Mrs. (Jane Haldimand) Marcet.

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time, from beins^ affected by the external cold.

Caroline. And the sea does not freeze, I suppose, because its^depth
is so great, that a frost never lasts long enough to bring down the
temperature of such a gpreat body of water to 40 degrees ?

Mrs, B. That is one reason why the sea, as a large mass of water,
does not freeze. But, independently of this, ^alt water does not
freeze till it is cooled much below|32 degrees, ahd with respect to
the law of condensation, salt water is an exceptipn, as it condenses
even many degrees below the freezing pointN (When the caloricof
fresh water, therefore, is imprisoned \y the ice on its surface, the
ocean still continues iJirowing off heat into the atmospher^ which
is a most signal dispensation of Providence to moderate tlmntensi-
ty of the cold in winter.

Caroline, This theory of the non-conducting power of liquids,
does not, I suppose, hold good with respect to air, otherwise the at-
mosphere would not be heated by the rays of the sun passing
through it ?

Mrs.B. Norisitheatedinthat way. The pure atmosphere is a
perfectly transparent medium, which neither radiates, absorbs, nor
conducts calonc, but transmits the rail of the sun to us without in
any way diminishing their intensity. \Tbe air is therefore not more
heated* oy the sun's rays passing throbeh it, than diamond, glasa,
water, or any other transparent mediuw' ^ v

Caroline, That is very extraordinary f Are glas^ windows not
heated then by the «an shining on them ?

^oshow still better that transparent media are not heated by
the tvys of the sun, throw the focus of a burning lens into a veatel
cf clear water. No effect on the temperature will be produced ;
but if an opake body, as a piece of cork be introduced under the
focus, the water at this point instantly begins to boil>— €.

313. Why does water first freeze at the surfoce ? ^
S14. Why does not the surfiK^ of the sea freeze? '
215. What moderates the intensity ^ the cold in winter ?

516. Is the atmosphefe heated by the rays of the sun passing
through it ?

517. }VM experimeni mmUnud in the nole, pr$Hi thai iran^a-
rent media are not heated by the rifys tf the tun} by Google


JIfrf . jB. Ko ; not if the glass be perfectly transpareDt. A most
conyincing proof that glass traosmits the rays of the sun without
being heatea by them, is afforded by the burning lens, which by con-
verging the rays jto a focus will set combustible bodies on fire,
without its own temperature being raised.

Emify. Yet, ^rs. B., if I hold a piece of glass near the fire, it is
almost immediately warmed by it ; the glass therefore must retain
some of the caloric radiated by the fire. Is it that the solar rays
alone pass freely through the g^lass without paying tribute ? It seems
unaccountable that the radiation of a common fire should have
power to do what the sun's rays cannot accomplish.

Mrs, B. It is not because tneirays from the fire hayemore power,
. but rather because they haye less, that they heat glass and other
transparent bodies. It is true, however, that as you approach the
source of heat the rays being nearer each other, the heat is more
condebsed, and can produce effects of which the solar rays, from
the great distance of their source, are incapable.— Thus we should
find it impossible to roast a joint of meat by the sun's rays, though
it is so easily done by culinary heat Yet Acaloric emanated from
burning bodi^which is commonly called etamary hetit, has neither
the intensity oor the Telocity of solar rays. All caloric, we have
said, is sapposed to proceed originally from the sun : but after hav-
ing been incorporated with terrestrial bodies, and ag[ain given out
by them, though its nature is not essentially altei^, it retains nei-
ther the intensity nor the velocity with which it first emanated from
that luminary ; it has therefore not the power of passing Ibrough
transparent mediums, such as glass and water, without bieing par-
tially retained by those bodies.

JEms/y. I recollect ^9Lt in the experiment on the reflection of
heat/tlie rlass screen which you interposed between the burning
taper- and mirror, arrested the rajs oc caloric, and suffered only
those of light to pass through iU

Caroline. Glass windows* then, thouf^ they cannot be heated by
the sun shininr on them, may be heated internally by a fire in the
room ? But, Mrs. B., since the atmosphere is not- warmed by the
sdar rays passing through it, how does it obtain heat ? for all tile
fires that are burning on the surface of the earth would contribute
very littletowards warming it.

jSms/y.fThe radiation of heat is not confined to burning bodies ;
for all bodies, you know, have that property : therefore, not only
every thingnipon the surface of the earth, but the earth itself, must
radiate hesT^and this terrestrial caloric, not havinjg, T suppose, suf-
ficient power to traverse the atmosphere, communicates heat to it.

