LIGHT AND RADIANT IIKAT. 
251 
potheses ; but the only means of deciding must consist in 
carefully ascertaining by experiment, and precisely fixing the 
properties essentially attached to caloric and light, connecting 
them in such points as afford resemblance, and opposing them 
in such as differ, so as, at length, to ascertain whether the same 
principle, always constant in iis nature, but acting differently 
upon our senses, and upon different bodies, may be capable of 
producing all the variety of effects we observe. The attention History of for- 
of several chemists and eminent philosophers has been directed mer inquiries, 
to this research. Thus, Mariotte discovered the obscure caloric 
radiating in the manner of light, and reflected in the same 
manner by metallic mirrors ; results which have been since con- 
firmed bv the experiments of Scheele and Pictet. M. Leslie 
and Count Rumford have particularly studied the influence 
which the nature of different substances, and state of their 
surfaces, enables them to exercise on the radiation of caloric 
when it enters bodies, or escapes from them. And lastly, 
M. Prevost, of Geneva, has comprehended all the phenomena 
of radiating caloric in an ingenious theory, which, if considered 
only as a systematic disposition (as the authorhimselfdoes)enables 
us to collect the phenomena under the same point of view, and 
connect the same by laws. Very lately M. Delafoche has added R at ji an t heat 
to these results a new fact, which seems, in a certain respect, passes through 
• . _ ' . . . t , . glass better 
to indicate a gradual and piogressive transition between caloric the higher the 
and light. It is, that the rays of obscure caloric pass with diffi- tem perature. 
culty through glass when they issue from a body of a tempera- 
ture below that of boiling water, but penetrate it more easily, 
and with a facility always increasing accordingly as the tempera- 
ture of the radiating body is more elevated, and approaches 
| more nearly to the state in which it becomes luminous. So 
that if we consider these experiments only, the modification, 
whatever it may be, which it is necessary to impress on theo'o- 
scure rays, to put them more and more into a state to pass 
through the glass, brings them also nearer and nearer to the 
state in which they must be to penetrate our eyes, and produce 
; the sensation of vision. M. Delaroche has found likewise, that Rays calo> 
the rays of light, which have already passed through a fir^t »ic which have 
S 2 plate 
