70 EXAMINATION OF THE THEORY OF M. BECQUEREL. 



nomena which may be explained on the supposition that the invisible tithonic rays es 

 cape by radiation from bodies which have been impressed by them, those bodies simul- 

 taneously reverting to their original condition. These results, and similar ones, have 

 of late years attracted much of the attention of experimental philosophers, but the in- 

 quiries involved are beset by numerous difficulties (Ap., 708, &c.). There is not, how- 

 ever, anything impossible, or even unlikely, in this secondary radiation. A phenome- 

 non of exactly the same kind is visible to the eye in the case of the phosphorescent rays, 

 when the glow of light by radiation escapes away from calcined oyster shells after 

 they have been illuminated by an electric spark. 



256. Dark, invisible rays thus exist in the sunlight, and carry on a variety of func- 

 tions, and control a variety of phenomena. Of solar principles, four different kinds 

 have been traced : rays of light, of heat, tithonic, and phosphorescent rays. The two 

 former are admitted to constitute recognised imponderable principles. What are the 

 latter two ? 



257. In the Philosophical Magazine (see AP., CH. XIII.) I have some years ago 

 brought forward the doctrine, that we are compelled to enlarge our catalogue of im- 

 ponderable principles, and include these tithonic rays in it. More recently I have of- 

 fered similar arguments in favour of the phosphorescent rays (Ap., CH. XVIIL). 



258. This brings me to offer some remarks on the opinion expressed by M. BEC- 

 QUEREL, that the phenomena now under discussion are due to the qualities of the re- 

 ceiving surfaces, and not to agents intrinsically different, coexisting in the solar beam. 

 That the same beam of light, falling on sulphuret of lime, causes it to phosphoresce ; 

 on chloride of silver, blackens it ; on the retina, gives rise to the phenomena of vision 

 and colour ; on a piece of black cloth, causes it to become warm. This opinion seems 

 to be surrounded with insurmountable difficulties, and, if admitted, would disturb some 

 of the best-established truths of science. 



259. No one can feel more strongly the absurdity of supposing that Nature has cre- 

 ated between forty and fifty elementary ponderable substances, all possessed of metallic 

 characters, and all so nearly alike that even a chemist is often puzzled to distinguish 

 them from one another. No one, upon satisfactory proof, would more willingly go 

 back to the alchemical doctrine in relation to these matters ; but so long as the evi- 

 dence on the constitution of these bodies rests where it does, the laws of chemistry 

 compel us to admit them to be simple and undecompounded. And, just in the same way 

 that I am willing to admit the existence of forty different simple metals, so, upon sim- 

 ilar evidence, I am free to admit the existence of fifty different imponderable agents, if 

 need be. Is there anything which should lead us to suppose that the imponderables 

 are constituted by Nature on a plan that is elaborately simple, and the ponderables on 

 one that is elaborately complex ? That the former are all modifications of one primor- 

 dial ether, and the latter intrinsically different bodies, more than a quarter of a hun- 

 dred of which have been discovered during the present century 1 



260. Before admitting the correctness of the hypothesis of M. BECQUEREL, that the 

 agent under consideration is one and indivisible, and that all the phenomena we dis- 

 cover are due to the receiving surfaces, and that there are as many spectra as there are 



