18 S. P. Langley — The History of a Doct/rme. 



" Light is merely a series of calorific 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 

 principle, 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 first to thus distinctly announce 

 this great generalization. 



The announcement passed apparently unnoticed, in spite of 

 his acknowledged authority ; and the general belief not merely 

 in different entities in the spectrum, but in a material caloric, 

 continued 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 " Encyclopaedia Britan- 

 nica," 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 Encyclopgedia " (edition of 1867), the doc- 

 trine 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 Mel- 

 loni's principle with equal emphasis, till J. W. Draper, in 1872. 

 Only sixteen years ago, or in 1872, it was almost universally 

 believed that there were three different entities in the spec- 

 trum, 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' analogy of the vibration of the air, and 

 sound ; but he makes no mention either of Descartes or of 

 Melloni, and speaks of the principle as leading to a modifica- 

 tion 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 

 welcomed it, its adoption among all physicists of note may be 

 said to be now universal, and a new era in our history begins 

 with it. I mean with the recognition that there is one radiant 

 energy which appears to us as "actinic," or " luminous " or 

 "thermal" radiation, according to the way we observe it. 

 Heat and light, then, are not things in themselves, but whether 

 different sensations in our own bodies, or different effects in 