•Xfrr. B. Your mference is extremely well drawn, Emily : but the
foundation on which it rests is not sound : for the fact is, that terres-
trial or culinary heat, though it cannot pass through the denser
transparent mediums, such as glass or water, without Toss, traverses
the atmosptneire completely ; so that all thefbeat which the earth
J - ■ -

218. What is culinary heat .'

219. Why does fire beat glass, when the sun does not ?

220. To what experiment is allusion here made illustrative of
1^ subject?

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radiates, tuileas it meet with clouds'" or znj foreigo bod^ to inter-
cept its passage, passes loto the distaot regions of the UDiverseS
Caroline What a pity that so mach heat should be wasted \^
Mrs, B> Before you are tempted to object to aoy laiir of nature,
reflect whether it may not prove to be one of the numberless dis-
pensations of Providence for our good If all the heat which the
earth has received from the sun since the creation, bad been accu-
mulated in it,/!fs temperature by this time would, no doubt, have
been more derated than any human being could have bore^j^

Cur&tmt. 1 spoke, indeed, very inconsiderately. But, IVTre. B.,
though the earth, at such a high temperature, might hare scorched
our feet, we should always have had a cool refreshing air to breathe,
since the radiation of the earth does not heat the atmosphere-

Emity, The cool air would have afforded but very insufficient
refreshment, whilst our bodies were exposed to the burning radia-
tion of the earth.

Mrs. B. Nor should we have breathed a cool air : for though it is
true that beat is not communicated to the atmosphere by radiation,
yet the air is warmed b^ <x)ptact with heated bodies, in the same
manner as solids or li^uids.rThe stratum of air which is immediately
in contact with the earth is^heated by it ; it becomes specifically
lighter, and rises, making way for another stratum of air, which is,
in its turn heated aad carried upwards; and thus each successive
stratum of air is warmed by coming in contact with the eart^ "fou
may perceive this effect in a sultry day, if you attentively merve
the strata of air near the surface of the eartn ; they, appear in con-
stant agitation ; for though it is true the air itself is invisible, yet the
sun shining on the vapours floating in it, render them visible, like
the amber dust in the water. The temperature of the sur&ce of
the earth is therefore the source from whence the atmosphere de-
rives its heat, though it is communicated neither by radiation, nor
transmitted from one particle of it to another by the conducting
power ; but every particle of air must come in contact with the
earth, in order to receive heat from \i»

Emily. Wind, then, by agitating the air, should coi^tribute to
cool the earth and warm the atmosphere, by bringing a more rapid
succession of fresh strata of air in contact with the earth ? and yet
in general wind feels cooler than still air.

Mrs, ^.ffiecause the agitation of the air carries off heat from the
sur&ce of our bodies more rapidly than still air, bv occasioning a
greater number of points of contact in a given timeS

* Every one has observed hojLPppressive the heat is on a foggy,
or cloudy day in the summer, ^he moisture of the fog absortis the
heat which the ea|[th radiates, and throws it back upon the earth

again, and upon us j-C,


221. What becomes of the heat which the earth radiates?

222. What would be the effect if all the heat which the earth has
received from the sun,8ince the creation,had been accumulated in it^

223. Why, in summer, is it particularly hot in cloudy, or foggy
weather ?

224. How it the air heated^ tfnot at hai been taid^ bythcrnyt of the
stmpattiM through ttf

225. Why it the wind cooliDg to our bodies? ^ i

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Emilif. Since it is from the earth, and not the sun, that the at-
mosphere receives its heat, I no longfer wonder that elevated regions
should be colder than plains and valleys. It was always a subject
of astonishment to me, that in ascending* a mountain and approach-
ing the sun, the air became odder instead of being more heated.

Mrs, B. At the distance of about a hundred millions of miles,
which we are from the sun, the approach of a few thousand feet
makes no sensible difference, whilst it produces a very considerable
effect with regard to the warming of the atmosphere at the surface
of the earth. ^

CaroHne. Yet as the warm air arises from the earth, and the
cold air descends to it, I should have supposed that heat would have
accumulated in the upper regions of the atmosphere, and that we
should havoJelt the air warmer as we ascended.

