August 17, 1888.] 



SCIENCE. 



79 



he seems to me the man, of all great students of our subject, who, 

 in reference to what he accomplished, made the fewest mistakes. 



Melloni is very great as an experimenter, and owes much of his 

 success to the use of the newly invented thermopile, which is partly 

 his own. I can here, however, speak only of his results, and of 

 but two of these, — one generally known ; the other, and the more 

 important, singularly little known, at least in connection with him. 



The first is the full recognition of the fact, partly anticipated Ijy 

 De la lioche, that radiant heat is of different kinds, that the invis- 

 ible emanations differ among themselves just as those of light do. 

 Melloni not only established the f.ict, but invented a felicitous terpi 

 for it, which did a great deal to stamp it on recognition, — the term 

 ' thermochrose,' or heat-color, which helps us 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. 1 

 translate this important statement as closely as possible from his 

 own words. They are that 



" 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 1S43, and ought, I think, to insure to Melloni the honor of 

 being first to distinctly announce this great principle. 



The announcement passed apparently unnoticed, in spite of his 

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

 ferent 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 ' 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 Encyclopedia ' (edi- 

 tion 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 re-asserted Melloni's 

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

 1S72, 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' 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 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 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 by the recognition 

 that there is one radiant energy which appears to us as ' actinic,' or 

 'luminous,' or 'thermal' radiation, according to the way we ob- 

 serve it. Heat and light, then, are not things in themselves, but 

 whether different sensations in our own bodies, or different effects 

 in other bodies, are merely effects of this mysterious thing we call 

 radiant energy, without doing more in this than give a name to the 

 ignorance which still hangs over the ultimate cause. 



I am coming down dangerously near our own time, — danger- 

 ously for one who would be impartial in dealing with names of 

 those living and with controversies still burning. In such a brief 

 review of this century's study of radiant energy in other forms than 



light, it has been necessary to pass without mention the labors of 

 such men as Pouillot and liecquerel in France, of Tyndall in Eng- 

 land, and of Henry in America. It has been necessary to omit all 

 mention of those who have advanced the knowledge of radiant en- 

 ergy as light, or I should have had to speak of labors so diverse as 

 those of Fraunhofer, of Kirchoff, of Fresnel, of Stokes, of Lockyer, 

 and many more. I have inade no mention, in the instructive his- 

 tory of error, of many celebrated experimental researches ; in par- 

 ticular of such a problem as the measurement of solar heat, great 

 in importance, but apparently most simple in solution, yet which 

 has now been carried on from generation to generation, each ex- 

 perimenter materially altering the result of his predecessor, and 

 where our successors will probably correct our own results in time. 

 I have not spoken of certain purely experimental investigations, like 

 those of Dulong and Petit, which have involved immense and con- 

 scientious labor, and have apparently rightly earned the name of 

 'classic ' from one generation, only to be recognized by the next as 

 leading to wholly untrustworthy results, and leaving the work to be 

 done again with new methods, guided by new principles. 



In these instances, painstaking experiments have proved insuffi- 

 cient, less from want of skill in the investigator than from his ig- 

 norance of principles not established in time to enable him to inter- 

 pret his experiments ; but, if there were opportunity, it would be 

 profitable to show how inexplicably sometimes error flourishes, 

 grows, and maintains an apparently healthy appearance of truth, 

 without having any root whatever. Perhaps I may cite one in- 

 stance of this last from my own experience. 



About fifteen years ago it was generally believed that the earth's 

 atmosphere acted exactly the part of the glass in a hotbed, and that 

 it kept the planet warm by exerting a specially powerful absorption 

 on the infra-red rays. 



I had been trained in the orthodox scientific church, of which I 

 am happy to be stdl a member ; but I had acquired perhaps an al- 

 most undue respect, not only for her dogmas, but for her least say- 

 ings. Accordingly, when my own experiments did not agree with 

 the received statement, I concluded that my experiments must be 

 wrong, and made them all over again, till spring, summer, autumn, 

 and winter had passed, each season giving its own testimony; and 

 this for successive years. The final conclusion was irresistible, that 

 the universal statement of this alleged well-known fact (inexplicable 

 as this might seem, in so simple a matter) was directly contradicted 

 by experiment. 



I had some natural curiosity to find how every one knew this to 

 be a fact ; but search only showed the same statement (that the 

 earth's atinosphere absorbed dark heat like glass) repeated every- 

 where, with absolutely nowhere any observation or evidence what- 

 ever to prove it, but each writer quoting from an earlier one, till I 

 was almost ready to believe it a dogma superior to reason, and rest- 

 ing on the well-known " Quod semper, quod ubique, quod ab omni- 

 bus, creditiim est." 



Finally I appear to have found its source in the writings of 

 Fourier, who, alluding to De Saussure's experiments (which showed 

 that dark heat passed with comparative difficulty through glass), 

 observes that if the earth's atmosphere were solid, it would act as 

 the glass does. Fourier simply takes this (in which he is wholly 

 wrong) for granted ; but, as he is an authority on the tbeory of 

 heat, his words are repeated without criticism, first by Poisson, then 

 by others, and then in the text-books ; and, the statement gaining 

 weight by age. it comes to be believed absolutely, on no evidence 

 whatever, for the next sixty years, that our atmosphere is a power- 

 ful absorber of precisely those rays which it most freely transmits. 



The question of fact here, though important. Is, I think, quite 

 secondary to the query it raises as to the possible unsuspected in- 

 fluence of mere tradition in science, when we do not recognUe it as 

 such. Now, the Roman Church is doubtless quite logical in believ- 

 ing in tradition, If these are recommended to the faithful by an in- 

 fallible guide ; but are we, who have no infallible guide, quite safe 

 in believing all we do. with our fond persuasion that in the scientific 

 body mere tradition has no weight ? 



In even this brief sketch of the growth of the doctrine of radiant 

 energy, we have perhaps seen that the history of the progress of 

 this department of science is little else than a chapter in that larger 

 hisior)' of human error which is still to be written and which, it is 



