78 



SCIENCE. 



[Vol. XII. No. 2i 



In spite of such criticism as this, the undulatory hypothesis of 

 light made rapid way, and carried with it, one would now say, the 

 necessary inference that radiant heat was due to undulations also. 

 This was, however, no legitimate inference to those to whom radi- 

 ant heat was still a fluid ; and yet, in spite of all, the modern doc- 

 trine now begins to make visible progress. 



A marked step is taken about i8n by a young Frenchman, De 

 la Roche, who deserves to be better remembered than he is, for he 

 clearly anticipated some of Melloni's discoveries. De la Roche in 

 particular shows that of two successive screens the second absorbs 

 heat in a less ratio than the first ; whence he, before any one else, 1 

 believe, derives the just and most important, as well as the then 

 most novel conception, that radiant heat is of different kinds. He 

 sees also, that, as a body is heated more and more, there is a grad- 

 ual and continual advance not only in the amount of heat it sends out, 

 but in the kind, so that, as the temperature still rises, the radiant 

 heat becomes light by imperceptible gradations ; and he concludes 

 that heat and light are due to one simple agent, which, as the tem- 

 perature rises yet more, appears more and more as light, or which, 

 as the luminous radiation is absorbed, re-appears as heat. Very 

 little of it, he observes, passes even transparent screens at low tem- 

 peratures, but more and more does so as the temperature rises. 



All this is a truism in 1888, but it is admirably new as well as 

 true in 1811 ; and if De la Roche had not been removed by an 

 early death, his would have not improbably been the greatest name 

 of the century in the history of our subject ; an honor, however, 

 which was in fact reserved for another. 



The idea of the identity of light and radiant heat had by this time 

 made such progress that the attempt to polarize the latter was 

 made in 1818 by Berard. We have just seen in Herschel's case 

 how the most sound experiment may lead to a wrong conclusion, if 

 it controverts the popular view. We now have the converse of this 

 in the fact that the zeal of those who are really in the right way 

 may lead to unsound and inconclusive experiment ; for Berard ex- 

 perimentally established, as it was supposed, the fact that obscure 

 radiant heat can be polarized. So it can, but not with such means 

 as Berard possessed, and it was not till a dozen years more that 

 Forbes actually proved it. 



At this time, however fairly we seem embarked on the paths 

 of study which are followed to-day, and while the movement of the 

 main body of workers is in the right direction, it is yet instructive 

 to observe how eminent men are still spending great and conscien- 

 tious labor, their object in which is to advance the cause, while the 

 effect of it is to undo the little which has been rightly done, and to 

 mislead those who have begun to go right. 



As an instance both of this and of the superiority of modern ap- 

 paratus, we may remark, — after having noticed that the ability of 

 obscure heat to pass through glass, if completely established, would 

 be a strong argument in favor of its kinship to light, and that De 

 la Roche and others had indicated that it would do so (in which 

 we now know they were right), — that at this stage, or about 1816, 

 Sir David Brewster, the eminent physicist, made a series of experi- 

 ments which showed that it would not so pass. Ten years later, 

 in view of the importance of the theoretical conclusion, Baden Pow- 

 ell repeated his observations with great care, and confirmed them, 

 announcing that the earlier experimenters were wrong, and that 

 Brewster was right. 



Here all these years of conscientious work resulted in establish- 

 ing, so far as it could be established, a wholly wrong conclusion in 

 place of a right one already gained. It may be added, that, with our 

 present apparatus, the passage of obscure radiant heat through glass 

 could be made convincingly evident in an experiment which need 

 not last a single second. 



We are now arrived at a time when the modern era begins ; and 

 in looking back over one hundred and fifty years, from the point of 

 view of the experimenter himself, with his own statement of the 

 truth as he saw it, we find that the comparison of the progress of 

 science to that of an army, which moves, perhaps with the loss of 

 occasional men, but on the whole victoriously and in one direction, 

 is singularly misleading ; and I state this more confidently here, 

 because there are many in this audience who did not get their 

 knowledge of nature from books only, but who have searched for 

 the truth themselves ; and, speaking to them, may I not say that 



those who have so searched know that the most honest purpose 

 and the most patient striving have not been guaranties against mis- 

 takes, — mistakes which were probably hailed at the time as suc- 

 cesses .' It was some one of the fraternity of seekers, I am sure, 

 who said, " Show me the investigator who has never made a mis- 

 take, and I will show you one who has never made a discovery." 