Mr8. B. iThe atmosphere you know, diminishes in density, and
consequently in weight, as it is more distant from the earth ; the
warm air, therefore, rises only till it meets with a stratum of air of
its own density ; and it will not ascend into the upper regions of the
^mosphere until all the parts beneath have been previously heat^
vEbc leng^th of summer, even in warm climates, does not heat the aiF
sufficiently to melt the snow which has accumulated during the win-
Fig. 6. ter on very high mountains, although tliey
are almost constantljr exposed to the heat
df th#snn*s rays, being too much ^leva-
ted to be often enveloped in cloudsl^

JEm%. These explanations ar^ very
satisfactory ; but allow me to ask you one
more question respecting the increased
iev^ty of heated liquids. You said that
when water was heated over the fire, the
particles at the bottom of the vessel as-
cended as soon as heated, in consequence
of their specific levity ; why does not the
'same effect continue when the water boils
and is converted into steam ? and why
does' the steam arise from the surface, in-
stead of thfUbottom of the liquid ?

Mrs. B,.«The steam or vapour does as-
cend from^e bottom, though it seems to
arise from the surface of the liquidjf We
shall boil some water in this Florence
flask, (Fig. 6.) in order that you may be
well acquainted with the process of ebul-
lition : you will then see, through the
^^^^ ^lass, that the vapour rises in bubbles
/%7V from the bottom. We shall make it boil
I / sfc,, by means of a lamp, which is more con-

B.iungwaterinafu.ko.era ^enieut for thb purposo than thechimney

Patent Lamp. Ute,

226. Why is it colder on high hills and mountains than it is in
^eys, since the former are nearer the sun than the latter, and
since also it is the nature of the air to rise as it becomes warmed ?

227. What illustration is mentioned to shew that the air is not
heated by the sun's rays passing through it ?

228. Does wate? boil from the top, or from the bottom of a vessel


£m%. I see some small babbles asoend, and a^g^reat many appear
all over the ioside of the flask ; does the water begio to boil already?

Jtfrf. B. No ; what you now see are babbles of air^hich were
cither dissolved in the water, or attached to the iaoer sQv&ce ^ the
flask, and which, being rarefied by the heat, ascend in the watm

Emily, But the heat which rarefies the air inclosed in the iviter
mast rarefy the water at the same time ; therefore, if it could re-
main stationary m the water when both were cold, I do not under-
stand why it^should not when both are equally heated.

Mrs. B, being' much less dense than watef^is more easily
yarefied ; Uie former, therefore, expands to a greaC extent, whilst
the latter continues to occupy nearly the same space ; for the wa-
ter dilates comparatively but very little without changing its state ,
and becoming vapour. Now that the water in the flask begins to
boiK observe wjiat large bubbles rise from the bottom of it.

EmUy, I see them perfectly ; but I wonder that they have suffi-
cient power to force themselves through the water.

Caroline, They miut rise, you knojL/Trom their specific levit/N

Mrs, B. You are right, Caroline /hot vapour has not in all If^
quids (when brought to the degree or vaporization )the powej of
overcoming the pressure of the less heated surfacelflvietals, for in-
stance, mercury exceptedf^evaporate only from ttw surface : there-
fore no vapour will ascendTrom them till the degree of heat which
is necessary to form it has reachai the surface ; that is to say, till
the whole of the liquid is brought to a state of ebullition.

Emily, I have ooserved that steam, immediately issuing from the
spout of a tea-kettle, is less visible (ban at a further distance from
it, yet it must be more dense when it first evaporates, than when
it begins to diffuse itself in the air.

M-s, B, When the steam is first formed, it is so perfectly dissolv-
eo by caloric, as to be invisible. In order, however, to anderstand
this, it will be necessary for me to enter into some explanation re-
specting the nature of ^lution. /Solution takes place whenever a
body is melted in a fluidJ In thisoperation the body is reduced to
such a minute state of division by the fluid, as to become invisible
in it and to partake of its fluidity ; but in common solutions this
happens without any decomposition, the body being only divided in-
to its integrant particles by the fluid in which it is melted.

Caroline, It is then a mode of destroying the attraction of aggre-

Mrs. B. Undoubtedly.— the two principal solvent fluids are^a-
ier and cahricy You may havft observed that if you melt -alt in wa-
ter it totally disappears, and the water remains clear and transpa-
rent as before ; yet though the union of these bodies appears so
perfect, it is not produced by any chemical combination ; both the

229. Whatcauses those babbles which ascend, and those which
gather on the inside of a vessel when water is heating .'
230 Why is air more easily rarefied than water ?

