S. P. Langley—The History of a Doctrine. 11 



when, for instance, years after Herschel's observation, we find 

 this cited as " demonstrating the existence of caloric ;" — which 

 was, it seems, the way it looked to a contemporary. 



In the year 1804 appeared what should be a very notable 

 book in the history of onr subject, written by Sir John Leslie, 

 whose name survives perhaps in the minds of many students 

 chiefly in connection with the " cube " which is still called 

 after him. 



Leslie, however, ought to be remembered as a man of orig- 

 inal genius, worthy to be mentioned with Herschel and Melloni ; 

 and his, too, is one of the books which the student may be 

 recommended to read, at least in part, in the original ; not so 

 much for the writer's instructive experiments (which will be 

 found in our text-books) as for his most instructive mistakes, 

 which the text-book will probably not mention. 



He began by introducing the use of the simple instrument 

 which bears his name, and a new and more delicate heat-meas- 

 urer (the differential thermometer) ; and with these, and con- 

 cave reflectors of glass and metal, he commenced experiments 

 in radiant heat, than which, he tells us, no part of physical 

 science then appeared so dark, so dubious, and so neglected. 

 It is interesting, and it marks the degree of neglect he alludes 

 to, that his first discovery was that different substances have 

 different radiating and absorbing powers. It gives us a vivid 

 idea of the density of previous ignorance, that it was left to 

 the present century to demonstrate this elementary fact, and 

 that Leslie, in view of such discoveries, says, " I was trans- 

 ported at the prospect of a new world emerging to view." 



Next he shows that the radiating and absorbing powers are 

 proportional, next that cold as well as heat seems to be radi- 

 ated, and next undertakes to see whether this radiant heat has 

 any affinity to light. He then experiments in the ability of 

 radiant heat to pass through a transparent glass, which transmits 

 light freely, and thinks he finds that none does pass. Radiant 

 heat with him seems to mean heat from non-luminous sources ; 

 and the ability or non-ability of this to pass through glass, is 

 to Leslie and his successors a most crucial test, and its failure 

 to do so a proof that this heat is not affiliated to light. 



Let us pause a moment here to reflect that we are apt to 

 unconsciously assume, while judging from our own present 

 standpoint where past error is so plain, that the false conclu- 

 sion can only be chosen by an able, earnest, conscientious 

 seeker, after a sort of struggle. Not so. Such a man is found 

 welcoming the false with rapture, as very truth herself. 



"What then," says Leslie, "is this calorific and frigorific 

 fluid after which we are inquiring ? It is not light, it has 

 no relation to ether, it bears no analogy to the fluids, real or 



