Mrs. (Jane Haldimand) Marcet.

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which it has when cold ; but it is from ihef change which the clay
has undergone by having been heated, that the indications of this
instrument are derived. This change consists io a beginning fur
sion, which tends to unite the particles of clay more closely, thus
rendering it less pervious or sp^iogy.^

/Clay is to be considered as a spongy body, abounding in inter-
stVces or pores, from its having contained water when soft. These
interstices are by heat lessened, and would by extreme heat be en-
tirely obliterateoN

Caroline, An^ow do you ascertain he degrees of contraction
of Wedgwood's pyrometer ?

Jdrt, B. The dimensions of the piece of clay are measured by a
scale gpraduated on the side of a tapered s^roove, formed in a brass
ruler ; the more the clay is contracted by the beat, the further it
will descend into the narrow part of tlie tube.

Before we quit the subject of expansion, i must obsep^e to you,
that, as li(]uids expand more readiljr than solids, sofelastic fluids,
whether air or vapourNare the most expansible of all Bodies.

It may appear extraordinary, tha^ll elastic fluids whatever, un-
dergo the s^e degree of expansioirfrom equal argumentation of

Emily. I rappose, then, that all elastic fluids are of the same den-
sity. ^

Jiirt. B. Very far from it ; Ahey vary in density, more than ei-
ther liquids or solids^ The uniformity of their expansibility, which
at first may appear lingular, is, however, readily accounted for.
For if the different susceptibilities of expansion of bodies arise
trom their various degrees of attraction of cohesion, no such dif-

however cold, still contain caloric^ Thus mercury freezes at 40°
below zero, but still contains calollc, an^ ^o on. The rising and
fdling of the water signifies the greater or less quantity of free ca«
loric as indicated by the thermometer.— C.

* According to the calculations of Saussure, the temperature
necessary to melt this clay is 1575° Wedgwood, which is a degree
ofheatgreatlybeyondour common furnaces. It is therefore most
probable that the clay contracts at lower temperatures by the loss
of moisture. — C.

132. Do all hodiet hovatvtr cold contain caloric ?

^33. In what manner is it that cla^ appeart to contract by beat ?

134. What bodies are most expansible ?

135. Are all elastic fluids equally expanded from equal augmen-
tations of temperature ?

136. Are all elastic fluids of the same density ? . ,t

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ferencecan be expected^ elastic *flaids, sioee in these the attrac-
tion ofcohesion does notexist, their particles being on the contrary
possessed of an elastic or repulsi?e povrer^ they will therefore all
be equally expanded by equal degrees of oslonc.

Emily. True; as there is no power opposed to the ekpansive
force of caloric in elastic bodies, its effect must be the same in all
' of them.

•Mr». B, Let us now proceed to examine tie other properties of
(r^ caloric.

^ree caloric always tends to diffuse itself equall/i that is to say,
wOen two bodies are of different temperatures, the warmer gradu-
ally parts with its beat to the colder, till they are both brought to
the same ternperarure. Thus, /^hen a thermometer is appli^ to a
hot bod^', it receives caloric ; when to a cold -ooe, it commuicates
part of :ts own. caloric, and this communication continues until the
thermometer nnd the body arrive at the same temperatureS

Emily, (old, then, is nothing but a negative quality, stffiply im*
plyinjr the absence of heat. * ^ ^

Mrs. B. Not the total absence, bulp diminution of he^ fgr'ive
know of no body in which some calorre may not be discoVered^

Caroline, But when I lay my hand on this marble table, I feel
it positively cold, and cannot conceive that there is any caloric in
it. j^

Mrs. B. The cold you experiencewonsists in the loss of caloric
that your hand sustains io ao attemprto bring its temperature to
an equilibrium with the marble% If yon lay a piece of ice upon it,
you will fiudthat a contrary effect will take place ; the ice will be
melted by the heat it abstracts from the marble*

Caroline. Is'it not in this case the air of the room, which being
warmer than the marble, melts the ice ?

Mrs. B. The air certainl\ acts on the surface which is exposed
to it, but the table melts that part with which it is in contact
' Caroline. But why does caloric tend to an equilibrium P ^t cai;^
not be on the same principle as other fluids, since it has no weighO

Mrs. B. Very true, Caroline, that is an excellent objection.
You might also, with some propriety, object to the term eawlibri"
tim being applied to a body that is without weight ; but I know of
no expression that would explain my meaning so well. You must
eoDsider it, however, in a figurative rather than a literal sense : its
strict meaning is an equal diffusion. We cannot, indeed, well say
by what power it diffuses itself equally, though it is not surprising
that it should go from the parts which have the most to those which
have the least The subject is best explained by a theory sugges-

137. How then can their uniformity of expansibility be account-
ed for r
138 How does caloric tend to diffuse itself ?
139.. How is this illustrated by a thermometer ?
140 What is cold?