231. When water begfins to boil why do large babbles rise from
the bottom ?

232. Has vapour alwayi the power of overcoming the pcsessure
of the less heated surface ?

233. What substances evaporate only from the sor&ce ?

234. When does solution take place ?

235. What are the two priacipal Bolrent fluids ? ^ ,

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nun CAMiome. ifi

sdtatdthewiterninaittiittclMa^^; ood if toh were to sepante
thenrby eraporating^ the latter jyou would fiod the salt in the same
stated beibffe. >

EnUly. I suppose that water is a solyent for solid bodies, and ca-
loric for liooids ?

Mrs. B, Liquids of course can only be conirerted into yapour by
caloric. But the solrent power of this arent is not at all confined
to that ciass of bodies ; a g^eat rariety of solid substances are dis-
8ol?ed by beat ; thus metaJs, which are insoluble in water, can be
diMolyed by intense heat, beinj^ first fused or conyerted into a li-
quid and then rarefied into an myisible yapour. Many other bod-
ies, such as salt, g^ros, &c. yield to either of these solvents.

Caroline, And that, no doubt, is the reason why hot water will
melt them so much better than cold water.

Mrs, B, It is so. (jCaloric may^ indeed, be considered as hayin?
in every instance, some share in the solution of a body by wate^
since water, howeyerlow its temperature may be, always contain^s
nu)re or less caloric.

Emdy, Then, perhaps, water owes its solvent power merely io
the caloric contained in it.

Mrs, B, That, probably, would be carrying^ the speculation too
far ; I should rather think that water and caloric unite their efforts
tp dissolve a body, and that the difficulty or facility of effecting^ this,
^epend both on Uie degree of attraction of aggregation to be over-
come, and on the arranrement of the particles which ^ more or
less' disposed to be divided and penetrated by the solventj

Emify. Bqt have not all liquids the same solvent powCT as water?

Mrs, B. ^e solvent power of other liouids varies accordingto
their nature, and that of the substances submitted to their actio^V-
Mobt of th^e solvents, indeed, difier essentially fhwn water, as^ffley
do not merely separate the integrrant particles of the bodies which
they dissolve, but attack their constituent principles bj the power
of chemical attraction, thus producing a true deoompo8ition\ These
more complicated operations we must consider in another pldke, and
confine our attention atpresent to the solutions by water and ca-

Cdrohne. But there area varielr of substances which, when dis-
solved in water, make it thick and muddy, and destroy its transpa-

^Mrs, B. In this case it is not a sdutioa, but simply a mixture.
I shall show you the diference between a solution and a mixture,
by putting some commod salt into one g^ass of water, and some
powder ofchalk into another ; both these substances are white, but
their dfect on the water will be very diffisrent.

CaroHne. Very different, indeed! The salt entirely disappears

S36. After salt has been dissolved in water, can they be separated
so as to have the salt in the same state, as before it was dissoiyed ?
By what means ?

237. Has caloric any influence in the solution of a body by wat«?

938. On what does die difficulty or ftuciHty of diasomng bodies

t39. Haye all liquids the same solvettt power as water?

240, How do these solvents diffisr imn water ?

241. WhatisthddiffiBreDcebttwMBaacdiitiimaAdaauxtnre?

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and leave Hie water tranwareat, whilst the ohalk ohanf^ it into
. an opaque Mquid like mil A

' Emily* And would laon|» of chalk and salt produce similar effects
on water ?

Mrs, B, Yes, but not so rapidly : salt is, indeed, soon melted,
though in a lamp : but ehalk, which does not mix so readily with
water would require a much greater lenji^ of time ; I therefore
preferred showing ^ou the ex|)eriment with both substances redu-
ced to powder, which does not in any respect alter their nature, but
facilitates the operation merely by presenting a greater quantity of
surface to the water.

I must not forget to mention a,ver3r curious circumstance res-
pecting solution, which is, that/a fluid is not nearly so much in-
creased in bulk by holding a boo^ in solution, as it would be, by
mere mixture with the body^

Caroline. How is that pos^le ? for two bodies cannot exist to-
gether in the same space.

Mrs. B. Two bodies may, by condensation, occupy less space
when in union than when separate, and this I can show you by an
easy experiment

i^his phial which contains some salt, 1 shall fill with water, pour-
ing it in quickly, so as not to dissolve much of the salt ; and when
it IS quite full I cork it. If 1 now sbake the phial till the salt is dis-
solved, you will observe that it is no longer fulA

Caroline. I shall try to add a little more sallT But now you see

Mrs. B. the water runs over. ^

Mrs. B. Yes ; but observe that€he last quantity of salt you put
in remains solid at the bottom, and^displaces the water ; for it has
already melted all the salt it is capable of holding in solution. This
is called the point of^turalion; and the water in this case is said to
be saturated with salt^

Emily. I think I viow understand the solution of a solid body by
water perfectly ; hut I have not so clear an idea of the solution of a
liquid by caloric.

