Mrs. (Jane Haldimand) Marcet.

Conversations on chemistry .. online

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w not/rozen.*

* To make this experiment striking, the glass containing' the wa-
ter and thermometer ou^ht to be kept perfectly still until the mercu-
17 sinks below the freezing point. Then agitate the water, or drop

317. How is latodt beat readered, sensible ?

318. How can water be made to freeze in a warm room ?

319. Why is a mixture of snow aad salt so intensely cold ?

320. Whea does water part with its latent heat ^

7* Digitized by Google



Mrs, B. That is always the case previous to the freezing^ of wa-
ter when it is in a state of rest Now it b^os to congeal, > and i^ou
msiy observe that the thermometer again rises to the freezing point.

CaroUne. It appears to me very strange that the thermometer
should rise the very moment that the water freezes ; for it seems to
imply that the water was colder before it froze than when in the
act of freezing*

Mrs, B, It is so ; and after our long dissertation on this circum-
stance, I did not think it would appear so surprising to you. Re-
flect a little, and I think you will discover the reason of it.

Caroline. It must be, no doubt,/f(e extraction of latent heat, at
the instant the water freezes, whicVraises the temperatur^

Mrs. B. Certainly : and if you now examine tne thermometer,
you will find that its rise was but temporary, and lasted only during
the disengagement of the latent heat — now that all the water is fro-
zen it falls "tprain, and will continue to fall till the ice and mixture
are all of an equal temperature.

Emihy. And can you show us any experiments in which liquids,
by being mixed, become solid and disengage latent heat P

Mrs. B. 1 could show you sc^veral, but you are not yet sufficient-
ly advanced to understand them well. I shall, however, try one,
which will afford j^ou a striking instance of the fact. The fluid which
you see in this phial consists of (a quantity of a certain salt called
muriat oflime^ dissolved in water. Now, if I pour into it a few drops
.of this other fluid, called sulphuric acid^ the whole, or very nearly
the whole, will be instantaneously converted into a solid mass^

Emily. How white it turns ! I feel the latent heat escaping for
the bottle is warm, and the fluid is changed to a solid white sub-
stance like chalk !*

Caroline. This is, indeed, the most curious eu||riment we have
seen yet. But pray what; is that white vapour which ascends from
• the mixture ?

Mrn. B. You are not yet enough of a chemist to understand that.
But take care, Caroline, do not approach too near it, for it has a
very pungent smell.

I shall show you another instance similar to that of the water,
which you observed to become warmer as it froze. I have in this
phiak^ solution of a salt called sulphat of soda, or Glauber's salt,
madeLvery strong, and corked up when it was hot, and kept without

into it a small piece of ice, and it instantly shoots into crystals, and
the thermometer rises.— C.

^&he sulphuric acid by its stronger affinity for the lime, takes it
from^he murioHc acid, unites with it, and forms «u/pAa<e of lime. —
Tl^ solidity is owing to' the insolubility of this last substanc6 in wa-
ten) The experiment succeeds well, if the water is saturated with,
th^^uriatc.— C.

32 !• Why does water become colder before freezing than it is in
the act of freezing ?

322. What example can you give of liquids becoming solid, by
being mixed, and disengaging latent heat f

324. HowU thu effect aecowUed/or in the note ?

324. What other instance of th^ extricatioii of latei|t heat is giv-
en, and how is it produced > Digitized by C^OOg I


agiUtion till it became cold, as yoa may feel the phial is. Noir
when 1 take out the cork and let the air fall upon it, (for being clo-
sed when boiling, there was a vacuum in the upper part,] observe
that the salt will suddenly crystallizej

Caroline. Surprising ! how beauUAilly the needles of salt hav^
shot through the whole phial !

Mrs, B Yes, it is very remarkable ; but pray do not forget the
object of the experiment. Feel how warm the phial has become by
the conversion of part of the liquid iuto a solid

Emily, Quite warm, I declare! this is a moat curious experiment
of the disenfijagement of latent heat. ^

Jdrs. B.rthe slaking of lime is another remarkable instance of
the extricatibn of latent hea7\ Have you never observed how quick-
lime smokes when water i»^ured upon it, and how much heat it
produces ?

Caroline, Tes ; but I do hot understand what change of state
takes place in the lime that occasions its gfiving out latent heat ; for
the ouick-lime, which is solid, is (if I recollect right) reduced to
powder by this operation, and is, therefore, rather expanded than
condensed. ^

J^rs. B, ut is from the water^ not (he lime, that the latent heat
is set free, xhe water incorporates with, and becomes solid in the
lime ; in consequence of which the beat, which kept it in a liquid
state is disengaged, and escapes in a sensible form.

