S. P. Langley — The History of a Doctrine. 9 



most explicit statement of the doctrine of heat as a mode of 

 motion. Here heat is defined (with the aid of a simile due, I 

 believe, to Boyle) as " any Agitation whatever of the insensi- 

 ble parts. Thus a Nail which is drove into the Wood by the 

 stroke of a Hammer does not appear to be hot, because its 

 immediate parts have but one common Movement. But 

 should the Nail cease to drive, it would acquire a sensible 

 Heat, because its insensible Parts which receive the Motion of 

 the Hammer now acquire an agitation every way rapid." We 

 certainly must admit that the user of this illustration had just 

 and clear ideas ; and the interesting point here appears to be, 

 that as Father Eegnault's was not an original work, but a mere 

 compendium or popular scientific treatise of the period, we 

 see, if only from this instance, that the doctrine of heat as a 

 mode of motion was not confined to the great men of an 

 earlier or a later time, but formed a part of the common pabu- 

 lum during the eighteenth century to an extent that has been 

 forgotten. 



Although Prevost gave us his most material contribution 

 about 1790, we have, it seems to me, on the whole, little to 

 interest us during that barren time in the history of radiant 

 energy called the eighteenth century, — a century in which sci- 

 ence wore the pedant's cap and gown, and her students read 

 the poem of Creation like grammarians, for its syntax ; — a 

 century whose latter years are given up, till near its very close, 

 to bad a priori theories in our subject, except in the work of 

 two Americans ; for in the general dearth at this time of ex- 

 periments in radiant heat, it is a pleasure to fancy Benjamin 

 Franklin sitting down before the fire, with a white stocking on 

 one leg and a black one on the other, to see which leg would 

 burn first, and to recall again how Benjamin Thompson (Count 

 Eumf ord) not only weighed " caloric " literally in the balance 

 and found it wanting, but made that memorable experiment in 

 the Munich founderies which showed that heat was perpetually 

 and without limit created from motion. 



It was in the last years of the century, too, that he provided 

 for the medal called by his name, and which, though to be 

 given for researches in heat and light, has, I believe, been 

 allotted in nearly every instance to men, who, like Leslie, 

 Malus, Davy, Brewster, Fresnel, Melloni, Faraday, Arago, 

 Stokes, Maxwell, and Tyndall, have contributed toward the 

 subject of radiant energy in particular. 



We observe that before this time the scientific literature of 

 the century scarcely considers the idea even of radiant heat, 

 still less of radiant energy ; so that we have been obliged here 

 to discuss the views of its physicists about heat in general, 

 heat and light in most minds being then distinct entities ; all 