141. Do we know of any substance in which some caloric may
not be found ?

142. Why do some bodies feel cold if we lay ourbs^ upon them?

143. What objection is there to the term equihbrium, when
speaking of the equal diffusion of caloric ?

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ted by Professor Prevost of Geneva, jurhich is now, 1 believe, gen-
erally adopted. ^

AccordiDg to this theory /caloric is composed of particles per-
fectly separate from each other, e^ry one of which moves with a
rapid v^ocity in a certain direction^ These directions vary as much
as imag^ation can conceive, the result of which is, that there are
rays or lines of the^ particles moving* with immense velocity in
every possible direction. Caloric is thus universally diffused, so
that when any portion of space happens to be in the neighbourhood
of another, which contain^^ more caloric, the colder portion re-
ceives a quantity of calorific rays from the latter, sufficient to re-
store an equilibrium of temperature, ^his radiation does not only
take place in free space, but extends aJSo to bodies of every kinajf"
Thus you may suppose all bodies whatever, constantly radiating ca-
loric ; those that are of the same temperature g^ve out and absorb
equal quantities^ so that no variation of temperature is produced in

thenu but when one body contains more tree caloric than anoth-
er,KDe exchange is ^ways in favour of the colder body, until an
equilibrium is effected) this you find to be the case when the mar-

ble table cooled your nand, and again when it melted the ice.

Caroline. This reciprocal radiation su prises me extremely ; I
thought, from what you first said, that the hotter bodies alone
emitted rays of caloric which were absorbed by the colder ; for it
seems unnatural that a hot body should receive any caloric from a
cold one. even though it should return a rreat quantity.

Jdrs. B. It may at first appear so, butyt is no more extraordina-
ry than that a candle should send forth rays of light to the suo,
which, you know, must necessarily happen)

Carolinfi. Well Mrs. B., I believe that imust ^ve up the point.
But I wish I could aee these rays of caloric ; I should then have
gpreater faith in them.

Jlrs. B, Will you give no credit to any sense but that of sight ?
You may feel the rays of caloric which you receive from any body
of a temperature higher than your own ; the loss of (he caloric
you part with in return, it is true, is not perceptible ; for as you
gain more than you lose, instead of suffering a diminution, you
are really making an acquisition of caloric- ^t is, therefore, only
when you are parting with it to a body of a rower temperature,
that you are sensible of the seigation of cold, because you then sus-

tain an absolute loss of caloric!


^This is true when applied to inanimate matter. (But if a live an**
imal is exposed to a degree of heat above the tenrfyferature of its
own body, it has the power of resistance ; and though the heat be
100 4^rees above that of the animal, it scarcely affects its temper-

144. What is professor Prevost's theory of caloric ?

145. What remark respecting live animais is made in the note ?

146. Do all bodies constantly radiate caloric ?

147. If one body contain more free caloric than another, vvhat is
the consequence t /

148. Does a hot body r^eive caloric from a cold one ?

149. How does Mrs. B. answer the objection to the reciprocal
radiation of caloric between bodies of different temperature t

150. What occasions the sensation of cold? ^ i

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Emily. And in this case we cannot be sensible of the small
quantity of heat we receire in exchange from the colder body, be-
cause it serves only to diminish the loss.

Mrs, B. Very well, indeed, Emily. Professor Pictet, of Gene-
va, has made some rery interesting experiments, which prove not
only/bat caloric radiates from all bodies whaieyer, but that these
raysinay be reflected, according to the laws of optics, in the same
manner as lighy I shall repeat these experiments before yon, hay-
ing procured mirrors* fit for the purpose ; and it will afford us an
opportiinity of using the differential thermometer, which is partic-
ularly well adapted for these experiments.— i place an iron bullet,
>» Fig, 5.

