454 Dr. Draper on a new Imponderable Substance, and a 



have been introduced into use have reference to that supposi- 

 tion ; such words as photography, photology, photometer, 

 are derived from this erroneous hypothesis, and lead us to 

 confound together things which ought to be kept essentially 

 distinct. 



As it is the object of this paper, and others which I am 

 shortly to publish, to call the attention of chemists to the ao-ent 

 that is involved in photographic results as a clearly established 

 and new imponderable substance, possessing striking ana- 

 logies with light and heat, yet differing as much from them 

 both as they do from each other, I am induced to propose 

 for it a proper name, and to endeavour to establish for it a 

 nomenclature that shall be free from ambiguity and keep the 

 description of its phsenomena separate from those of light. 

 Whilst therefore I show that it undergoes radiation, reflexion, 

 refraction, polarization, absorption, interference, &c. under the 

 laws to which its radiant companions light and heat are sub- 

 ject, I wish to claim for it a separate and independent ex- 

 istence, to introduce it into the natural family of imponderable 

 agents, with light, heat, and electricity. In that family it stands 

 as the fourth member. Is there any reason that the progress 

 of knowledge should not make known to us multiplied forms 

 of imponderable substances as well as of ponderable matters? 

 This agent differs from light and heat, as much as lead differs 

 from zinc or tin. 



When novel effects, brought about by novel causes, are met 

 with, the purposes of science require new corresponding terms. 

 In the case of the chemical rays of light it is so. I have ex- 

 perienced the need of a nomenclature of the kind from my 

 earliest experiments. It is a rule of which modern philoso- 

 phers know the value, that such names ought to be free from 

 all attending hypothesis ; for if this be not complied with, it 

 soon comes to pass, as knowledge advances, that terms in- 

 volving theoretical ideas lose much of their significance. 



The chemical rays are associated with the rays of light, 

 accompanying them in all their movements, originating with 

 them, and unless disturbed continuing to exist along with 

 them. But should a compound beam like this fall upon a 

 sensitive surface, the chemical rays sink into it, as it were, 

 and lose all their force, and the rays of light are left alone. 

 Photographic results thus resulting from the reposing of the 

 chemical rays on the sensitive surface are not however in 

 themselves durable, as will be shown in this paper, for the 

 rays escape away under some new form. 



Tithonus was a beautifulyouth whom Aurora fell in love 

 with and married in heaven. The Fates hadmadehim immortal, 



