170 Count Rumford's Enquiry concerning the Nature of Heat, 
thermometer, at the same temperature, which is exposed to their 
action. 
If heat be a vibratory motion of the constituent particles 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 pul- 
sations 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 vibrations of' the 
constituent particles of such bodies are, at that time, isochronous 
with the vibrations of the constituent particles of the body from 
which these undulations proceed. But, to return to our ex- 
periment. 
Suppose now that, instead of this ice-cold body, another much 
colder, at the temperature of freezing mercury, for instance, be 
placed in the focus of the mirror A, and that a thermometer at 
the temperature of freezing water be placed in the focus of the 
mirror B; what might be expected to be the result of this ex- 
periment ? — 'That the thermometer would fall, in consequence of 
its being cooled by the accumulation of frigorific rays proceeding 
from this very cold body. 
Now this is what actually happened, in the celebrated expe- 
riment of my ingenious friend Professor Pictet, of Geneva. 
Several attempts have been made to explain the result of that 
experiment, on the supposition that caloric has a real or material 
existence, and that radiant heat is that substance, emitted and 
sent off in right lines, in all directions, from the surfaces of hot 
bodies. But none of these explanations appear to me to be 