Jlfr. PkUi^sfSpparatusfor the RfJUclionofHeaiA

[A A, md B B, CoDttTC Mirrcnvt fijud ou lUiids O, Hatted Bolkt, pke*d ia tfa* focus of th»
Jiinor A. D, TlMnnoncter* with iu balk pUccd im the xmm of tb* llinvr B. l 3 3 4, ny% 9 {
Ci||phc HMiiatiof from th« Bolkt. aod ftJIJHT oo ebe Bfirror A. 56 7 8, tb* mom raji rtflocted
from the Mirror A to the Mirror B. 9 10 1 1 13, tlio saiim njt rtflected bj the M invr B. to the

about two inches in diameter, and heated to a degree not sufficient
to render it luminous, in the focus of this large metallic concave
mirror. The rays of heat which Tall on this mirror are reflected,
agreeably to the property of concaye mirrors, in a parallel direc-
tion, so as to fall on a similar mirror, which, you see, is placed op-
posite to the first, at the distance of about ten feet ; tnence the
rays conyerffe to the focus of the second mirror, in which I place
one of the bulbs of this thermometer. Now, obserye in what man-
ner it is affected by the caloric which is reflected on it from the
heated bullet — ^The air is dilated in the bulb which we placed in
the focus of the mirror, and the liquor rises considerably in the op-
posite leg.

* Mirrors made of common tinned iron show this experiment
yery well. They may be 10 or 12 inches in diameter, ana about 2
inches deep. They must be planished with a hammer haying a
eonyex face, and afterwards polished with a piece of backskin, and

151. What do Professor Pictet*s experiments on caloric prore ?

162. What is the object of figure 5 ^

153. How ffould you explain the experiment repreiented iD.tbts


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Emily. But would not tbe same effect take place, if the rays of
caloric from the heated bullet fell directly on the thermometer,
without ^e assistance of the mirrors ?

Mrs, B' The effect would in that case be so trifling', at the dis-
tance at which the bullet and the thermometer are from each oth-
er, that it would be almost imnerceptible. The mirrors, you
know, gi^atly increase the effect,J|K,v collecting a lar^e quantity of
rays in a focusj^lace your hand in'the focus of the mirror, and you
will find it mtlch hotter there than when you remore it nearer to
the bullet.

Emily. That is ycry true ; it appears extremely singular to feel
the heat diminish iti approaching tne body from w^ich it proceeds.

Caroline And the mirror which produces so much heat, by con-
remng the j^ys, is itself quite cold.

iMru B.^he same number of rays that are dispersed orer the
surface ofthe mirror are collected by it into the focu» but if you
consider bow large a suHace the mirror presents to toe rays, and,
consequently* how much they are diffused in comparison to what
they are at tne focus, which is a little more than a point, 1 think you
can no longer wonder that the focus should be so much hotter than

Q^e principal use of the mirror in this experiment is, to proye
that^e calorific emanation is reflected in the same manner as

Carolme. And the result, I think, is yery conclusiye.

Jlfrt. B. The experiment may be repeated with a wax taper in-
stead of tbe bullet, with a yiew of separating the light from the
caloric. For this purpose a transparent plate of glass must be in-
terposed between the mirrors ; for light, you know, passes with
great facility through <rlass,'^ whilst the transmission or caloric*is
almost wholly impeded %y it; We shall find, howeyer, in this ex-
periment, that some few of the calorific rays pass through the glass
together with the light, as the thermometer rises a little ; but, as
soon as the glass is remoyed, and free passage left to the caloric, it
will rise considerably higher.

EmUv^ This* experiment, as well as that of Dr. HerschelFs,
proyes^at light and heat may be separated^ for in the latter ex-
perimefit the separation was not perfect, any inore than that of Mr.

CaroKfM. 1 should like to repeat this experiment, with the dif-
ference of substituting a cold body instead of a hot one, to see
whether cold would not be reflected as well asl^at. \

Mr$. B. That experiment was proposed towlr. PictetTby an in-
credulous philosopher like yourself, and he immediately 4ried it by
substituting a piece of ice in the place of a heated bullet.

CaroUne* Well, Mrs. B., and what was the result ?

154. Why do mirrors increase the effect in this experiment ?

156. Why does a metallic mirror feel cold when placed before

166. What is the use of the mirror in the experiment ?

15T. What wbetanee ahnort wholly impedes the traoBmissioo of

168. Wh«t^k>es^iipfeye?

159. What phUotophereuppoied that cold migfat be reflected?


Mrs. B. That ire shall see ; 1 hare procured some ice for the
Emily, The thermometer fallsconsiderably !
Caroline. And does not that prore that cold is not merely a neg'
alwt quality, iroplyiog: simply an inferior deg^ree of heat f The
cold must be ponlwe^ since it is capable of reflection.

