NEW NOMENCLATIVE HiOPOSED. 159 



property was mainly due to the violet rays. SEEBECK observed that chloride of silver, 

 exposed to the spectrum, varied its colour with the colour of the space in which it was 

 held, and during the present century a very large amount of new observations has been 

 accumulated. A new art, Photography, has come into existence. 



656. The general supposition that obtains is, that the effects in question are due to 

 the rays of LIGHT ; hence all the words that have been introduced into use have refer- 

 ence to that supposition; 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 



657. As it is the object of this paper, and others which I shall shortly publish, to call 

 the attention of chemists to the agent that is involved in photographic results as a 

 clearly-established and new imponderable substance, possessing striking analogies 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 nomen- 

 clature that shall be free from ambiguity, and keep the description of its phenomena 

 separate from those of LIGHT. While, 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 subject, I wish to claim for it a separate and 

 independent existence, 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 multi- 

 plied 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. 



658. 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 experienced the need of a nomenclature of the kind from my earliest 

 experiments. It is a rule of which modern philosophers 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 involving theoretical ideas 

 lose much of their significance. 



659. 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 

 ravs 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. 



660. " Tithonus was a beautiful youth whom Aurora fell in love with and married 

 in heaven. The fates had made him immortal; but, unlike his bride, in the course of 

 events he became feeble and decrepit, and, losing all his strength, was rocked to sleep 

 in a cradle. The goddess, pitying his condition, metamorphosed him into a grasshop- 

 per." Mythological Dictionary. 



661. The fact and the fable agree pretty well; and, indeed, the playful coincidence 