Carolinf. I always thought that the heat originated in the lime.
It seems very stran{;e that water, and cold water too, should con- '
tain so much heat.

Emily, After this extrication of caloric the water must exist in a
state of ice in the lime, since it parts with the heat which kept it li-

Jtrs. B. It cannot properly be called ice, since ice implies a de-
gree of cold, at least e^ual to the freezing point Yet, as water, in
combining with lime, gives out more heat than in freezing, it must
be in a state of still greater solidity in the lime than it is in the form
of ice ; and you may have observed that it does not moisten or liqui-
fy the lime in the smallest degree.

Emily. But, Mrs. B. the smoke that rises is white ; if it was on-
ly pure c^alooc which escaped, we might feel, but could not see it.

Jtfr*. jB./frhis whit^ vapor is formed by some of the particles of
lime in a state of fine dust, which are carried. off by the calori^

Emily. In all changes of state, then, a body either absorbs or
disengages latent heat ?

Mrs. B, Ydu cannot exactly say absorbs latent heat, as the heat
hecomes latent only on beitag confined in the body ; but you may
''^y* generally, that bodieson passing from a solid to a liquid form,
y from the liquid state to\hat of vaporAabso^ heat ; and that
when the reverse takes place heat is disefl^agedT^

♦ This rule, if not universal, admits of very few exceptions.

325. What other instance is mentioned of the extrication of la-
tent heat !

326. Whence proceeds the heat in the slaking of lime i*

327. Why is the smoke that rises in the slaking of lime, white ?

328. When do bodies absorb beat ?

When is beat disengaged ? ^.^,.^^, ,^ Google


Emily. We can now, 1 think, account for the ether boiling, and
the water freezing^ in vacno, at the same temperaturcf

•Mr*. B, Let me hear how ^ou explain it.

Emily. The latent heat which the water g^re out in freezing,
was immediately absorbed by the ether, during its conversion into
vapor ; and therefore, from a latent state in one liquid, it passes
into a latent state in the other. *

«A/r4. B. But this ooty partly accounts for the result of the ex-
periment ; it remains to be explained why the temperature of the
ether, while in a state ofebulhtion, is brought down to the freezing
temperature of water.-At is because the ether, during its evapora-
tion, reduces its own teiqperature, in the same proportion as that of
the water by converting its free caloric into latent heat ; so that
though one liquid boils, and the other freezes, their temperatures
remain in a state of equilibrium!%

Emily. But why does not water, as well as ether, reduce its own
temperature by evaporating ?

Mrs. B. The fact is, that it does, though much less rapidly than
ether. Thus, for instance you may often have observed, in the heat
of summer, how much any particular dpot may be cooled by water-
ing, though the water used for that purpose beas m arm as the air
itself. Indeed^ much cold may be producedVSy the mere evapo-
ration of waterj^at the inhabitants of India, b^vailing themselves
of the most favSrable circumstances for this process which their
warm climate can afford, namely, the cool of the night, and situa-
tions most exposed to the night breeze, succeed in causing water
to freez&though the temperature of the air be as high as 60 de-
grees. (The water is pot into shallow earthen trays, so as to expose
an extensive surface to the process of evaporation, and io the morn-
ing, the water is found covered with a thin cake of ice^ which is
collected in sufficient quantity to be used for purposes of/luxury.

Caroline. How delicious it must be to dnnk liquids so cold in
those tropical climates ' But, Mrs. B. could we not try that exper-
iment ?

Mrs. B. If we were in the country, I have no doubt but that we
should be able to freeze water, by the same means, and under sim-
ilar circumstances. But we can do it immediately, upon a small
scale, in tIHa very room, in which the thermometer stands at 70 de-
grees. For this purpose we need onlymlaee some water in a little
cup under the receiver of the air-pumpVFig. 8, and exhaust the air
from it. What will be the consequence, Caroline ?

Caroline. Of course the water will evaporate more quickly, since
there will no longer be an atmospheric pressure on its surface ;
but will this be sufficient to make the water freeze P

Mrs, B. Probably not, because the vapor will not be carried off

t See page 65.

