THE WONDERFUL EFFECTS OF HEAT—(_continued_).
“But heat,” said Humphry to himself, as he reviewed his previous
experiments, “not only _expands_ the bulk of different bodies, but it
_changes their form_, rendering certain solids liquid, and converting
liquids into vapours. Let us see now what occurs during such changes.”
Accordingly, the youth proceeded to pound some ice, and to cool it down
in a tumbler by means of a freezing mixture to the temperature of zero.
The tumbler was then inserted in a bath of tepid water, the temperature
of which was maintained, by an argand-lamp beneath it, constantly at
60°. A thermometer having been plunged in the ice, the quicksilver was
observed to mount rapidly in the tube until it reached 32°, when the
ice began to liquefy. But, though another thermometer in the water-bath
showed that the liquid there was still 60° hot—the same temperature,
indeed, as when the ice was first immersed in it—nevertheless
the thermometer in the tumbler remained _stationary_ at 32°—the
freezing-point. Nor did it begin to rise until the whole of the ice was
melted; after which it mounted gradually, and ultimately settled at 60°,
the temperature of the surrounding water.
Humphry was astonished at the effect. “Why did the thermometer,” he asked
himself, “remain fixed at the freezing-point until the whole of the ice
was melted? The heat from the surrounding water must have been entering
the ice as much when it began to dissolve, as it did before the thawing
occurred, or even afterwards. How, then, came it that the thermometer was
not affected by it?”
The eager boy was so puzzled with the mystery, that he could not rest
till he had tested the result by another experiment.
Having procured two small glass globes, he filled them both with the same
quantity of water. The liquid in one he froze, and that in the other
he cooled down to 33°, so that it might be as near as possible to the
temperature of the first, without being solid. When the ice in the one
globe had just begun to melt, and the thermometer in that vessel marked
32°, the two globes were plunged into a water-bath, the temperature of
which was kept at 47° throughout the experiment.
The vessels, therefore, were, as near as possible, under similar
conditions of temperature, within and without, and with similar
contents—except that the one contained _ice_, and the other _water_.
The progress of the heating was then noted.
In the globe which contained the _water_, Humphry found the thermometer
rose in half an hour to 40°. In the globe, however, which contained the
_ice_, no less than 10½ hours elapsed before the whole was melted, and
the temperature of the resulting liquid raised to 40°, like the other; so
that the rate of heating in the ice-vessel was 21 times slower than in
that which held the water.
“How can this be?” mused the boy-philosopher. “The actual amount of heat
received by the two must have been uniform during the whole time, and
yet the ice took 10½ hours to have its temperature raised to 40°, while
the ice-cold water needed only ½ an hour to acquire the same heat. How
extraordinary!” he inwardly exclaimed; “what can be the cause of it?
For,” said the lad, as he jotted the figures down in his note-book,
“the water in the globe had its temperature raised 7° in half an hour,
consequently that must have been the rate at which the warmer liquid in
the external bath was giving out its heat to the two globes. Accordingly,
in 10½ hours it must have given 10½ times 7°, or 147° in all, to the one
containing the ice. Consequently the _ice_ required 140 more degrees of
heat to raise its temperature to 40° than the _water_ did.”
Humphry still doubted the accuracy of his conclusions; for they were so
marvellous and unexpected by him, that he sat for a long time considering
by what experiment he could bring the matter to a positive test. “If,”
he inwardly exclaimed, “the ice really received 140° more heat than the
water, what became of them? The two were each, in the end, of the same
temperature; and the thermometer didn’t show, nor could I feel, that the
one had imbibed more heat than the other.”
Presently the boy started to his feet, for a sudden thought had struck
him. He would take 12 oz. of pounded ice, and upon this he would pour
the same weight of hot water, at a temperature of 172°; so that the
difference between the heat of the ice and that of the water would be
172°-32°, or exactly 140°, and this was precisely the quantity of heat
that the ice in the previous experiment had received over and above the
water.
Humphry wondered again and again, as he prepared the experiment, what
the result would be; and when he mixed the warm water with the ice, he
was overjoyed to find that the temperature produced was only 32°—the
same as that of the pounded ice itself. Hence it was plain the water had
lost exactly 140° of its heat, and, moreover, that these had _entirely
disappeared_, since the temperature of the colder substance remained the
same as at the outset of the experiment.
