ties altefation of gravity, the 
pet to each ther th i 
» the substance itself, producing a species 
Fr ae teem ge 
HEAT. 
; mime ll eae with re- 
i or part rises to 
surface, while the part sinks to the bottom. 
It is obvious, then, that heat may pane a to pass 
fluids in two ways; it may ei be trans- 
nitted from particle to particle, as is the case with so~ 
, or it may be conveyed by a change of gravity in 
c i circula~ 
tion among its parts. Many facts that fall under com- 
mon observation, shew that this circulation takes place 
in partially heated fluids, and prove that it is much 
easier to cause heat to pass upwards through them, 
than in the contrary direction; but the impossibility 
of heat being propagated downwards through fluids, 
was not admitted until after the experiments of Rum- 
ford: These experiments, which are perhaps the most 
ingenious of any which he ever performed, have been 
generally regarded as sufficient to prove the general 
principle, to which we have already alluded, that when 
any portion of a fluid is heated beyond the tempera- 
ture of the remaining part, it rises to the surface, in 
‘consequence of its comparative levity ; and if the heat 
be ome the bottom of the vessel, which is usual- 
ly the case, the successive portions of the fluid, as 
they receive the heat, rise in their turn to the upper 
part, until the whole acquires one uniform temperature. 
-On the contrary, if the heat be applied to the top of 
the fluid, it is only the upper stratum which becomes 
heated, the remainder retaining its former temperature. 
‘Rumford placed a portion of ice on a certain quantity 
of boiling water, and found that it was melted in about 
8 minutes ; but when the same substances and appa- 
ratus were used, except that the ice was fixed at 
the bottom of the water, several hours elapsed, and yet 
the ice was not completely thawed. To render the ope- 
ration still more striking, it was so managed that wa- 
ter was made to boil in the upper part of a cylindrical 
vessel, while the lower part remained full of ice. Es- 
says, Vol. ii, p. 241, et seq. ‘ 
‘We have already mentioned the experiments in which 
it was found that heat passed less readily through so- 
lids, when they were of a spongy or porous texture, 
than through those that were more dense and compact. 
The same kind of effect was found by Rumford to take 
place in fluids when substances were mixed with them, 
which, by their viscidity, or other analogous property, 
would prevent the motion of their particles, and thus 
put a stop to the circulation, which we have described 
as-conveying the heat through their different portions, 
He compared the time which was necessary for a quan- 
tity of pure water to change its temperature, by a cer- 
tain number of degrees, with the same bulk of water 
when mixed with down or with starch ; and he found 
the times to be as the numbers 6, 94, and 11. He af- 
terwards went on to shew, that by increasing the quan- 
tity of matter in the fluid, the difficulty of the passage- 
of heat through. it was proportionally increased. ‘,*;th 
part of its weight of down added to water, produced 
- a retardation equal to 75, while ,,th of the same sub- 
‘stance produced an effect equal to 95: (Essays, vol. ii. 
p. 221, et seq.) The general principle, therefore, ap- 
pears to be fully established, that fluids transmit heat, 
or suffer it to pass among their parts, in a different way 
from that in which heat is transmitted through solids, 
not by its being given off from one icle and recei- 
ved by another, but by all the particles coming suc- 
cessively to the source of caloric, and individually re- 
eeiving it from the heating body. Whatever impedes 
the intestine motions of the fluid, and prevents this cir- 
‘culation of its particles from taking place, stops the pas- 
4 
’ 
675 
sage of heat through it, and confines its effect to the 
portion which first received it. 
_ The facts ep forward by Rumford may be con- 
sidered as —— ecisively proving the general prin- 
ciple ; but it still been questioned, whether there 
is in fluids that absolute and complete non-conducting 
power which he attributes to them. Some experiments 
to prove that they really pees a small degree of a pro- 
conductin: wer, have been ormed by Dr 
ope, Mr Nicholson, Mr Murray, Thomson, and 
Mr Dalton. Their plan was to communicate heat to 
fluids in a direction different from that in which the 
supposed currents would act, as by applying it to the 
upper surface, and by their using every precaution to 
prevent its being carried downwards by the sides of 
the vessel containing the fluid, or by any other coun- 
teracting cause. The ex 
‘ot Mew 
—_—~ 
iments appear, upon the or only in » 
whole, to prove their position, and thus to modify the small de- 
conclusions of Rumford ; but perhaps in all of them 8*e- 
there may still be some sources of inaccuracy, which 
it is very difficult to guard against. The process of 
Mr Murray appears, upon the whole, to be the most un- Murray's 
exceptionable. 
whi 
ject of his experiment; for ice, as long as it remains 
unmelted, being a perfect: non-conductor of heat, the 
objection must be removed, which depends upon 
the conducting power of the vessel itself. The other 
arrangements made by Mr Murray were very ingeni- 
ous, and oppose to be well adapted for preventing the 
e of heat in any manner except through the ac- 
tual fluid ; yet heat seemed to be certainly transmit- 
ted from the upper surface to the lower part, so as 
to affect a thermometer fixed near the bottom: (Ni- 
He formed a hollow cylinder of ice, experi- 
served as a vessel in which to contain the sub. ments. 
cholson’s Journal, vol. i. p. 421.) If, then, we are brought- 
to conclude, that fluids possess some degree of con- 
ducting power, it next remains for us to inquire, what 
is the relative de in which it exists in different 
kinds of fluids? his, hewever, is a question which 
it is impossible for us to answer with any accuracy ; 
because we have no correct means of learning how far 
the communication of heat to them depends upon their 
proper conducting power, and how far upon the mo- 
tion between their particles. It may be inferred, from 
some experiments of Rumford’s, that mercury is a better 
conductor of heat than oil or water ; and this might be 
expected to be the case, It is natural to suppose, that 
mercury, so far as it is a fluid substance, conveys calo- 
ric, by internal motions am its particles, like other 
fluids, yet that it still retains its metallic property, and 
may conduct it from particle to particle. 
The subject of the conducting power of liquids has 
Brewster's 
very recently been investigated by Dr Brewster, who experi- 
has removed every doubt that could remain respecting 
the existence of this property in fluid bodies, and has 
pointed out a simple method by which the conducting 
ments on 
the con- 
ducting 
er of 
power of all transparent fluids may be rendered visible guids. 
to the eye, and easily measured, without the aid. even 
of a thermometer. The apparatus by which these ex- 
periments were made, is shewn in Plate CCLXXXIX. prare 
Fig. 8. where ABCD is a tin vessel about a foot long, ccLxxxrx. 
having two openings E, F, in which two pieces of Fig. 8. 
parallel glass are firmly cemented. A heated iron GH 
is suspended by a wire LM, and a stand RT, having a 
small circular aperture P, about one-eighth of an inch 
in diameter, is placed. very. near the plate of glass F, 
so as to be seen distinctly by an eye at E. This aper- 
ture is capable of being raised and depressed by a finger 
screw S, The vessel is now filled with water, or any 
other fluid, nearly to the top, and the eye being placed 
