528 
fweat makes its appearance at the furface 
of the fkin. This watery fluid not only 
reflets very powerfully the calorific rays 
of the fun, which fall on its polifhed fur- 
face, but alfo, by its evaporation, gene- 
rates cold. When the fun is gone down, 
the fweat difappears ; the oi] at the fur- 
face of the fkin retires inwards; and the 
fkin is left in a fate very favourable to the 
admiffion of thofe feeble frigorific rays 
which arrive from the neighbouring ob- 
jects.” 
As refulting from other experiments 
the Count infers that the warmth of any 
kind of fubftance ufed as clothing, or its 
power of preventing our bodies from be- 
ing cooled by the influence of furrounding 
colder bodies, depends much on the poli/b 
of its furface: for upon careful examina- 
tion, it will be found that thofe fubfances 
which fupply us with the warmeft cover- 
ings, as furs, feathers, filk, &c. are not 
only fmooth, but highly polifhed: it will 
alfo be found, other circumftances being 
equal, that thofe fubftances are the warmett 
which are the fineft, or which ‘are com- 
pofed of the greateft number of fine polith- 
ed detached threads or fibres... The fine 
white fhining fur of a Ruffian hare is 
much warmer than coarfe hair; and fine 
filk, as fpuo from the. filk-worm, is 
warmer than the fame filk twifted together 
into coarfe threads. 
Formerly Count Rumford confidered 
the warmth of natura! and artificial cloth- 
ing as depending principally on the ob- 
-ftacle it oppofed to the motions of cold air 
by which the hot body is furrounded ; but 
by a patient examination of the. fubject, 
he is now convinced, that the efficacy of 
radiation is much greater than he had 
fuppofed it to be. Only a vety {mali 
part of the heat which a hot body appears 
to lofe, when it is cooled in the air, is in 
fa& communicated to that fluid ; a much 
greater portion of it being communicated 
to other furounding bodies at a diftance. 
If the hypothefis of the communication, 
or rather generation, of heat and cold b 
vadiation, as explained by this philofo- 
pher, be true, it will, he conceives, en- 
able him to illuftrate in a fatisfa&tory 
manner, what has been called the oz-coz- 
duding power of tran!parent fluids, with 
refpect to heat: for if heat be really com- 
muni¢ated, or excited, in the manner thus 
deferibed, it is evident that a perfectly 
tranfparent Buid can receive heat only at 
its jurfece; and confequentiy that heat 
cannot be propagated in fuch a fluid by 
ccmmunication from one particle of the 
fluid to another. 
Proceedings of Learned Societies, 
[ Nov. ly 
It cannot be afcertained whether any of 
the fluids with which we are acquainted 
are perfecly tranfparent : but pure waters 
and air, and moft other fluids which are 
tran{parent to the light, poffefs, probably, 
a high degree of traniparency in regard to 
calorific and frigorific rays; or give a free 
paflage to them when they have once pafi- 
ed their furfaces. An even or polifhed 
furface has been found to facilitate very 
much the reflection of the rays of light; 
the Count therefore afks, ** May it not 
alfo, in all-cafes, have an equal tendency 
to facilitate the reflection of calorific and 
frigorific rays ?”” ; 
In fome experiments with large cylin- 
drical veffels, where they were expofed 
naked to cool, in the air, their . furfaces 
were polifhed, and they were a long time; 
in cooling. But when the furface of the 
veflel was blackened, or covered with other 
fubftances, the veflel was found to cool 
more rapidly. A large proportion of the 
frigorific rays from the {urrounding colder 
bodies were, in the former cafe, reflected’ 
at the poliflied furface of the metallic 
veflel ; but, in the latter cafe, more of 
them were abforbed. 
When a large drop of water rolls abouts 
without being evaporated, upon the flat 
furface of a piece of red-hot iron, the furs. 
face of the drop is polifhed ; and the calo- 
rifle rays being mottly refle&ed the water 
is heated very little. Ifthe iron be le 
ot, the water penetrates the pores of the 
oxide which covers the metal—the drop. 
ceafes to have a polifhed furface—acquires 
heat very rapidly—and is foon evape- 
rated. . +k 
Jf a drop of water be placed on a clean 
and polifhed furface of a metal not fo 
eafily oxidable as iron, it will retain its 
{pherical form and polifhed furface, un- 
der a lower degree of temperature than on 
iron. oe) 
If a large drop of water be put care= 
fully into a clean. filver fpoon, previoufly 
heated very hot, the drop will fupport the 
heat a confiderable time; but, after the 
{poon has been fuffered to cool down near= 
ly to the temperature of boiling water, a 
drop of water will be evaporated inftan- 
taneoufly. Hence it is inferred that un- 
der high temperatures, air is attracted by 
metal, fo much more ftrongly than water, 
that even the weight of a drop of water 
is not fufficient to force away the ftratum: 
of air which covers, and adheres to, the: 
furface of a metal on which the drop re- 
pofes ; but, at lower temperatures, this 
does not feem to be the cafe. : 
We fhall conclude this part of the. 
Count’s 