Mrs. B. It is probably of a similar nature ; but as caloric is an
invisible flui^, its action as a solvent is not so obvious as that of wa-
ter. Caloric, we may conceive, dissolves water and converts it in-
to vapour by the same process as water dissolves salt ; tbatjis to say,
the particles of water are so minutely divided by the caloric as to
become invisible. Thus, you are now enabled to understand why
the vapour of boiling water, when it first issues from the spout of a
kettle is invisible ;Dt is so, because it is then completely dissolv-
ed by caloric. But the air with which it comes in contact, being
much colder than the vapour, the latter yields to it a quantity of
its caloric. The particles of vapour being thus in a good measure
deprived of their solvent, gradually collect, and become visible in
the form of steam, which is water in a state of imperfect solution^
and if you were further to deprive it of its caloric, it would reiurti
to its original liquid state.

242. Are fluids equally increased in bulk by the solution and the
mixture of a solid?

243. What experiment proves that they are not ?

244. When is a solvent saturated ?

245. Why is vapour less visiUe on first rising from a liquid, than
after having ascended a distance from it ? r^^^^u

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CaroUne, That I understand rer? well. If you hold a «>ld plate
over tk tea-urn, the steam issuing from it will be immediately\5on.
verted into drops of water by parting with its caloric to the plate :
but in what state IS the steam when it becomes inyisible by beine
dinused m tbeair?
^^'f^'rll '^ °°* merely diffused, but is again dissolved by the air.
T?*^^ Tbeair, then, has asolrent power, like water and caloric.
Jttrs.B, This was formerly believed to be the case. But it ap-
pears from more recent enquiries that the solvent power of the at-
mosphere depfindssoleljppon the caloric contained in it) Sometimes
llhe watery vapour diffused in the atmospher^is but im|ferfect!y dis-
solved, as is the case in the formation of clollde and fogs ; but if it
gets mto a region sufficienilj warm, it becomes perfectly invisible.
JLmtfy Can any water be dissolved in the atmosphere without
having been previously converted into vapour by boiliDg ?

Mrs, B, Unquestionably : and thisconstitutes the difference
between vaporisation and evaporation OYater, when heated to the
boiling point, can no longer exist in the form of water, and must
necessarily be converted into vapour or steam, whatever may be
the state and temperature of the surrounding medium ; this is called
vaporization. But the atmosphere, by means of the caloric it con-
tains, can take up a certain portion of water at any temperature,
and hold it in a state of solution. This is simply evaporation^ Thus
the atmosphere is continually carrying off moisture from the sur-
face of the earth, until it is saturated with it.

Caroline. This is the case, no doubt, when we feel the atmos-
phere dam t^

Mrs. B,^n the contrary, when the moisture is well dissolved it
occasions no humidity ; it is onlv when in a state of imperfect so-
lution and floatiofif in the atimgphere, in the form of watery vaponr,
that it produces dampnessl «T)i8 happens more frequently in win-
ter than in summer; for iheiower the temperature of the atmos-
phere, the less water it can dissolve ; and in reality it never con-
tains so much moisture as in a dry, hot, summer^s da^

Caroline. You astonish me ; but why, then, is the air so dry in
frosty weather, when its temperature is at the lowest ?

Emily. This, I conjecture, proceeds not ^ much from the mois-
ture being dissolved, ^nrom its being frozen^* is not that tbecase^
Mrs. B. It is ; and^he freezing of the waftery vapour which the

* In cold climates, where there is not a cloud to be seen, andlb^
sun rises in all his glory, the air is sometimes full of little particles
of ice fflisteninj^ in every direction, and forming a most beauti/ul
spectacle. This is owing to the condensation, and freezing of^e
T^cles of water in the air, by the intense cold. — C.

^ ■ " i » I ■ I < I Mil II ■!. I I I I, , i H i .

246. Upon what does tiie solvent power of the atmosphere dqpei^df
547. What causes fogs ? ' -

248. What is the diference between vaporatton and eraporizatioa^

249. Why does the atmosphere sometimes feel damp?

250. When does the atmosphere eontain most moisture, io sua-
wer or winter ?

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 7 of 43)