76 



SCIENCE. 



[Vol. XII. No. 289 



us, but is not perhaps unaccountable in view of the then current 

 thought. 



I have said that the progress of science is not so much that of an 

 army as of a crowd of searchers, and that a call in a false direction 

 may be responded to, not by one only, but by the whole body. In 

 illustration, observe that during the greater part of the entire eigh- 

 teenth century this doctrine was adopted by almost every chemist 

 and by most physicists. It had quite as general an acceptance among 

 scientific men then as the kinetic theory of gases, for instance, has 

 now, and, so far as time is any test of truth, it was tested more 

 severely than the kinetic theory has yet been ; for it was not only the 

 lamp and guide of chemists, and to a great extent of physicists 

 also, but it remained the time-honored and highest generalization 

 of chemico-physical science for over half a century, and it was 

 accepted not so much as a conditional hypothesis as a final guide 

 and a conquest for truth which should endure always. And now 

 where is it ? Dissipated so utterly from men's minds, that, to the un- 

 professional part of even an educated audience like this, ' phlogiston,' 

 once a name to conjure with, has become an unmeaning sound. 



There is no need to insist on the application of the obvious moral 

 to hypotheses of our own day. I have tried to recall for a moment 

 all that ' phlogiston ' meant a little more than a hundred years ago, 

 partly because it seems to me, that, though a chemical conception, 

 physics is not wholly blameless for it, but chiefly because before it 

 quitted the world it appears to have returned to physics the wrong 

 in a multiplied form by generating an offspring specially inimical to 

 true ideas about radiant heat, and which is represented by a yet 

 familiar term. I mean ' caloric' 



This word is still used loosely as a synonvme for heat, but has 

 quite ceased to be the very definite and technical term it once was. 

 To me it has been new to find that this so familiar word ' caloric,' 

 so far as my limited search has gone, was apparently coined only 

 toward the last quarter of the last century. It is not to be found in 

 the earliest edition of Johnston's Dictionary, and, as far as I can 

 learn, appears first in the corresponding French form in the works 

 of Fourcroy. It expressed an idea which was the natural sequence 

 of the phlogiston theory, and which is another illustration that the 

 evil which such theories do lives after them. 



' Caloric ' first seemingly appears, then, as a new word coined by 

 the French chemists, and meant originally to signify the unknown 

 cause of the sensation heat, without any implication as to its nature. 

 But words, we know, though but wise men's counters, are the 

 money of fools ; and this one very soon came to commit its users 

 to an idea which was more likely to have had its origin in the mind 

 of a chemist at that time than of any other, — the idea of the cause 

 of heat as a material ingredient of the hot body ; something not, it 

 is true, having weight, but which it would have been only a slight 

 extension of the conception to think might one day be isolated by a 

 higher chemical art, and exhibited in a tangible form. 



We may desire to recognize the perverted truth which usually 

 underlies error, and gives it currency, and be willing to believe that 

 even ' caloric ' may have had some justification for its existence ; 

 but this error certainly seems to have been almost altogether per- 

 nicious for nearly the next eighty years, and down even to our own 

 time. With this conception as a guide to the philosophers of the 

 last years of the eighteenth century, it is not, at any rate, surprising 

 if we find that at the end of a hundred years from Newton the 

 crowd seems to be still going constantly farther and farther away 

 from its true goal. 



Although Provost gave us his most material contribution about 

 1790. we have, it seems to me, on the whole, little to interest us dur- 

 ing that barren time in the history of radiant energy called the eigh- 

 teenth century, — a century whose latter years are given up, till near 

 Us very close, to bad a priori theories in our subject, except in the 

 work of two Americans ; for in the general dearth at this time, of 

 experiments in radiant heat, it is a pleasure to fancy Benjamin 

 Franklin sitting down before the fire, with a white stocking on one 

 leg and a black one on the other, to see which leg would burn first, 

 and to recall again how Benjamin Thompson (Count Rumford) 

 not only weighed ' caloric ' literally in the balance and found it want- 

 ing, but made that memorable experiment in the Munich foun- 

 deries which showed that heat was perpetually and without limit 

 created fiom motion. 



