INFLUENCE ON RECENT SCIENCE 95 



laws are not correct generalizations; and whose con- 

 clusions are unlike the phenomena of nature; which, 

 nevertheless, anticipates an idea to be advanced again, 

 after a century and a half. This hypothesis of light 

 contains, as a corollary, the germ of the kinetic theory 

 of heat. He discards the notion, which then generally 

 prevailed, that heat was a sort of mysterious substance 

 called caloric, for which indeed there was no place in 

 his universe, and defines it as the oscillatory agitation 

 of terrestrial particles, set up by the pressure of light. 

 The pores of such bodies are tortuous, and the pressure 

 of light on one end of such a gross and irregularly 

 shaped particle may be greater than on the other end 

 and so cause a tipping motion. Such a tilt would give 

 an oscillatory motion to a particle, similar to a violin 

 string when bowed, and he believed its momentum 

 affects us as heat. An oscillation of this sort would 

 communicate itself to neighboring particles and there- 

 fore explains the conduction of heat from one part of 

 a body to another; also, it accounts for the expansion 

 of bodies by heat, since vibrating particles usually re- 

 quire more space than quiescent ones; but, on the 

 other hand, they might be so shaped and arranged as to 

 occupy less space, when in motion, and such a body 

 should contract if heated. Such an effect was unknown 

 at that time, but we ought to claim that it was bril- 

 liantly verified when, in later years, water was ob- 

 served to contract when heated from zero to four 