329. Why does water freeze and ether boil in vacuo ?

330. Why does the ground become cooled h^ watering in sum-
mer, though the water used be as warm as the air itself f

331. How is ice often produced in India, where the temperature
is not below 60 degrees ?

332. How is water made to freeze under a glass receiver, as il-
lustrated in figure 8 ? n \

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fast enough; but this will be acconiplished without difficulty if we
introduce into the receiver, Fig 8, in a saucer, or other large shal-
low vessel some strong sulphuric acid, a substance which has a
^reat attraction for water, whether in the form of vapor or in the
liquid state. This attraction is such that the acid will instantly ab-
sorb the moisture as it rises from the water, so as to m»ke room for
Fig. 8.

9. The air poap and neeirer for Mr. L.esli«'t t xperiment C. a Micer with lulf hurit?
■eid. B. % g]uM or atrthBn ctip contuidag ■nMtn, D. aiUnd for th« cup with its !•;■ made •!'
CUaa. A. A T^armoneter.

the formation of fresh vapor ; this will of course hasten the pro-
cess, and the cold produced from the rapid evaporation of the wa-
ter, will in a few minotes, be sufficient to freeze its surface^f We
shall now exhaust the air from the receiver. /

Emily. Thousarfds of small bubbles already arise through the
water from the internal surface of the cup ; what is the reason of

Mrs. B. These are bubbles of air which were partlv^ttached to
the vessel, and partly diiSfused in the water itself ; andmiey expand
and rise in consequence of the atmospheric pressure bemg^removedr)

Caroline, See, Mrs. B. ; the thermometer in the cup is sinking
fast : it has already descended to 40 degrees !

Emily The water now and then violently agitated on the surface
as if it were boiling ; and yet the thermometer is descending fast !

•Mrs, B. You may call it hoilinsr if you please, for this appear-
ance is, as well as boiling,/6wing io the rapid formation of vapof^
but here, as you have just Observed, it takes place from the surfac^
for It is only when heat is applied to the bottom of the vessel that the
vapor is formed there. —Now crystals of ice are actually shooting
all over the surface of the water.

* This experiment was first devised by Mr. Leslie, and has since
been modified in a variety of forms.

333. When the air is exhausted from the receiver why do bub-
bles rise through the water from the internal surface of the cup ?

334. Why does the water appear as if boiling ?^ ,,Google


Caroline. How beautiful it is ! The surface is now entirely fro-
zen—but the thermometer remains at 32 deg^rees.

J\lr8. B. And so it will, conformably with our doctrine of latent
heat/until the whole of the water b^ frozen^ but it will then again
beg-in- to descend lower and lower,^ consequence of the evapora-
tion which goes on from the surface of the ice!\

Emily, This is a most interesting experimCTt ; but it would be
still more strikiog if no sulphuric acid were required.

•Mrs. B. I will show you a freezing instrument, contrived by Dr.
WoilastODi upon the same principle as Mr. Leslie's experiment, by
which water may be frozen by its own evaporation alone, without
the assistance of sulphuric acid.

Fig. 9.

Thistube, which,asyou ^'' w»i»"t<"»'« crjophon*

see is terminated at each
extremity by a bulb, one
of which is half full of wa-/
ter,is internally perfectly^
exhausted of air; the con-

sequence of this is, that the water in the bulb, is always much dispo-
sed to evaporate. This evaporation, however, d^s not proceed suf-
ficiently fast to freeze the water; but if the empty ball be cooled
by some artificial means, so as to condense quickly the vapor which
rises from the water, the process may be thus so much promoted as
to cause the water to freeze in the other ball. Dr. Wollaston has
called this instrument Cryophoms.

Caroline. So that cold Pig. jq.

seems to perform here
the same part which the
sulphuric acid acted in
Mr. Leslie's experi-

J^rs. B. Exactly so ;
put let us try the ex-

Emily. How wiil you
cool the instrument ?i
You have neither ice"
nor snow.

Mrs. B. True ; but
we have other means of
effecting this.'" You re-
collect what an intense
cold can be produced ^

bv the evaporation of e- P« 8. (Fit . IC) Dr. Mawefi mode of mlBg the Cryopbo-
«1.A..;» «« r«K».te*A^ ^« ^ "■• Wo. I, and 2, the differtnt parn of No. 3,

tner m an exnausted re- ,«„ separate^

ceiver. We shall in- /

'*' This mode of making the experiment was proposed, and the
particulars detailed, by Dr. Marcet, in the 34th toI. of Nicholson's
Journal, p. IIQ.

33J. How long in this experiment will the thermometer remain
at 32 degrees, or at the freezing point ?

