10 S. P. La/ngley — The History of a Doctrine. 



the ways for pilgrims to this special shrine of truth being 

 barred, like those in Banyan's Pilgrim's Progress, by the two 

 unfriendly giants who are here called Phlogiston and Caloric, 

 so that there are few scientific pilgrims who do not pay them 

 toll. 



The last years of this century were destined to see the most 

 remarkable experiments in heat made in the whole of the hun- 

 dred ; for the memoir of Rumford appeared in the Philo- 

 sophical Transactions for 1798 ; and in the very year 1800 

 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 progres- 

 sively 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 generation. 



I hardly know more remarkable papers than these of Her- 

 schel'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 second-hand. 



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

 the minds of his contemporaries. Herschel's observation is to 

 us almost a demonstration of the identity of radiant heat and 

 light ; but now, though the nineteenth century is opening, it is 

 with the doctrine in the minds of most physicists, and perhaps 

 of all chemists, that heat is occasioned by a certain material 

 fluid. Phlogiston is by this time dead or dying, but Caloric is 

 very much alive, and never more perniciously active than now, 



