THE HISTORY OF A DOCTRINE. 219 



time, but formed a part of the common pabulum during the 

 eigliteentli 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 Rumf ord appeared in the " Philo- 

 sophical Transactions" for 1798; and in the very year 1800 ap- 

 peared 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 ob- 

 serves, 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 dif- 

 ferent 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 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 fugi- 

 tive escape from his grasp, and not to be again fairly caught till 

 the next generation. I hardly know more remarkable papers than 

 these of Herschel's in the "Philosophical Transactions" for 1800, 

 or anything more instructive in little men's successes than in this 

 great man's failure, which came in the moment of success. I 

 would strongly recommend the reading of these remarkable 

 original memoirs to any physicist who knows them only at sec- 

 ond hand. 



One more significant lesson remains, in the effect of this on the 

 minds of his contemporaries. Herschel's observation is to us al- 

 most a demonstration of the identity of radiant heat and light ; 

 but now, though the nineteenth century is opening, it is with the 

 doctrine still in the minds of most physicists, and perhaps of all 

 chemists, that heat is occasioned by a certain material fluid. Phlo- 

 giston is by this time dead or dying, but caloric is very much alive, 

 and never more perniciously active than now, when, for instance, 

 years after Herschel's observation, we find this cited as " demon- 

 strating the existence of caloric," which was, it seems, the way it 

 looked to a contemporary. 



