Effects of 
Heat. 
Effects of 
-heat. 
Effect upon 
the sensa- 
tions, 
2. Expan- 
sion. 
678 
who wish for full information on this subject may con- 
sult ‘the Philosophical Transactions for 1816, ‘part i. 
p. 46—114, and 156—179. 
SECT. II. 
The Effects of Heat. 
We now arrive at the second of the great divisions 
into which we proposed to arrange our materials, the 
effects of heat. The principal of these arefour. 1. Tem- 
perature, or the production of the sensation of heat ; 
2. Expansion, or the simple augmentation of the bulk 
of a body, without altering its condition ; 3. A change 
in the physical state of a body, by which a solid is con- 
verted into a fluid, and a fluid into a gas; and, 4. Ig- 
nition, or the operation of heat by which a body is ren- 
dered luminous. 
1. Temperature is often used rather as a generic term, 
applicable to all the phenomena that bear a relation to 
heat, than as a specific term, proper to designate any 
one of them in:particular. It is, however, not unfre- 
quently used to express the power of exciting the sen- 
sation of heat, or the effect which it produces on the 
animal. body ; and there seems to be some pinion 2 in 
establishing this restriction, because the word heat, which 
might otherwise be employed, as we have already re« 
marked, is liable to the objection of being sometimes 
used to express the cause, and at other times the effect. 
The temperature of a body varies according to the 
quantity of heat which it contains, and may be said to 
be in the direct ratio of this quantity, although the in- 
dication of temperature, which we derive from the sen- . 
sations, is much too vague to enable us to lay down an 
exact scale of admeasurement. This proportion of ef- 
fect, however, only applies to the same body, or to 
those of a similar kind ; for it has been found that the 
same quantity of heat affects the temperature of diffe- 
rent bodies in very different degrees. These different 
effects depend partly upon the capacity of bodies for 
heat, and partly upon their conducting power ; a point 
to which we have already referred in a former part of 
the article. The sensation of temperature is also very 
much influenced by the state of the nervous system, 
independent of the quantity of heat in the substance 
exciting it. It is well known that when the hand is 
cold, a body that is applied to it will appear hot, when 
the same body would excite the sensation of cold, had 
the hand been previously exposed to a higher tempera- 
ture. This subject belongs more to physiology than 
to natural philosophy in general ; we shall therefore 
only remark, that we seem always to compare our pre 
sent feelings with our former ones, and to refer our 
sensations, not to. any invariable standard, but to what 
we have felt just previous to the present impression. 
2. The effect of heat upon the sensations, although the 
one which must have been first attended to, and that 
which constantly offers itself to our observation, is found 
to be so inaccurate as a measure of its quantity, that we 
always have recourse, for this purpose, to the second of 
its effects, which we enumerated above, expansion. Ac- 
cording to one of the characters of heat, which we have 
already described, that of distributing itself e ually 
among bodies whenever two substances are brought to- 
gether, which differ in their temperature, the one gives 
off a portion of its heat to the other, and that which re- 
ceives it becomes expanded, or has its bulk increased 
in all its dimensions. This expansion continues as long 
as the body in question maintains its elevated tempera 
ture ; but when the heatis withdrawn, it begins to con- 
tract, and by degrees it acquires its former bulk, This 
. we shall find it to be, in the first of these’ fluids, 
HEAT. 
expansion occurs, with very few é&ceptions, in all bo- 
dies ; but it differs serail in , Itisby much 
the greatest in aeriform fluids:of all kinds. It is\less in 
poner he although still considerable, in liquids ; while 
in solids, it is'so small as not tobe perceived; exceptby 
‘contrived to 
the intervention of an tus expressly 
render it visible. Not only these general-classes of bo~ 
dies, but many ofthe individual substances ized 
under each of them, have very different: ees of ex« 
‘pansive power, which has: been’ frequently made the 
subject of experiment.) | cee) web etteeedion 
The different gases have been submitted to numerous 
trials, for the purpose of ascertaining ‘their relative ex~ of g 
ayoukelesh 
= ensane simon een attention to the 
subject. It was er investigated by 5, Monge, 
and Saussure ; and lastly, with much A ver racy, 
by Guyton. (Ann. Chim. t. i. p. 256. et, seq.) Their 
results, although various, agreed-in the main conelusion, 
that each of the gases had a specific power of expansion 
peculiar to itself, and which is uniform for each: of the 
gases at the same rature. But, notwithstanding 
the weight of these authorities, a farther examination of 
the subject by Mr Dalton and M, Gay-Lussac has led’ Exper 
Xp 
to an opposite conclusion, that all the aeriform fluids ment 
suffer the same expansion from the same addition of 
heat. Mr Dalton’s experiments indicate, that 100 parts 
of any gas, in passing from the freezing to the ae 
as increase to 137 parts, which is nearly +45 for 
egree: (Manch. Mem. vol. v. p. 598.) M. Gay-Lussae’s 
experiments were published a short time 
or co-operation, they are to be regarded 
ginal, and they lead to conclusions so 
as to afford the strongest evidence of their correctness. 
(Ann. Chim. t. xliii. :p.. 187. et seq.) The source of error 
in the experiments had been performed previous to 
those of Dalton and Gay-Lussac, appears to have arisen. 
from a quantity: of aqueous vapour that had been mixed a ot 
with the gases. See Expansion. © © 
The expansion of liquids by heat differs, in many re- ‘The © 
from that of 3 itis 
proportion to the quantity of caloric that is added. to 
the heated body. If we observe the effects produced 
upon mercury, water, and alcohol, by the same addition 
of heat, and when they are all at the: of 50°, 
ual 
to ;i5 of its volume, in the second +}, and in ‘the 
~;- The comparative changes in bulk are also found 
to vary for the same fluid at different temperatures, 
contrary to what has been observed with respect to 
aeriform bodies, the expansion proceeding, in a gradu~ 
ally increasing ratio, as the tem re advances. 
Thus at the temperature of 50°, the expansion produced 
by a single degree of heat, is less than what would be 
produced by a single degree of heat at the temperature 
of 100°. Many accurate experiments, of which we have peluc’s ex. 
perform- perim 
given an account in our article Expansion, were: 
ed on this subject by Deluc. He found, that in fluids the 
rate of expansion is frequently i r near the two.ex- 
tremes of freezing and boiling, and more equable in the 
middle of thescale; and hence hededuceda practical rule, 
that those fluids are the best adapted for measuring the 
degrees of heat, which have the largest interval between 
the limits of freezing and boiling., The general result 
of our examination. of the expansive power of different 
fluids, leads us to conclude that it 
with their density, but it seems rather to be related to 
the quantity of heat necessary to convert them into the 
gaseous state. Thus ether is probably the most expan~ 
to 
those of Mr Dalton; but being made without concert» 
aa 
spects, : less in amount, it varies ‘sion o 
much in different fluids, and it is found not to be in 44 
no.connexion - 
Dalto 
Gay- 
SAC. 
“Cee 
