1 1 8 Inquiry concerning the Nature of Heat, 



however sensible it may be, will not be apparently- 

 affected by the rays from the hot body. 



This experiment, which is of ancient date, has often 

 been made, and always with the same results. 



Let us now suppose the hot body to be removed 

 from the focus of the mirror A, and that a colder body 

 be substituted in place of it. And, in the first place, 

 we will suppose the temperature of this- colder body to 

 be that of freezing water, or just equal to that which 

 reigns in the room. 



As the rays which bodies at the same temperature 

 send off from one to the other have no tendency to 

 increase or to diminish the temperature of those bodies, 

 the concentration of rays in the focus of the mirror B, 

 proceeding from the ice-cold body placed in the focus 

 of the mirror A, can have no effect on a thermometer, 

 at the same temperature, which is exposed to their 

 action. 



If heat be a vibratory motion of the constituent par- 

 ticles of bodies, and if the rays which sensible bodies 

 send off in 'all directions be undulations in an ethereal 

 elastic fluid by which they are surrounded, occasioned 

 by those motions ; as the pulsations in this fluid must 

 be isochronous with the vibrations by which they are 

 occasioned, these pulsations or undulations can neither 

 accelerate nor retard the vibrations of other bodies at 

 the surfaces of which they arrive, provided the vibra- 

 tions of the constituent particles of such bodies are, at 

 that time, isochronous with the vibrations of the con- 

 stituent particles of the body from which these undula- 

 tions proceed. But to return to our experiment. 



Suppose now that, instead of this ice-cold body, 

 another much colder at the temperature of freezing 



