123 
and the Mode of its Communication. 
calorific radiations of hot bodies have been so well known ; but, 
if we consider the matter with attention, our surprise will cease. 
Those radiations by means of which the temperatures of neigh- 
bouring bodies are gradually changed and equalized, are not 
sensible to our feeling, unless the intervals of temperature be 
very considerable ; and the constitution of things is such, that 
while we are often exposed to the influence of. bodies heated 
several thousand degrees (as measured by the thermometer) 
above the mean temperature of the surface of the skin, it is 
very seldom that we have opportunities of experiencing the 
effects of the radiations of bodies much colder than ourselves ; 
and we have no means of producing degrees of cold which bear 
any proportion to the intense heats excited by means of fire. 
From the result of the experiment of which an account has 
just been given, it is evident, that we should be just as much 
affected by the calorific fays emitted by a cannon bullet at the 
temperature of 160 degrees of Fahrenheit’s scale, ( =64 degrees 
above that of the blood,) as by the frigorific rays of an equal 
bullet, ice cold, placed at the same distance ; and that a bullet 
at the temperature of freezing mercury, could not affect us much 
more sensibly, by its frigorific rays, than an equal bullet at the 
temperature of boiling water would do, by its calorific rays ; but, 
at these comparatively small intervals of temperature, the radia- 
tions of bodies are hardly sensible, and could never have been 
perceived, much less compared and estimated, without the assis- 
tance of instruments much more delicate than our organs of 
feeling. Hence we see how it happened, that the frigorific ra- 
diations of cold bodies remained so long unknown. They were 
suspected by Bacon ; but their existence was first ascertained 
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