We have seen the whole scientific body, as regards this particular 

 science of radiant energy, moving in a mass, in a wrong direction^ 

 for a century ; we have seen that individuals in it go on their inde- 

 pendent paths of error ; and we can only wonder that an era should 

 have come in which such a real advance is made as in ours. 



That era has been brought in by the works of many, but more 

 than by any other through the fact that in the year 180 1 there came 

 into the world at Parma an infant who was born a physicist, as 

 another is born a poet ; nay, more ; who was born, one might say, 

 a devotee of one department of physics, — that of radiant heat ; 

 being affected in his tenderest years with such a kind of precocious 

 passion for the subject as the childish Mozart showed for music. 

 He was ready to sacrifice every thing for it ; he struggled through 

 untold difficulties, not for the sake of glory or worldly profit, but 

 for radiant heat's sake ; and when fame finally came to him, and 

 he had the right to speak of himself, he wrote a preface to his col- 

 lected researches, which is as remarkable as any thing in his works. 

 In this preface he has given us, not a summary of previous memoirs 

 on the subject, not a table of useful factors and formula, not any 

 thing at all that an English or American scientific treatise usually 

 begins with, but the ingenuous story of his first love, of his boyish 

 passion for this beloved mistress ; and all this with a trust in us his 

 readers which is beautiful in its childlike confidence in our sym- 

 pathy. 



I must abbreviate and injure in order to quote ; but did ever a. 

 learned physical treatise and collection of useful tables begin like 

 this before } 



" I was born at Parma, and when I got a holiday used to go in- 

 to the country the night before and go to bed early, so as to get up- 

 before the dawn. Then I used to steal silently out of the house, 

 and run, with bounding heart, till I got to the top of a little hill, 

 where I used to set myself so as to look toward the East." There, 

 he tells us, he used, in the stillness of nature, to wait the rising sun, 

 and feel his attention rapt, less with the glorious spectacle of the 

 morning light itself than with the sense of the mysterious heat 

 which accompanied its beams, and brought something more neces- 

 sary to our life and that of all nature than the light itself. 



The idea that not only mankind, but nature, would perish though 

 the light continued, if this was divorced from heat, made a pro- 

 found impression, he tells us, on his childish mind. 



The statement that such an idea could enter with dominating 

 force into the mind of a child will perhaps seem improbable to 

 most. It will, however, be comprehensible enough to some here, I 

 have no doubt. 



Is there some ornithologist present who remembers a quite in- 

 fantile attraction which birds possessed for him above all the rest 

 of the animated creation ; some chemist whose earliest recollec- 

 tions are of the strange and quite abnormal interest he found as a 

 child in making experimental mixtures of every kind of accessible 

 household fluid and solid ; some astronomer who remembers when 

 a very little creature that not only the sight of the stars, but of any 

 work on astronomy, even if utterly beyond his childish comprehen- 

 sion, had an incomprehensible attraction for him .' 



I will not add to the list. There are, at any rate, many here who 

 will understand and believe JVIelloni when he tells how this radiant 

 heat, commonplace to others, was wonderful to his childish thought, 

 and wrought a charm on it such that he could not see wood burn 

 in a fireplace, or look at a hot stove, without its drawing his mind, 

 not to the fire or iron itself, but to the mysterious effluence which it 

 sent. 



This was the youth of genius ; but let not any fancy that genius in 

 research is to be argued from such premonitions alone, unless it can 

 add to them that other qualification of genius which has caused it. 

 to be named the faculty of taking infinite pains. Melloni's subse- 

 quent labors justified this last definition also ; but I cannot speak 

 of them here, further than to say, that after going over a large part 

 of his work myself, with modern methods and with better apparatus , 