Then came the question—What had become of the lost heat? Where had it
gone? What effect had it produced?
There was but one answer! and this Humphry was not long in divining.
The heat which had disappeared had _combined_ with the ice, and its
effect had been, not to raise its _sensible_ temperature, but simply to
convert the solid body into a liquid one, so that the caloric had become
_latent_, or imperceptible to the senses, as well as incapable of being
detected by the most delicate thermometer.
“So, then,” cried the boy, as the new thought flashed upon him, “a
_liquid is merely a solid, whose particles are kept asunder by so much
heat, which is insensible to us_. In this piece of wax the particles are
held together by a certain force, which is called the ‘attraction of
cohesion,’ so that when I press upon it I am incapable of separating one
part of it from another. If, however, I apply but a little heat to the
substance, the cohesion is soon destroyed, and the particles, instead
of then firmly adhering one to another, become free to move, and are,
consequently, easily separable; so that a rod can be plunged into it when
liquefied, and moved about in it with little or no difficulty.”
“Yes,” he repeated, “a liquid is merely a solid, whose particles are kept
asunder by heat.”
Humphry was so pleased with the result he had arrived at, that he varied
the experiment in a number of different forms.
First, he took some spermaceti (a substance which melts at 112°), and
found that a thermometer plunged in this (as in the ice) remained
stationary at the melting-point until the whole was liquefied, and that
it was not till then that the temperature could be raised above the point
of liquefaction.
Next he tried the same experiment with some lead, and found that, though
he heated a ladle-full nearly red hot, the temperature of the whole was
immediately cooled down again to the fusing-point by the addition of a
piece of the solid metal.
Hence the law was manifest, _that in all cases of liquefaction a certain
quantity of heat, not indicated by the thermometer, is absorbed, or
disappears—this heat being withdrawn from surrounding bodies, and so
leaving them comparatively cold_.
Accordingly, now that Humphry had ascertained that liquefaction itself
was a source of cold, he proceeded to try what degree of cold he could
produce by causing certain substances to melt rapidly.
For this purpose he took some snow and sprinkled a little salt over it,
when he observed that the two solids immediately formed a liquid; and
then, plunging a thermometer into the mixture, he beheld the quicksilver
sink and sink, until it had nearly reached zero, so that it fell from
32° to 0°. Then the boy made a mixture of 5 parts of smelling salts, 5
parts saltpetre, and 16 parts water, and plunging the thermometer into
this, he found that it sank from 50° (the temperature of the room in
which the experiment was made) to 10° (which is several degrees below
freezing-point). With a mixture, consisting of equal parts of saltpetre,
ammonia, and water, the thermometer fell 6° lower than in the previous
experiment—or from 50° to 4°. Again, with 5 parts of Glauber’s salts
and 4 parts of oil of vitriol and water, the temperature sank 1° lower
still—or from 50° to 3°. Further, having finely powdered some of the
crystals of the last-mentioned salt, he drenched them with muriatic
acid, when the salt dissolved to a greater extent than it had previously
done in the water, and the consequence was that the temperature fell
even lower than before—or from 50° to 0°, while the vessel in which the
mixture was made became covered with hoar frost; and when some water in a
tube was plunged into the liquefied salt, it was speedily converted into
a mass of ice.
Humphry was now anxious to see whether the heat which is absorbed, and
becomes latent or insensible during the liquefaction of bodies, really
_remains_ in the liquid, and whether it is given out again or emitted
during their solidification.
To satisfy himself upon this point he prepared two vessels, one full
of water and the other full of brine, the temperatures of the liquids
in each being, at first, precisely 52°. On a very cold day, when the
thermometer stood at 22° (_i.e._ 10° below freezing-point), the boy
exposed the fluids in these two vessels, with thermometers in each, to
the open air, and found that they both gradually parted with their heat
to the surrounding atmosphere, and were soon cooled down to 32°; then the
water began to freeze very slowly, and during this the thermometer in the
water-vessel remained perfectly stationary. The brine, however (which
does not congeal till its temperature sinks to 4°), continued to cool on,
the thermometer in it sinking without interruption, until it gradually
reached the temperature of the external air, or 22°.