Mrs. B. So it at first appeared to VI r. Pictet ; but upon a little
cs^osideration he fouj^ that it afforded only an additional proof of
foe reflection of heat) this I shall endegyour to explain to you.
^ According to Mr. rrqrost's iheory/we suppose that all bodies
whaterer radiate caloricjl the thermometer used in these exper-
iments, therefore, emits calorific rays in the same manner as any
other substance. When its temperature is in equilibrium with that
of the surrounding bodies, it receires as much caloric as it parts
with, and no change of temperature is produced. But when we
iqtroduce a bodr of a lower temperature, such as a piece of ice,
which parts with less caloric than it receires, the consequence is,
that its temperature is raised, whilst that of the surroundmg bodies
is proportionally lowered

Emily. If, for instance, I was to bring a large piece of ice into
this room, the ice would in time be melted, by absorbing caloric
from the genera] radiation which is going on throughout the room ;
and as it would contribute rery little caloric in return for what is
absorbed, the room would necessarily be cooled by it.

JUrs. B. Just so ; and as iu consequence of the mirrors, a more
considerable exchange of rays takes place between the ice and the
thermometer, than between these and any of the surrounding Bod-
ies, the temperature of the thermometer must be more lowered than
that of any other adjacent object.

Caroline, I confess I do not perfectly tmderstand your explan-

Mrs, B. This experiment is exactly similar to that made with
the heated bullet ; for,nf we consider tne thermometer as the hot
bodr (which it certainl^ is in comparison to the ice,) jou may then
easily understand that it is by the loss of the calorific rays which
the tnermometer sends to the ice, and not by any cold rays receir-
ed from it, that the fi\\ of the mercury is occasioned ; for the ice,
far from emitting rays of cold, sends forth ra^ of caloric, which
diminish the loss sustained by the thermometer^

Let us say, for instance, that the radiation df the thermometer
towards the ice is equal to 10,, and that of the ice towards the
thermometer to 20 ; the^exchange in faror of the ice is as 20 is
tolO, or the thermometer absolutely loses 10, whilst the ice gains

Carole. But if the ice actually sends rays of caloric to the
thermometer, must not the latter Ml still lower when the ice is re-
mored? • ^x'

Mrs. B* ^o\(ior the space which the ice occupies, admits rays
— - _ .

160. What did his experiment prore ?

IQI . What is probaUe respecting the use of the thermometer in
this eKperimeot, aecordtng to Mr. Prerost^s ^eory ?

162. What similarity is there between this experiment ahd that
of ^ faei^ bciHet ?

163. Since the ice tends rays of caloric to tfae^eniicmieter,wnt
not the theimometer fidl if the ice ia remorcd '^^^^^^^ ^^ Google


from all the surroaodingr bodies to pass through it : and those betog
pf the same temperature as the thermometer, will not affect it, be-
cause as much beat now returns to the thermometer as radiates
from itl^

Carolme, I must confess that you ba^e explained this in so batis-
factory a manner, that I cannot help being conirinced now that cold
has no real claim to the rank of a positive being

Jtfr«. B. Before 1 conclude the subject of radiation, I must ob-
serve to you, thalyCliffereot bodies for rather surfaces,] possess the
power 01 radiatmg^caloric in very aifferent d^preesS

Some curious experiments have been made bv<^ VI r.' Leslie on
this subject, and it was for this purpose that he invented the differ-
entiail thermometer ; with its assistance he ascertained that^lack
surfaces radiate most, glass next, and polished surfaces the least of

imily. Supposing these surfaces, of course, to be all of the same

Mrs. B. Undoubtedly. I will now show you the very ingen-
ipus apparatus, by means of which he made these experiments.
G^his cubical tin vessel, nr canister, has each of its sides externally
covered with different materials; the one is simply blackened ;
the next is covered with white paper ; the third with a pane of
^lass, and in the fourth the polished tin surface remains uncovered-
We shall fill this vessel with hot water, so that there can be no
doubt but that all its sides will be of the same temperature. Now
let us place it in the focus of tme of the mirrors, making each of
its sides front it in successionX We shall beg^n with Uie blaok
surface.* ^

Caroline, It makes the thermometer which is in^e focus of the
other mirror rise considerably. Let us turn the paper sur&ce
towards the mirror. The thermometer falls a little, therefore of
course, this side cannot emit or radiate so much caloric as the black-
ened side.