336. Why will it begin and continue to descend as soon as ail the
water is frozen? 337. What isthe object of figures 1,2. and 3.'

** )ogre


close the bulb in this little bag^ of fine flannel, (Fi^. 1.) then soak
it in ether, and introduce it inro the receiver of the air-pump. (Fig.
3.) For this purpose we shall find it more convenient to use a crjo-
phorus of this shape, (Fi^. 2.) as its elongated bulb passes easily
througb a brass plate which closes the tup of the receiver. If we
now exhaust the receiver quickly, you will see in less than a minute,
the water freeze in the other bulb out of the receiver.

Emily, The bulb already looks quite dim, and small drops of wa-
ter are condensing on its surface.

Caroline. And now crystals of ice slioot all over the water. This
b, indeed, a very curious experiment ! ^

Mt9. B. You will see, some other day, that,A>y a similar method,
eren quicksilver may be frozen^ But we canni»t at present indulge
in any further digression. /

Having advanced so far on the subject of heat, I may now eive
you an account of the calorimeterySn instrument invented by La-
voisier, upon the principles just exUained, for the purpose of esti-
mating the specific heat of bodies jTlt consists of a vessel, the inner
surface of which is lined with ice, toSs to form a sort of hollow globe
of ice, in the midst of which the body, whose specific heat is to be
ascertained, is placed!^ The ice absorbs caloric from this body, till
it has brought it down to the freezings point ; this caloric converts
into water a certain portion of ice which runs out through an aper-
ture at the bottom of the machine ; and the quantity of ice changed
to water is a test of the quantity of caloric which the body has given
out in descending from a certain temperature to the freezing point.
Caroline. In this apparatus, I suppose, the milk, chalk, and lead,
would melt different quantites of ice, in proportion to their differ-
ent capacities for caloric.

Mrs. B. Certainly ; and thence we are able to ascertain, with
precision their respective capacities for heat, /cut the calorimeter
affords us no more idea of the absolute quantky of heat contained
in a body, than the thermometei^ for though by means of it we
extricate both the free 'and combined caloric, yet we extricate
them only to a certain degree, which is the freezing point ; and we
know not how much they contain of either helow that point.

Emily, According to the theory of latent heat, it appears to me
that the weather should be warm when it freezes, and cold in a thaw ;
for latent heat is liberated from every substance that it freezes, and
such a large supply of heat must warm the atmosphere ; whilst du-
ring a thaw, that very quantity of free heat must be tsdcen from the
atmosphere and return to a latent state in the bodies which it thaws.
Mrt. B, Your observation is very natural ; but consider that in
^ frost the atmosphere is so much colder than the earth, that all the
caloric which it takes from the freezing bodies^is insufficient to raise
its temperature above the freezing point ; otherwise tlie frost must
cease. But if the quantitv of latent neat extricated does not destroy
the frostyft serves to raoderate the suddeooess of the change of tern*

338. Can quicksilver he frozen ?

339. What is the calorimeter, and what is its vie ?

340. Of what does it consist ?

341. Does the calorimeter indicate the abselate quantity of beat
<2ontained in a body ?

342. What effect is produced on the temperature of the atmos-
**cre,by the attraction of lateot heat from the winter frosts f


perature of the atmosphere, at the commencement both of frost and
of a thaw. In the first instance, its extrication diminishes the serer-
ity of the cold ; and in the latter, its absorption moderates the
warmth occasioned bv a thaw ; it even^ometimes produces a dis-
cernible chill, at the breaking up of frosu

Caroline, Bat what are the general cAses that produce those
sudden changes in the weather, especially from hot to cold, which
we often experience ?

J^rs. B, This question would lead us into meteorological discus-
sions, to which I am by no means competent. One circumstance,
howeyer, we can easily understand. When the air has passed over
cold countries, it will probably arrive here at the temperature much
below our own, and then it must absorb heat from every object it
meets with, which will produce a general fall of temperature.

Caroline, But pray, now that we know so much of the effects of
heat, will you inform us whether it is really a distinct body, or, as I
have heard, a peculiar kind of motion produced in bodies f

Mrs, B, As I have before told you, there is yet much uncertain-
ty as to the nature of these subtle agents^ But I am inclined to
consider heat not as mere motion, but asfe separate substance. —
Late experiments, too, appe^ to make it a compound body, con-
sisting of the two electriciti^\ and in our next conversation I shall
inform you of the principal fdlfts upon which that opinion is founded.