Now it was plain that both liquids, in cooling, were alike parting with
their heat to the colder air. Why, then, should one and not the other
_suddenly cease giving out caloric_, and refuse for a certain time to be
cooled down to the same level as the atmosphere around it?
The only explanation of the problem was, that the water, during the
process of freezing, was parting with the 140° of heat which Humphry
had before seen were necessary to retain it in a liquid form, and that
it was the evolution of this amount of caloric which served to keep the
temperature of the water for a considerable time at 32°—notwithstanding
the cooling effect of the surrounding atmosphere.
Still this experiment hardly satisfied the boy. He wanted to have some
_sensible_ proof of the evolution of heat from water during the act of
freezing. It was true, there was no way of accounting for the fact of the
thermometer remaining stationary in one vessel while it was continually
sinking in the other, except by supposing that the latent _insensible_
heat was being given off from the water as it passed into the solid
form. Humphry desired, however, to _see_, as it were, the heat so given
off—that is to say, he wished to behold the thermometer _rise_ instead of
merely _remaining fixed at one point_.
Accordingly it struck the lad, that if he was to keep some water in
a vessel perfectly still, and to prevent even the air from agitating
its surface, he might be able to cool it down some few degrees below
the freezing-point, and then he should be able to see if, in the act
of freezing afterwards, the thermometer would really rise as the water
solidified. So successfully did Humphry perform this experiment, that
he was enabled, by great care, to cool some water in a vessel down to
22° without freezing it. Then he felt a thrill dart through his frame as
he agitated the water, and beheld it immediately shoot into a thousand
transparent crystals—whereupon the thermometer in the vessel instantly
began to rise, and soon stood at 32°, thus showing that the water had
acquired 10° of heat almost in an instant.
Whence came this heat, then?
But one answer was possible. It was manifest that the water, in the act
of solidifying, _gave out_ heat, even as in the act of liquefying ice
_absorbed_ it.
“But do all things,” said Humphry to himself, “give off heat as they
pass from a liquid to a solid form? And is solidification, therefore,
a _heating_ process to surrounding bodies, in the same manner as
liquefaction (from the absorption of heat at such times) is a _cooling_
one?”
The youth had heard that, if a strong solution of Glauber’s salts be
poured, while hot, into a flask, and corked tightly down, it will remain
liquid when cold, and that on removing the cork it will immediately shoot
into a fibrous mass of crystals. He wished to see, therefore, whether, in
the act of solidification, heat is evolved from such a solution.
Humphry was not long in preparing the requisites for the experiment, and
was then delighted to find, as he watched the crystals, immediately that
he withdrew the cork, dart from the surface downwards, the temperature of
the substance became so much increased, that the bottle which contained
it grew sensibly warm in his hand, though it was perfectly cold before.
Now the lad knew, that in making the solution he had added 2 ounces
of water to 3 ounces of the salts, and as the whole of this became
solidified on opening the bottle, it was clear that the elevation of
temperature arose principally from the _solidification of the water_ in
the crystalline mass.
Next Humphry added a saturated solution of tartaric acid to some strong
liquid ammonia, and found, immediately that the two fluids were poured
together, a solid substance was thrown down, and considerable heat
evolved.
Again, the lad was aware that, on mixing powdered plaster of Paris with
water, there is a like increase of temperature at the moment of the
composition “setting;” or, in other words, when the water with which the
plaster has been mixed passes into the _solid_ form.
Further, he could now perceive that the great increase of heat which
occurs during the slacking of lime is due merely to the same cause,
viz. the _solidification of the water poured upon it_. Consequently it
was plain that water, in passing from the _liquid_ to the _solid_ form,
invariably _evolves_ the heat which is necessary to retain it in a liquid
state, and which as it passes, on the other hand, from the _solid_ to the
_liquid_ state, it as invariably _absorbs_.
Not only, however, is there an increase of temperature when water becomes
solid, but Humphry found the same result to ensue, even when the same
liquid is _condensed_. On mixing 4 parts of strong oil of vitriol
with 1 part of water, cooled down to the freezing-point, he perceived
that the two together occupied considerably less space than they did
alone, and that the mixture rose rapidly from 32° to 212°, or from the
freezing to the boiling point. The same result ensued when 1 part of
snow was substituted for the 1 part cold water; but, strange to say,
when the proportions were reversed, so that there were 4 parts of snow
to 1 part of oil of vitriol, _intense cold_ instead of intense heat was
produced—the _increase_ of temperature in the one case arising from
the _condensation_ of the water from the snow, and the _decrease_ of
temperature, on the other hand, being due to the _liquefaction_ of the
snow itself.
