THE HISTORY OF A DOCTRINE. 387 



He uses the then all-powerful " Edinburgh Review '* for his pul- 

 pit, and notices Young's great memoir as follows : " This paper 

 contains nothing which deserves the name either of experiment or 

 discovery ; and it is, in fact, destitute of every species of merit. 

 . . . The paper which stands first is another lecture, containing 

 more fancies, more blunders, more unfounded hypotheses, more 

 gratuitous fictions, . . . and all from the fertile yet fruitless brain 

 of the eternal Dr. Young. In our second number we exposed the 

 absurdity of this writer's ' law of interference,' as it pleases him 

 to call one of the most incomprehensible suppositions that we re- 

 member to have met with in the history of human hypotheses." 



There are whole pages of it, but this is enough ; and I cite this 

 passage among many such at command, not only as an example 

 of the way the undulatory theory was treated at the beginning of 

 this century in the first critical journal of Europe, but as another 

 example of the general fact that the same thing may appear in- 

 trinsically absurd, or intrinsically reasonable, according to the 

 year of grace in which we hear of it. The great majority, even 

 of students of science, must take their opinions ready-made as to 

 science in general ; each knowing, so far as he can be said to know 

 anything at first hand, only that little corner which research has 

 made specially his own. The moral we can all draw, I think, for 

 ourselves. 



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 ra- 

 diant heat was still a fluid ; and yet, in spite of all, the modern 

 doctrine now begins to make visible progress. 



A marked step is taken about 1811 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 ab- 

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

 else, I 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 

 gradual 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 temperature rises yet more, appears more and more 

 as light, or which, as the luminous radiation is absorbed, reappears 

 as heat. Very little of it, he observes, passes even transparent 

 screens at low temperatures, but more and more does so as the 

 temperature rises. All this is a truism in 1888, but it is admirably 



