438 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



and cold body and those of two strings, one of which is 

 emitting a musical sound and compelling the other to 

 execute synchronous movements. The ether in the one 

 case and the air in the other are the media through which 

 their motions pass. 



Equilibrium of temperature takes place when the mol- 

 ecules of the substances concerned are in synchronous 

 and equal vibration. A hot body in presence of a cold 

 one compels the latter to hasten its rate of motion, its 

 own rate all the time declining, and this continues until 

 both have the same frequency ; then equilibrium of tem- 

 perature results. The theory of the exchanges of heat is, 

 therefore, only an expression for the exchanges of vibra- 

 tions through the ether. 



But temperature in thermotics is the equivalent term 

 for brilliancy in optics. Both refer to compound quali- 

 ties, depending not only on frequency of vibration, but 

 also on its amplitude. As the degree of heat of a mass 

 rises, the mass expands, the increase in its volume indi- 

 cating that not only do its parts vibrate more swiftly, 

 but also that their individual excursions are increased. 

 It follows, therefore, that every mass will have a deter- 

 minate volume for every degree of heat, the volume in- 

 creasing as the temperature rises. On this view the ex- 

 planation of the expansion of bodies by heat is that their 

 parts are not only vibrating more quickly, but also that 

 the individual excursions are greater. 



The atoms of the chemical elements differ in weight. 

 We therefore should not expect that the ethereal vibra- 

 tions would throw them into movement with equal facil- 

 ity, but that some would yield more readily than others. 

 Is not this what we express in chemistry by the term 

 specific heat ? a body, the capacity of which is great, re- 

 quiring a prolonged application of ethereal pulses before 

 a consentaneous motion is reached, and in its turn im- 



