Class of Chemical Rays analogous to theRays of Dark Heat. 459 



ways, transiently and permanently, exactly after the manner 

 of heat. 



The same result is obtained when other sensitive surfaces 

 are employed, the period of time differing for different bodies. 

 Guided, therefore, by the analogy of heat, I perceive that bo- 

 dies have a relation to this imponderable agent corresponding 

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

 The specific tithonicity of bodies is the prime function on 

 which their sensitiveness depends. Under this point of view 

 the sensitiveness is inversely as the specific tithonicity. 



The circumstances under which this experiment is made 

 serve also to show that metallic bodies are non-conductors of 

 tithonicity. 



This contrasts remarkably with their action towards heat. 

 Having exposed a sensitive plate a b to light until it would 

 whiten if mercurialized, as before ; and having prepared a se- 

 cond, c d (fig. 2), in total darkness, without 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 covered by the first, will also remain unchanged ; but both 

 on those parts that have looked towards each other will whiten. 

 From this I infer, that the portion of the first not over- 

 shadowed by the second does not whiten because its tithoni- 

 city escapes away under the form of dark tithonic rays. 



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 

 peculiar chemical force, and have affected the second plate. 



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. 



This experiment proves in a clear and undoubted manner 

 the total physical independence of tithonicity and light. 



Hence the absolute necessity of some such nomenclature 

 as that proposed, — the chemical rays of light is a misnomer. 

 On the surface of a sensitive plate that has been suitably 

 exposed, as heretofore, 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 



