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XLI. On Calorific Radiation. By Mr. Henry MEIKLE. 
To Mr. Tilloch. 
London, April 1, 1819. 
Sir, — Tae nature of heat, like that of light, is still in dark- 
ness ; for although many of its effects are pretty familiar to us, 
yet the nature of the cause is wholly unknown. Our ignorance 
in this respect does not arise from want of diversity of opinion 
on the subject, since almost every author entertains his own fa- 
vourite notions. But to enumerate all the wild speculations that 
have from first to last been let loose on the public, would be an 
endless as well as an useless task. Suffice it to say, that what- 
ever discordance apparently prevails among the different hypo- 
theses, if is some consolation, that they generally agree in the 
main point—their grand common tendency being only to fortify 
more strongly the impenetrable barrier that has hitherto defended 
the nature ‘of heat. Indeed it may be fairly questioned, if even 
the most capacious and enlightened human mind be in any de- 
gree adequate to comprehend the nature of heat, although it 
were fully unfolded to view. The same may be said of many 
other things. Like the unlettered child fondly handling the my- 
sterious page, he would gape, and gaze, and wonder, in vain. Be 
this as it may, the little success that has uniformly attended the 
numerous attempts to explain this hidden principle, indisputably 
proves they were premature. For the present it is surely of in- 
finitely more use, carefully to register the observed effects. 
No simple hypothesis has ever been framed satisfactorily to 
explain all the different phenomena. The more common one— 
that heat is a substance and cold the absence of it, does not quite 
account for every appearance. The difficulty of giving a suffi- 
cient reason, on this hypothesis, for the apparent reflection of 
cold, has even induced some to create a system of frigorifie rays 
to operate as a deserved check on the capricious anomalies of 
heat, which have so wantonly derided every honest effort to ex- 
plain them. 
The supposed inexplicable case alluded to is, that when a trun- — 
cated hollow cone of metal, ABC D (fig. 3, Plate III.) open at 
the ends, with its inner surface polished, has a spherical vessel E, 
containing a freezing mixture, placed at the wider ena, and the 
ball of an air-thermometer F at the narrower end, it is found 
that the thermometer is more cooled, or ascends higher, in this 
state of things, than when it and the cold body mutually change 
places. 
Passing some whimsical explanations as foreign to the subject, 
this effect is usually ascribed to the concentration of the rays of 
cold 
