142 PROGRESS OF SCIENCE IN THE CENTURY. 



Motion is the title of Tyndall's well-known work 

 (1862), yet Descartes, Amontons, Boyle, Francis 

 Bacon, Ilooke, and Newton already looked upon heat 

 as a mode of motion. Of course, in the seventeenth 

 century, this theory rested upon somewhat slender 

 experimental evidence, else the doctrine could hardly 

 have been cast to the winds by the eighteenth cen- 

 tury philosophers." 



The Fiction of Imponderable Matter. Even in 

 the eighteenth century, it could not but be noticed, 

 when the habit of weighing began, that a body which 

 had been heated was no heavier than it was before. 

 Therefore a fiction had to be invented, the well- 

 known fiction of the " imponderables." Heat, or 

 rather " caloric," was a substance, but it was an im- 

 ponderable substance. The further difficulty that 

 heat may be produced in abundance apart from all 

 fire or combustion, even by rubbing two pieces of ice 

 together, and that it may in other cases disappear 

 beyond trace, seems to the modern outlook quite fatal 

 to the material theory of heat, but the difficulty does 

 not appear to have oppressed the natural philoso- 

 phers of the eighteenth century. It must be recalled 

 that the doctrine of the indestructibility of matter 

 dates from Lavoisier and that it was not fully ap- 

 preciated till much later. With this and the doctrine 

 of the conservation of energy now clearly before us, 

 the materiality of heat seems like a contradiction in 

 terms, but this is to be wise after the event 



Let us therefore consider how the old Newtonian 

 idea was re-habilitated, how it has come to be an 

 elementary fact in physics that heat depends upon 

 motion of the particles of a body, and is a form of 

 energy, not a kind of matter. 



Rumford's Experiments. The first strong blow 