Mrt, B, Before we proceed further, it will be necessary to give
you some account of certain properties of electricity, which have
of late years been discovered to have an essential connexion with
the phenomena of chemistry.

Caroline. ItisELECTRiciTv^if I recollect right, which comes
next in our list of simple substances ?

Mrs, B, I have placed electricity in that list, rather from the ne-
cessity of classing it somewhere, than from any conviction that it has
a right to that situation ; for we are as yet so ignorant of its intimate
nature, that we are unable to determine, not onlyAyhether it issim-

glc or compound, but whether it is in fact a material agent; or, as
ir H. Davy has hinted, whether it may not be merely a property
inherent in matter^ As, however, it is necessary to adopt some
hypothesis for the*&pIanation of the discoveries which this agent
has enabled us to make, I have chosen the opinion, at present most
prevalent, which supposes the existence of ^o kinds of electricity,"^
distinguished by the name of ^sitive and fUgative electricity^

*rrhe electricity extricated by the metals is commonly called

343. What is heat now supposed to be ?

344. What subject is to be cimsidered in this conversation ?

345. What are the uncertainties as to the nature of electricity r
J*- U?.^ ™*"y lf»'»ds of electricity are there ?

347. What are they called? o,,:™.., Google


Caroline. Well, I most confess, I do not fed neairljr so interested
io a science in which so mnch uncertainty prerails as in those
which rest upon established principles. I nerer was fond of elec-
tricity, because, howerer beautiful and curious the phenomena it
exhibits may be, the theories, by which they were explained, ap-
peared to me so yarious, so obscure and inacfequate, that I always
remained dissatisfied. 1 was in hopes that the new discoveries in
electricity had thrown so great a light on the subject, that erery
thing respectiDg it would now hare been clearly explained.

Mrs. B, That is a point which we are yet far from baWng attain-
ed. But* in spite of the imperfection of our theories, you will be
amply repaid by the importance and novelty of the subject. The
number of new facts which have already been ascertained, and the
immense prospect of discovery which has lately been opened to us,
will, I hope, ultimately lead to a perfect elucidation of this branch of
natural science ; but at present yon must be contented with study-
ing the effects, and in some degree explaining the phenomena, with-
out aspiring to a precise knowledge of the remote cause of electri-

You have already obtained some notions of electricity ; io our
present conversation, therefore, I shall confine myself to that part
of the science which is of late discovery, and is more particularly
connected with chemistry.

It was a trifling and accidental circunv^oce y^ich first gave rise
to tliis new branch of physical science. MSalvani7)a professor of nat-
ural philosophy at Bologna, being engaged (about 20 years ago)
in some experiments on muscular irritability, observed, that when 4
piece of metal was laid ojq the nerve of a frog, recently dead, whilst
the limb supplied by that nerve rested upon some other metal, the
limb suddenly moved, on a communication being made between the
two pieces of metaij

Emily, How is tms communication made f

^rs, B. Either by bringinjp^ the two metals into contact, or by
connecting them by means of a metallic conductor. But without
sabjecting a frog to any cruel experiments. I can easily make you.
sensible of this kind of electric action mere is a piece of zinc,
{one of the metals 1 mentioned in the lisfof elementary bodies) —
put it under your tongue, and this piece of silver upon your tongue,
and let both the metals project a little beyond the tip of the tougue
—very well ; . now make the projecting parts of the metals tomch
each other, and you will instantly perceive a peculiar sensation.i

Emily. Indeed I did ; a singular taste, and I think a degree of
heat ; but I can hardly describe it.

Mrs. B. The action of these two pieces of metal on the tongue is,
I believe precisely similar to that made on the nerve of a frog, r
shall not detain you by a detailed account of the theory by which
Galvani attempted to explain this fact, as it was soon overturned

348. What is the difference between electricity and Oahanism ?

349. From whom did Galvanism receive its name ^

350. What gave rise to the branch of physical science called
Galvanism ?

351 . What simple experiment is mentioned,that can be made up^
OB the tongue to lUnstrate this subject f

Digitized by LjOOQIC



bj rabtequant ezperimeots, which prored that Golranifiii (the name
this new power had obtaiDed)wa8 nothinf more than electricitj.
(^dTani Bopposed that the virtue of this new agent resided in the
iienres of the froff\ but Volta, who prosecuted this subject with
much greater suc^ss, showed that tb&4>henomena did not depend
on the organs of the frog, but upon Ue electrical agency or the

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 10 of 43)