by Dr 
the conducting y faculty would over advan- 
»which might be derived abt ae 
the increase of the 
d power. Therevare several reasons which in- 
duce us to su that this latter paseo is affected 
Sa demtentabant ‘npusienie spebidebcaninedt 
Y nature of 
the su ce’ through its whole thickness. It does 
not that any experiments have yet been 
Seed pbgerwhich we can learn in what ‘oka thie bas 
lance: wer exists, Rumford’s general conclusions en- 
tirely coincide with Professor Leslie’s. He found that the 
_. same metallic cylinder, which, when filled with water, 
' required 55 minutes to cool through a certain number 
of degrees, oo rere ‘only 48 minutes when covered 
with a layer of glue, 836 when covered with linen, and 
34 when coated with a varnish of lamp black. Phil. 
Trans. 1804, p. 90. . 
When bodies are surrounded either by a fluid or by 
air during their cooling, the rate of the process will be 
considerably affected by the motions that take place 
among the particles of the investing medium: The 
action of a current of air in promoting refrigeration is 
| too. well known to require illustration ; and this must 
“evidently bean effect entirely independent of radia- 
‘tion, and in a great de at least of the conducting 
power of the air. ithout deciding the question, 
whether air be! an absolute non-conductor, it is render- 
ed» probable, both from circumstances that fall under 
daily observation, as well as from the direct experi-' 
ments of Rumford’ and others, that air is not'a good: 
conductor of heat, ‘and’ that one of the most effectual 
methods of retaining the temperature of a body, is to’ 
surround it with a stratum of air, so confined, that no 
internal motion can take place among its particles. 
ro. © Hitherto’heat has appeared only as a simple sub- 
sof stance, —— of being communicated through the 
co- parts of solid bodies with different degrees of velocity, 
according to their conducting power ; the presence of 
the heat being in every case marked by an expansion of 
‘the ‘solid body corresponding with the temperature. 
These views, however, though universally received both 
‘among chemists and natural philosophers, have been 
etely disproved by some recent experiments made 
by Dr Brewster on the p' ion of heat along glass, 
obsidian, semiopal, muriate of soda, fluor spar, alum, 
gum copal, rosin, horn, amber, tortoise shell, and other 
substances. We shall endeavour to lay before our rea- 
ders as brief and icuous an account as we can of 
the new properties of heat which were discovered in 
the course of these experiments, without anticipating 
any of the phenomena, which more properly belong to 
the’subject of opti 
cs. 
Let a vheror glass, ABDC, (Fig. 1.) having MN 
(Fig. 2.) for its section, be placed with its edge CHD 
jon a hot iron, or be in any way to a source 
of heat. The heat will be slowly communicated through 
the substance of the glass in the direction HGFE, and 
when the heat has reached E, the temperature will be 
greatest at H, diminishing gradually towards E. The 
glass will, therefore, eingho 3 to the common doctrine, 
be in a state of sion, as shewn in Fig. 3. being most 
dilated at H, and least dilated at E. The case, however, 
is very different. When the heat has entered the glass 
at the edge CD, the parts of the glass between the edge 
and the dark line at G are in a state of expansion, dimi- 
nishing towards G, and at the same instant: the parts 
of the glass between E and F are thrown into a similar 
state of expansion, while the intermediate portions be- 
tween F and G are thrown into a state of contraction, 
the lines AFB and CGD being the limits between the 
5 
: HEAT. 
. it will be contracted, as shewn in F 
suffered no mechanical change. 
which are shewn in Fig. 4. are 
fore the heat has reached the point F. When 
—— of the glass plate HE is very small, EF is 
to HG, and the contractions and expansions are al 
simultaneously produced ; but when HF is two or 
inches, EF is always much than HG, and the 
expansion between E and F is less distinctly seen, being 
spread over a greater space." From these results it follows, 
4. ‘That the heat expands an adjacent portion of the 
ss where it does exist in a sensible state. 
If the plate of glass ABDC, instead of being heated, 
is brought to an uniform ‘tem re by immersion in 
boiling water. and is then allowed to cool in the air, all Prater 
the effects, which we have described, are exactly re- ccuxxxrx. 
versed during cooling, as shewn in Fig. 5. the parts of 
the glass which were formerly contracted being now 
expanded, and vice versa. 
If the expansion between H and G is either increased 
or diminished by a variation in the source of heat, the 
other expanded and contracted ons suffer a similar 
change ; so that there must always be an equilibrum 
among the forces, by which these opposite mechanical 
states are produced. 
Let the plate of glass ABDC, Fig. 6. be cut with a 
diamond through its centre O, by a line MN, but not se- 
into two parts, and in this state let it be placed 
with its edge CHD on a hot iron as before. It will now 
exhibit the very same phenomena as those which we have 
already described, just as if it had never been touched 
with the diamond. When it is in this state of contrac. 
tion in the middle; and of expansion at the let it 
be suddenly broken ome at the line MON, and it 
will be found that each of its halves ABNM, MNDC, 
Fig: 6. are thrown into the same state of contraction 
an nsion, as when the plate was unbroken. Be- 
tween 
and f, g and O, O and f’, g’ and H, the 
will be exparited while between f and g, and f’ and g’ 
ig. 7. 
Hence, it is obvious, that the tolineeniont and expan- 
sion, or rather the phenomena wv which they are indi- 
cated, are produced by some fluid, which is decom- 
posed by heat into two fluids possessed of ite po- 
larity, in the same manner as the electric fluid of the 
tourmaline is decomposed by the action-of heat, or the 
etic fluid of a piece of iron by the action of a load- 
stone. All the phenomena, indeed, which are exhibit- 
ed during the passage of heat through a plate of glass, 
are precisely the same as the phenomena which take 
place in the communication of magnetical and electri- 
cal polarity ; and if we admit the existence of two fluids 
as the agents by which the phenomena of electricity 
and magnetism are produced, we are compelled to make 
the same admission in the case of the propagation of 
heat along glass. It is not unlikely that the ena 
may be produced by the mutual action of and la- 
tent heat. In our article Optics we shall have occa- 
sion to resume this subject at fn length, and to give 
drawings and descriptions of the various a pheno- 
mena which are’ produced by the singular state into 
which’ glass'and other bodies are thrown during the 
propagation of heat through their substance. hose 
Fig. 5. 
Fig. 6. 
Fig. 7. 
