with a power to render objects visible, and that there are different
degrees of heat in the prismatic spectrum of these invisible rays.
This being established, the question now, according to the original
enunciation, is in fact, "Whether some of these heat-making rays
may not have a power of rendering objects visible, superadded to
their own already established power of heating bodies ?" From a ge-
neral and comparative view of those among the preceding experiments
which apply to this question, we gather that no kind of regularity
takes place among the proportions of the luminous and heating rays
which are stopped in their passage, and that hence it might be rea-
sonably inferred that heat and light are entirely unconnected. Yet,
not to evade the above hypothesis, the Doctor enters into 'a more
minute investigation of the subject, and shows that, admitting, ac-
cording to the supposition, that the same rays being both huminous
and calorific, may in their passage through certain media be so af
fected as to produce the very discordant results observed in the ex-
periments, it is yet evident, on a due comparison of those results,
that no given proportion that may be ascribed to this operation of
the transmitting media, will anyways account for the general phe-
nomena; the degrees of heat being in some instances greatly redun-
dant, and in others as much deficient, both ways deviating from any
given proportion. Thus it is that he reduces his opponent to the di-
lemma of either maintaining that the same agent may under different
circumstances produce effects perfectly dissimilar, such as heat with-
out light, decreasing heat and increasing light, or the reverse; or
else to admit that there actually is a difference between the rays that
give light, and those which produce heat.
A more direct proof of the difference of the two sorts of deduced from the manifest results of the experiments, in which the stoppage of one sort of rays does by no means occasion the stoppage of the other sort. In investigating this subject the Doctor contro- verts a conjecture that the phænomena observed may be ascribed to a peculiar texture or configuration in the diaphanous substances, which produce differences in the transmission of the rays, though there be no difference in the rays themselves. This hypothesis also is minutely investigated, and its contradiction with the experiments being pointed out, its rery foundation seems in.fact to be wholly subverted
Lastly, another direct proof of the difference of the two sorts of rays, is deduced from a comparative view of the results of some of the experiments, from which it appears that the stoppage of heat is. in general gradually extending as far as five minutes in time, whereas the suppression of light hitherto appears to be instantaneous. This, together with various other arguments derived from the transmission of terrestrial heat, which cannot be properly explained in a manner sufficiently concise for this place, seem to evince that in fact the law by which heat is transmitted is essentially different from that which directs the passage of light, and that hence there is every reason to believe that the rays of heat are different from those of light.
is rays
