220 THE POPULAR SCIENCE MONTHLY. 



In the year 1804 appeared wliat should be a very notahle book 

 in the history of our 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 original 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-measure 

 (the differential thermometer) ; and with these, and concave re- 

 flectors of glass and metal, he commenced experiments in radiant 

 heat, than which, he tells us, no part of physical science then ap- 

 peared so dark, so dubious, and so neglected. It is interesting, 

 and it marks the degree of neglect he alludes to, that his first dis- 

 covery was that different substances have different radiating and 

 absorbing powers. It gives us a vivid idea of the density of pre- 

 vious ignorance, that it was left to the present century to demon- 

 strate this elementary fact, and that Leslie, in view of such dis- 

 coveries, says, " I was transported at the prospect of a new world 

 emerging to view." 



Next ho shows that the radiating and absorbing powers are. 

 proportional, next that cold as well as heat seems to be radiated, 

 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 uncon- 

 sciously assume, while judging from our own present standpoint 

 where past error is so plain, that the false conclusion 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 f rigorific 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 imaginary, of magnetism and electricity. But why 

 have recourse to invisible agents ? Quod petis, hie est. It is 

 merely the ambient AIR." 



The capitals are Leslie's own, but ere we smile with superior 