It was in the last years of the century, too, that he provided for 

 the medal called by his name, and which, though to be given for 

 researches in heat and light, has, I believe, been allotted in nearly 

 every instance to men, who, like Leslie, Mains, Davy, Brewster, 

 Fresnel, Melloni. Faraday, Arago, Stokes, Maxwell, and Tyndall, 

 have contributed toward the subject of radiant energy in particular. 



We observe that till Rumford's time the scientific literature of 

 the century scarcely considers the idea even of radiant heat, 

 still less of radiant energy ; so that we have been obliged here to 

 discuss the views of its physicists about heat in general, heat and 

 light in most eighteenth-century minds being distinct entities. We 

 must remember, then, to his greater honor, that the idea of radiant 

 heat as a separate study has before Rumford scarcely an existence ; all 

 the ways for pilgrims to this special shrine of truth being barred, 

 like those in Bunyan's allegory, by two unfriendly monsters who 

 are called Phlogiston and Caloric, so that there are few scientific 

 pilgrims who do not pay them toll. 



The doctrine of caloric is, however, even then recognized as a 

 chemical hypothesis rather than one acceptable to physicists, some 

 of whom still stand out for vibratory theories even through the 

 darkest years of the century ; and, further, we may find, on strict 

 search, that the old idea of heat as a mode of motion has not so 

 utteriy died that it does not appear here and there during the last 

 century, not only among philosophers, but even in a popular form. 



In an old English translation of Father Regnault's compilation 

 on physics, dated about 1730, I find the most explicit statement of 

 the doctrine of heat as a mode of motion. Here heat is defined 

 (with the aid of a simile due, I believe, to Boyle) as " any Agita- 

 tion whatever of the insensible parts. Thus a Nail which is drove 

 into the Wood by the stroke of a Hammer does not appear to be 

 hot, because its immediate parts have but one common Movement. 

 But should the Nail cease to drive, it would acquire a sensible 

 Heat, because its insensible Parts which receive the Motion of the 

 Hammer now acquire an agitation every way rapid." We certain- 

 ly must admit that the user of this illustration had just and clear 

 ideas ; and the interesting point here appears to be, that as Father 

 Regnault's was not an original work, but a mere compendium or 

 popular scientific treatise of the period, we see, if only from this 

 instance, that the doctrine of heat as a mode of motion was not 

 confined to the great men of an earlier or a later time, but formed 

 a part of the common pabulum during the eighteenth century to an 

 extent that has been singularly forgotten. 



The last years of the eighteenth century were destined to see the 

 most remarkable experiments in heat made in the whole of the 

 hundred ; for the memoir of Rumford appeared in the Philosophical 

 Transactions for 1798 ; and in the very year iSoo appeared in the 

 same place Sir William Herschel's paper, in which he describes 

 how he placed a thermometer in successive colors of the solar 

 spectrum, finding the heat increase progressively from the violet to 

 the red, and increase yet more beyond the red where there was no 

 color or light whatever ; so that there are, he observes, invisible 

 rays as well as visible. More than that, the first outnumber the 

 second ; and these dark rays are found in the very source and fount 

 of light itself. These dark rays can also be obtained, he observes, 

 from a candle or a piece of non-luminous hot iron, and, what is 

 very significant, they are found to pass through glass, and to be 

 refracted by it like luminous ones. 



And now Herschel, searching for the final verity through a 

 series of excellent experiments, asks a question which shows that 

 he has truth, so to speak, in his hands, — he asks himself the great 

 question whether heat and light be occasioned by the same or 

 different rays. 



Remember the importance of this (which the querist himself fully 

 recognized) ; remember, that, after long hunting in the blindfold 

 search, he has laid hands, as we now know, on the truth herself, 

 and then see him — let go. He decides that heat and light are not 

 occasioned by the same rays, and we seem to see the fugitive escape 

 from his grasp, not to be again fairly caught till the next genera- 

 tion. 



I hardly know more remarkable papers than these of Herschel's 

 in the Philosophical Transactions for 1 800, or any thing more in- 

 structive in little men's successes than in this great man's failure, 

 which came in the moment of success. I would strongly recom- 



