 

 for four or five hours in a dark room, then mercurialize, and it will be found that the 

 portion on which the glass has been placed will whiten powerfully, but all the rest will 

 remain unchanged. 



697. This, therefore, proves that colourless glass is nearly opaque to the dark tithonic 

 rays, a result observed, also, in the case of the dark rays of heat. 



698. I made a comparative trial of the relative permeability of colourless plate-glass 

 and common writing-paper. A sensitive surface was exposed until it had slightly, but 

 very plainly commenced to turn brown. On one portion I now laid a piece of clear 



