Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 5, BUR-CHI

470
Caloric.
CHEMISTRY.
Ml
Bad con-
d udors.
548
Their ufe.
M9
Metals the
belt con-
dadtori.
paflcs with diiBculty, or very flowly, are faid to be had
condu^lors. The motion of caloric from one body
to another, or through the fame body, is not altoge¬
ther in proportion to their denfities, as might be fup-
pofed Irom the inftance of the communication of calo¬
ric through wood and iron, jult mentioned. Calo¬
ric is conducted very flowly through a more porous
firbthmce, fuch as a mafs of cork, of wool, of feathers,
or furs. It is on account of the flownefs with which
heat is conducted in thefe fubllances, that fome of them
are employed in the colder feafons of the year, or in
cold countries, as materials for clothing. The heat
being flowly conduced through fuch fubftances, they
prevent the heat of the body from being diflipated, or
cut oft the communication between the warm body and
the cold air. And thus we fee a wife proviflon of na¬
ture, in furniflring all animals which are inhabitants of
the colder regions of the earth, with a. thick covering
of fur or feathers. According to the experiments of
Count Rumford, the conducing power of fur, feathers,
filk, and wool, diminilhes in proportion to the finenefs
of their texture.
Metallic fubftances are the beft conduftors of calo-
but among the metals it would appear that there
ric
is conftderable variety in their conducing power, and
this is not in proportion to their denlity, as appears
from the experiments of Dr Ingenhoufz on the follow¬
ing metals, which are fet down in the order of their
*Journ. de condufting power *.
■Pbyf.il$9'
Silver,
Gold,
Copper,
Tin,
Platina,
Iron,
Steel,
Lead.
Conducting ^ ^et experiments were made on the conducting
power of ° power of different woods, by Profeffor Mayer of Er-
woods. langen, of which the following are the refults, eorapar-
f Annal. de ed with the conducting power of water f.
vol.
xxx. p. 43. Water, 10.0
Ebony, 21.7
Crab apple, 27.4
Afh, 38.0
Beech, 32.1
Hornbean, 32.3
Plum tree, 32-5
Female oak, 32.6
Pear tree, 33.2
Birch, 34.1
Oak, 36.3
Pitch pine, 37.5
Alder, 38.4
Pine, 38.6
Fir, 38.9
Lime tree, 39.0
The experiments of Guyton {hew, that the conducing
\ Ibid. vol. power of charcoal is to that of fine fand nearly in the
ixvi.p. 227. proportion of 2 to 3 J.
5. Fluid bodies, as well as folids, are condu&ors of Cakr
caloric; but they are found to conduct it fo flowly, -v
that it was at firft fuppofcd they did not poffefs thisp,^1
power at all, that is, that the caloric was not conduct-
cd from particle to particle in fluids, firoilar to what
happens in folid bodies. This opinion feemed to be fup-
ported by the nature and conftitution of fluids, in which
the particles have free motion among each other, fo
that when one fet of particles acquire an additional
quantity of caloric, their fpecific gravity is neceffarily
diminiflied, and they naturally change place with thofe
other particles of the fluid which have been lefs heat¬
ed and are eonfequently heavier. Thefe different ap¬
pearances which were obferved in the heating of fluids
led Count Rumford, who made many ingenious expe¬
riments on this fubject, to conclude, that fluids are
heated, or conduct caloric, in a. different manner from
folids. In a fpirit of wine thermometer, which was
placed in a window to cool,--he obferved the fluid in
the tub© m rapid motion. There were two* currents
going in- different diredtion^, the one afeending, anti
the other defeending. The defending current oc¬
cupied the fides of the tube, and the afeending current
the middle. The currents were ©wing to the change
in the fpecific gravity of the particles, which being
heated became lighter, and refc to the top ; the heavier
particles at the fame time defeended. The particles
which afeended having reached the fides or top of the
tube, gave out their caloric, became fpecifically hea¬
vier, and again fell to the bottom. The motion of the
currents was confiderably increafed by the application
of a cold body to the fides-of the tube. The count al-
fo repeated the experiment with linfeed oil, and alfo
with water, in the latter of which he diffolved potafli,
to bring its fpeeific gravity to that of amber, fmall
pieces of which he introduced, to obferve the currents
more diftin&ly. Thefe experiments were followed with
the fame- refult. When the temperature was inereafed
or diminiftved, the currents were fet in motion, and only
ceafed when the temperature became equal to the fur-
rounding bodies.
In prosecuting this fubjecl, the count made other
experiments, to afeertain how far the heating or cool¬
ing of fluids is affected by a difference of fluidity. The
thermometer which he employed in thefe experiments,
has a copper bulb and a glafs tube, and it was filled
with linfeed oil. This was placed in the centre of a
brafs cylinder, and the fpace between the fides of the
cylinder- and the thermometer, was 0.25175. The
thermometer being fecured, the cylinder was filled with
2276 grs. of pure water, and held in melting fnow, till
the thermometer fell to 3 2°. It was then immerfed
in boiling water, and the thermometer rofe from 3 2°
to 200° in 597". The calorie wrhich raifed the ther¬
mometer muft have been communicated to it through
the water in the cylinder. The experiment was then
varied, by boiling 192 grs. of ftarchin the water in the
the cylinder. The thermometer now required ncp"
to rife from 3 2° to 200°. The fame experiment was
repeated by mixing 192 grs. eider down with the fame
quantity of water, and alfo with a quantity of ftewed
apples. The refult of thefe experiments will be feen # ^ ^
in the following tables
Effay 7-
Time