HEAT: 
of 0" of his scale was equivalent to the 1077° of Fahrenheit's 
‘ it so far extended. The d 
ch was arbitrary, were each of them equal to 130° 
of Fahrenheit’s ; and the highest temperature which he 
of ally measured was 160° of his pyrometer, corre- 
sponding to above 21,800° of Fahrenheit, which was 
iias 1 to be the temperature of a particular air furnace 
which he employed in his experiments: (Phil. Trans. 
1782, p. 305, et seq.) This pyrometer has generally 
een ded as a very valuable addition to our phi- 
losophical apparatus; bat there is one circumstance 
which unfortunately much impairs its use—the difficul- 
ty of procuring the same kind of clay on which Wedge- 
wood operated for the test pieces, as the mass which he 
originally employed is exhausted ; and it does not ap- 
‘pear certain, whether the imitation of it that has been 
sitice prepared, is possessed of precisely the same phy- 
sical properties. See Curmistry, p. 152. 
Several other en have been invented, de- 
. pending upon different principles, and possessing diffe- 
rent degrees of accuracy. Muschenbroek, Ferguson, 
Ellicot, and Smeaton, have all exercised their ingenuit 
upon this point ; but the only apparatus which we shall 
farther notice is that of Guyton. In this instrument a 
long rod of platina is fixed in an horizontal groove, form- 
ed in a mass of clay, that had been hardened by expo- 
sure to a strong heat ; one end of the red extends beyond 
_ the groove, and presses against another rod of platina, 
which acts as a lever; its longer arm being extended, 
so as to form an index, which moves upon a graduated 
arc. As the rod is more or less expanded by the heat, 
the index is carried along the scale, and serves to point 
out very minute variations in the bulk of the metal: 
Ann. Chim. tom. xlyi. p. 276.) We are not certain whe- 
er this instrument has ever been employed in England; 
but it appears to possess many advantages as an ac- 
curate measure of high temperatures. See Pyrometer, 
8. We now arrive at the third of the effects of heat, 
the change which it produces in the state of bodies, con- 
verting, according to circumstances, a solid into a li- 
quid, or a liquid into an aeriform fluid. The effect of 
caloric, which we last described, consists in introducing 
a portion of heat between the particles of bodies, by 
which they are, to a certain extent, separated from each 
other, so as to experience an increase of bulk, without 
having their cohesion materially impaired. If, how- 
ever, the addition of heat be continued beyond a cer- 
tain limit, the particles are removed still farther from 
each other, until at length they are so far separated, as 
to lose their cohesive power, and to become easily move- 
able among themselves in all directions. By this change 
of form, the substance in question undergoes a com- 
ete alteration in its physi roperties, and not un- 
os de in its chemical action upon other bodies. 
‘The effect is produced by destroying the balance be- 
tween the expansive power of heat, and the force of at- 
traction ; the former tending to remove the particles of 
bodies to a distance from. each other, the latter to retain 
them in close contact, But although the attractive 
force is conceived to be very much weakened in bodies 
when they assume the fluid state, it is not entirely de- 
stroyed, but is still exerted with considerable energy. 
It is also to be remarked, that after a solid has re- 
ceived an addition of heat, so as to convert it into a 
fluid, a still farther quantity of heat may be given to 
it, which kas the simple effect of expansion. If, how- 
ever, we continue to increase the quantity of heat after 
the expansion has advanced to a certain length, the 
body again assumes a new form, and becomes a gas; 
VOL, X, PART Il. 
the size of ~ 
681 
and it now, as before, after this third change, by add- 
pe Sema 2} suffers merely an ion in its 
volume. But the expansion in these toren cases, a8 
has been already observed, differs considerably in its 
degree, being small in the first instance, in the 
second, and much more so in the last, This peculiar 
quality of assuming the different states of solidity, li. 
quidity, and elastic fluidity, may be considered as a 
property common to all kinds of matter; for it would 
appear, that if we had the power of producing, at plea- 
sure, temperatures sufficiently high and sufficiently low, 
every substance that usually appears under the solid 
form might be vaporized, and every substance that 
usually appears as a vapour might be rendered solid, 
When we , therefore, of any body being natural. 
ly in the solid, or naturally in the liquid state, we mean 
nomore than that, under the ordinary temperature of 
the atmosphere, or under the circumstances in which it 
9 presents itself to our notice, it is solid or 
uid, 
Effects 
Mews, 
——_—o 
There is, indeed, another obstacle to altering the some bo- 
state of bodies, besides the difficulty of emeaney dies not ca- 
high or very low temperatures, that by a great 
ition pable of 
or subtraction of heat, their chemical constitution is af- ‘** 
cwne 
fected, and they are either decomposed, or are dispo- °* 
sed to enter into new combinations. Organised sub- 
stances in general, both of animal and vegetable origin, 
when subjected to high temperatures, are converted 
into their ultimate elements ; and, on the contrary, in 
some cases, a great reduction of temperature causes 
the constituent parts of certain bodies to separate from 
each other, before they are brought into the solid form, 
These, however, are to be regarded rather as inciden- 
tal circumstances, interfering with particular. opera- 
tions, than as any fundamental exceptions to the gene- 
ral law that has been laid down. It would 
ear, Sir James 
from the experiments of Sir James Hall, that many of Hall's ex- 
these apparent exceptions may be brought under the periment 
eneral rule, merely by employing strong pressure or 
Sten mechanical means. Thus it had nee supposed 
impossible to melt the carbonate of lime, because at an 
elevated temperature the carbonic acid escaped in the 
form of gas; but when the substance was subjected 
to strong compression, so as to prevent this escape of 
gas, the fusion was easily accomplished, 
The suddenness of the change which occurs when 
the body receives so much. heat as to convert it into 
the liquid state, shews that some other circumstance 
takes place besides the mere addition of caloric. And 
the same remark applies to vaporization ; for here again 
no indication of the change of state can be observed, 
until the actual change occurs in its full extent. This 
point we shall soon ‘consider more at length; but, in 
the mean time, before we quit the subject of the change 
of bodies from the solid to the liquid form, we must of- 
fer a few observations upon some phenomena that oc- 
cur when we reverse the operation, and convert them 
from the liquid to the solid state. 
dergo-this change, without having their chemical con- 
stitution materially altered, have a fixed point at 
which they are said to freeze, congeal, or become solid ; 
and except under certain circumstances, this point is 
invariably the same. This is so much the case with 
re to water, that the freezing point of this fluid 
has been employed as one of the fixed degrees which 
regulate the graduation of the whole scale, 
tis, however, ible, by certain m 
cool water seve: 
All. bodies that un- Freeing 
point of 
bodies. 
Cooling 
water be- 
ent, tO low the 
degtees below its freezing point, freezing 
without rendering it solid; according to the experi- P 
4R 
