HEAT. 691 
; — 
of first, Does heat possess, any properties, or any relation 
to other bodies, which can strictly be called chemical ? 
And, secondly, What are these relations? When we 
_ speak of a substance being possessed of chemical pro- 
perties, we mean, that by its being separated from, or 
added to another substance, the two substances, in the 
case of their decomposition, or in the other case, the 
compound of the two, manifest different’ chemical re- 
lations from, those which they possessed before’ the 
pers pon- By some philosophers it has been supposed 
this change takes place with respect to heat, that 
when it enters into combination with a body, as for 
example with oxygen, and forms oxygen gas, a com- 
pound is nt ) possessed of new erties that 
are not merely mechanical, while at the sume time the 
heat itself has entirely lost its former qualities by its 
union with the oxygen, and, like other bodies after a 
chemical composition, has imparted to the compound 
mpote quite different from its own. If we again 
pose the oxygen gas, as by tniting’ its base to 
carbon, the heat is procipitatens and resumes all its 
Soemnes. peopertion, which had been entirely destroyed, 
or rendered latent, while it remained in the combined 
state. But although there may a to be a consi- 
derable degrée of plausibility in this method of reason- 
ing, it has not been generally a ed in, The 
great objection which has been w ‘inst the opi- 
nion that heat unites chemically to bodies, is that it 
may, in all cases, be separated from them, merely by a 
reduction of temperature, without the intervention of 
a more powerlul affinity ; a circumstance which is con. Nature af 
ceived to be essential to a proper chemical decomposi- _ est. 
tion. With respect to the new properties which heat ““Y"” 
imparts to other substances, by its union with them, 
these have been said to be merely mechanical, or to 
such as must necessarily result from these i 
being placed at a ter distance from each other, 
As to the heat itself, there is undoubtedly something 
more analogotis to chemical union, in the changes 
which it experiences ; as by entering into com 
with other ies, its characteristic properties are all 
destroyed. Yet this involves a discussion rather about 
words than ideas, and in a great measure 
upon the meaning which we attach to certain sobwet 
expression. The fact is, that heat seems to have elec- Genera! 
trive attractions for different bodies, independent of conclusion, 
any of their relations to other substances ; it com P 
bines with them, and produces a change in their state, 
which gone had be regarded as merely mechanical ; 
pee tha lg ese combinations its action is 1 
and it no | r possesses the properties it viousl 
exhibited. Epon the hi. vn ome ‘nclitted to rd 
opinion, that the effects of heat are not referable to the 
usual laws of chemical affinity, but must be referred, 
partly to tliose of mechanical impulse, and partly to 
what we may denomiinate specific action, 7. ¢. an action 
peculiar to. itself, and essentially different from what 
we can observe in other natural objects. . See our ar« 
ticles Coup and Expansion.  (#) 
APPENDIX, 
In the preceding article, we have given an account 
of the experiments of Dr Herschel, Sir H. Englefield, 
and M. , from which it follows, that.there exists 
beyond the red extremity of the spectrum.a set of invi- 
sible rays, which affect the thermometer more power- 
pg Be even the red light of the spectrum. 
: experiments were repeated, under the most fa- 
vourable circumstances, by our celebrated countryman 
Sir H. Davy, who favoured us with an account of them 
’ ‘when we had the pleasure of seeing him at Geneva. 
Availing himself of the fine climate and the serene 
‘sky of Italy, he resolved to inquite into the cause 
of the difference between the results. obtained by Dr 
Herschel and Berard; the former having placed the 
point of maximum intensity beyond the redrays, while 
the latter found the heat greatest within the extremity 
of the red space. It occurred to Sir Humphry, that 
thermometers with circular balls of the size used by 
, must necessarily shew ae maximum of heat in 
‘be within the red space, as a cooling : must al- 
ways be exerted by that part of the bulb. which is out 
‘of the limits of the ‘calorific rays; and hence the dimi- 
nution of temperature which this occasioned would 
compensate for the greater effect produced by the invi- 
sible rays. In order to remove this source of error, Sir 
H. Davy employed extremely slender thermometers, 
‘not more than ;;th of an inch in diameter, with very 
bulbs filled with air confined by a coloured fiuid ; 
and in this way the bulb was affected only ‘by the invi- 
‘sible rays. general results of these experinients, 
which were i ae also at Geneva, confirmed ‘those 
of Dr Herschel, and may be considered as removing 
the objection of Professor Leslie, that, in our climate 
particularly, thete must always be a concentration of Appendi 
eeiennciie Nave beyond the red extremity of the spec- seas 
trum. ‘ 
We regard it, therefore, as a point established in 
physics; that there isa distinct effect. produced upon 
the thermometer beyond the red extremity of the spec- 
trum; but we cannot admit for a moment the conclu« 
sion which has been universally drawn from it, that it 
is produced by a separate set of heat-making rays, re- 
fracted by the 8 and having a refrangibi- 
lity than red light. Dr Herschel’ attem tO Feat is in- 
prove that invisible culinary heat is also susceptible of capable of 
refraction like the invisible rays, and M. Prevost and permeating 
a ees aeewne y padlin oa to i that sis. 
is kind of is e of permeatin We 
have no doubt che tes opinion will be found to be 
erroneous, atid we think it is ‘demonstrated * that invi- 
sible culinary heat is incapable of refraction, or of per- 
meating glass like light, and hence we cannot suppose 
that the invisible rays of the sun are capable of being 
refracted by glass. If this shall be found to be the 
ase, it will follow that the invisible heat of the sun is 
a_ substance ing properties ‘essentially different 
“from those of invisible culinary heat. As radiant heat, 
therefore, possesses very different pro ies from light, 
we think it will be found, that light, Fike all other mat- 
ter, whether , fluid, or solid, is capable of being 
heated, and ‘that the solar rays are nothing more than 
heated light. “Hence the least reftangible rays, having 
the greatest velocity, or momentum, will produce the 
most powerful calorific effect. We shall have occasion 
to resume this subject in another part of our work, 
See Oprics. Ep. Te 
# See Phil, Trans, 1816, p. 105 
