392 THE POPULAR SCIENCE MONTHLY. 



welcomed it, its adoption among all physicists of note may l)e 

 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 en- 

 ergy 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 sensa- 

 tions 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 Becquerel in France, of Tyn- 

 dall in England, and of Henry in America. It has been necessary 

 to omit all mention of those who have advanced the knowledge of 

 radiant energy as light, or I should have had to speak of labors 

 so diverse as those of Fraunhofer, of Kirchhoff, of Fresnel, of 

 Stokes, of Lockyer, and many more. I have made no mention, in 

 the instructive history of error, of many celebrated experimental 

 researches ; in particular 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 experimenter 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 ex- 

 perimental investigations, like those of Dulong and Petit, which 

 have involved immense and conscientious labor, and have appar- 

 ently rightly earned the name of " classic " from one generation, 

 only to be recognized by the next as leading to wholly untrust- 

 worthy results, and leaving the work to be done again with new 

 methods, guided by new principles. 



In these instances, painstaking experiments have proved in- 

 sufficient, less from want of skill in the investigator than from 

 his ignorance of principles not established in time to enable him 

 to interpret his experiments ; but, if there were opportunity, it 

 would be profitable to show how inexplicably sometimes error 

 flourishes, grows, and maintains an apparently healthy appear- 

 ance of truth, without having any root whatever. Perhaps I may 

 cite one instance of this last from my own experience. About 

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

 phere acted exactly the part of the glass in a hot-bed, and that it 

 kept the planet warm by exerting a specially powerful absorption 

 on the infra-red rays. 



