THE HISTORY OF A DOCTRINE. 391 



helps lis to remember that, as the visible and apparently simple 

 emanation of light is found to have its colors, so radiant heat, 

 the invisible but apparently simple emanation, has what would 

 be colors to an eye that could see them. This result is well known 

 in connection with Melloni. 



The other and the greater, which is not generally known as 

 Melloni's, is the generalization that heat and light are effects of 

 one and the same thing, and merely different manifestations of it. 

 I translate this important statement as closely as possible from 

 his own words. They are that " Liglit is merely a series of calo- 

 rific indications sensible to the organs of sight, or Vice Versa, the 

 radiations of obscure heat are veritable invisible radiations of 

 light." The italics and the capitals are Melloni's own. He wishes 

 to have no ambiguity about his announcement behind which he 

 may take shelter ; and he had so firm a grasp of the great prin- 

 ciple that, when his first attempts to observe the heat of the moon 

 failed, he persevered, because this principle assured him that 

 where there was light there must be heat. This statement was 

 made in 1843, and ought, I think, to insure to Melloni the honor 

 of being the first to distinctly announce this great principle. The 

 announcement passed apparently unnoticed, in spite of his ac- 

 knowledged authority ; and the general belief not merely in dif- 

 ferent entities in the spectrum, but in a material caloric, con- 

 tinued as strong as ever. If you want to see what a hold on life 

 error has, and how hard it dies, turn to the article " Heat," in the 

 eighth edition of the " Encyclopsedia Britannica," where you will 

 find the old doctrine of caloric still in possession of the field in 

 1853 ; and still later, in the generally excellent " English Ency- 

 clopaedia " (edition of 1867), the doctrine of caloric is, on the whole, 

 preferred to the undulatory hypothesis. It is very probable that 

 a searcher might find many traces of it yet lingering among us ; 

 so that Giant Caloric is not, perhaps, even yet quite dead, though 

 certainly grown so crazy, and stiff in the joints, that he can now 

 harm pilgrims no more. 



So far as I know, no physicist of eminence reasserted Melloni's 

 principle till J. W. Draper, in 1872. Only sixteen years ago, or in 

 1872, it was almost universally believed that there were three dif- 

 ferent entities in the spectrum, represented by actinic, luminous, 

 and thermal rays. Draper remarks that a ray consists solely of 

 ethereal vibrations whose lost vis viva may produce either heat 

 or chemical change. He uses Descartes's analogy of the vibration 

 of the air, and sound ; but he makes no mention either of Des- 

 cartes or of Melloni, and speaks of the principle as leading to a 

 modification of views then " universally " held. Since that time 

 the theory has made such rapid progress that, though some of the 

 older men in England and on the European continent have not 