Emily, This is verv surprising ; for the side^ are exactly of the
same size, and must be of the same temperature. But let us try
the glass surface.

jSr8, B. The thermometer continues falling, and with the plain
surface it falls still lower ; these two surfaces therefore radiate less
and less.

Caroline I think I have found out the reason of this.

Jdrs. B, I should be very happy to hear it, for it has not yet, (to
my knowledge) been accounted n>r.

^* The radiating newer of different surfaces may be shown thus.
vTake a common half pint tin cup, scour one side bright, and paint
or smoke the other blaok. Place this in the focus of the mirror,
and the thermometer will rise or fsdl as its sides are changedlVC.

164. Why will it not ?

165. Do all surfaces radiate caloric in equal demes ?

166. What surfaces radiate roost caloric, and what ones least i

167. What illustration is given of the different radiations of dif-
ferent surfoces?

.168. flow U U Haiedin the noU, ^tU the radiating power of dif
ferent iurfaceemt^f heefunm?

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rSES GAKttlC. 45

Caroline. The water within the ▼estelmdiially cools, and the
thermometer m consequence gradually falls.

Jlrs. B, It is true that the water cools, hut certainly in much less
proportion than the thermometer descends, as you will perceive if
you now change the tin surface for the black one.

Caroline. I was mistaken, certainly, for the thermometer rises
again now that the black surface fronts the mirror

JIrs B. And yet the water in the vessel is still cooling, Car-

Emily. I am surprised that the tin surface should radiate the
least caloric, for a metalhc vessel filled with hot water, a silver
tea-pot for instance, feels much hotter to the hand than one of black

Mrs. B. (Jhat is owing to th^ different power which various bod-
ies possesfi Wr conducting calon<^va property which we shall pre-
sently examine. Thus, although 4r metallic vessel feels warm to
the handf^vessel of this kind is known to preserve the heat of the
liquid within, better than one of any other materials^ it is for this
reason that silver tea-pots make better tea than thoil of earthen-

Emily. According ro these experiments, lurht coloured dresses,
in cold we ather, should keep us warmer thanrolack clothes, Wi»'
^|SMl(il!h radiate »o much more than the fonher)

Mrs. B And that is actually the case. ^

Emily. This property, of different surfaces to radiate in different
degrees, appears to me to be a,t vwance with the equilibrium of
caloric ; since it would imply jhaffthose bodies which radiate most
must ultimately become coMesT^ ^

Suppose that we were to vary this experiment, by using two
metallic vessels full of boiling water, the one blackened, the other
not ; would not the black one cool the first ?

Caroline. True ; but when they were both breu^ht down to the
temperature of the room, the interchange of calonc between the
canisters and the other bodies of the room being then equal, their
terof>erature would remain the same* ^

Emily. I do not see why that should be the case ; for^ different
surfaces of the same temperature radiate in different degrees when
heated, why should they not c^tmue to do so when cooled down
to the temperature of the roon^

Mrs. B. You have started ^difficulty. Emily, which certainly
requires explanation, ft is found by experiment, that the power
of absorption corresponds with, and is proportional to, that of radia-
tion : so that under equal temperatures, bodies compensate for the

169 Wbv will a silver tea-pot or any metallic vessel filled with
hot water, feel much hotter to the hand than one of black earthen
ware ?

170. Why will a silver tea-pot make better tea than an earthen

171. Why is a light colored dress warmer than a black one in
winter ?

\TZ. And why a light colored one colder than a black in the
summer ?

173. What difficulty is meQtkmed respecting the above theory of
the radiation of calonc ?

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greater loss they sustain io consequence of their greater radiation
by their greater absorptioo ; so that if you were to make your ex-
periment io an atmosphere heated like the canisters, to the tem-
perature of boiling water, though it is true that the canisters would
radiate in different degrees, no change of temperature would be
produced io them J^ecause they would each absorb caloric in pro-
portion to their re^»ective radiation^

Emily. But would not the canirfers of boiling water also absorb
caloric m difTereni deforces in a room of the common temperature ?
Mrs. B Undoubtedly they would But the various bodies io
the room would not, at a lower temperature, furnish either of the
canisters with a sufficiency q$ calonc to compensate for the loss
they undergo; for, suppose a black canister to absorb 400 rays
of caloric, ivhilst the metallic <>ne absorbed only ^00; yet if the
former radiate 800, whilst the latter radiates only 400, the black
canister wil' be the first ceoled down to the temperature of the
room. But from the moment the equilibrium of temperature has
taken place, the black canister, both receiving and giving out 400

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 5 of 43)