Moreover, the boy was aware that a piece of soft iron, when hammered,
becomes intensely heated; and he had heard that when a bar of red-hot
iron is passed through a rolling-mill, its temperature is so much raised
that it is rendered nearly white-hot by the extreme pressure, and the
consequent condensation of the particles.
_Sudden expansion_, on the other hand, Humphry found to be a cooling
process; and this is one of the reasons why high-pressure steam, on
issuing from a small aperture, instead of scalding the hand as ordinary
steam would, scarcely feels warm, even though its temperature be some
hundred degrees higher than the vapour at a low pressure; for as the
compressed steam escapes into the atmosphere, its instantaneous
expansion so far cools it, that it is deprived of all power of burning.
Moreover, at the fountain of Hiero, in Hungary, a part of the machinery
for working the mines consists of a column of water 260 feet high, which
presses upon a large volume of air, enclosed in a tight reservoir, so
that the air within is greatly condensed by the enormous weight of the
water; and that when a pipe communicating with this reservoir is suddenly
opened, the condensed air rushes out with extreme velocity, and then
instantly expanding, absorbs so much heat as to precipitate the moisture
it contains in a shower of snow—a hat held in the blast being immediately
covered with it. So strong, however, is the current of condensed air,
that the workman who holds the hat is obliged to lean his back against
the wall to retain it in its position.
Another illustration of the cooling effect produced by the sudden
expansion of condensed air is afforded in the fact, that if the blast
from an air-gun be directed upon a delicate thermometer, the temperature
will be found to be lowered at the moment of the discharge.
* * * * *
As yet Humphry had dealt only with the effects produced by the conversion
of solids into liquids, or liquids into solids, and there still remained
for him to learn what results ensued when _liquids were changed into
vapours, or vapours into liquids_.
Accordingly he proceeded to heat some water in an open vessel, and found,
as the temperature gradually rose, that the vapour continued to form on
its surface till the thermometer reached 212°, when the liquid became
violently agitated. But then, although the fire was kept up beneath the
vessel as strong as at first, and _the heat continued to flow into it
as before_, the quicksilver in the thermometer became stationary, and
remained so until the whole of the liquid had been dissipated in the form
of steam.
Here, then, was another instance of the absorption of heat during a
change of form; and it was evident, that as water required a certain
amount of temperature in order to convert it from a _solid_ into a
_liquid_ state, so did it need a proportionate supply of heat to change
it from a _liquid_ into a _vapour_.
“The heat absorbed during the boiling passed off, perhaps, in the steam,”
thought Humphry.
On testing the temperature of the vapour, however, it was found to be no
hotter than that of the water during its ebullition.
What, then, had become of the heat which had been added since the boiling
commenced?
Why, it had been rendered _latent_ or _insensible_, being necessary for
retaining the liquid in a more rarified form; for as a liquid is but a
solid whose particles have been separated, and so made free to move, by
the latent heat existing between them, so a vapour is merely a liquid,
whose atoms have been driven farther asunder, and made still more easily
separable, by the heat imbibed during the process of vaporization. It
was evident, therefore, that the production of vapour is attended with
a loss of sensible heat, and that, as in the case of liquefaction,
heat _disappears_ in order to constitute the liquid, so in the case of
evaporation a considerable quantity of heat becomes _latent_ in the
vapour.
To render this part of the subject still more clear, Humphry filled
a flat-bottomed tin vessel with a definite quantity of water, at the
temperature of 50°. Then, having placed the vessel upon a heated plate,
he found that in 4 minutes it had acquired a temperature of 212°, and
began to boil, whilst in 20 minutes the whole had evaporated, having been
dissipated in the form of steam. The water, therefore, had received 212°
- 50°, or 162° of heat in 4 minutes, which is at the rate of 40½° in
each minute. The heat, however, continued to flow into the water at the
same rate during the whole 20 minutes, so that the entire amount of heat
received must have been 40½° × 20, or 810°, and this had become _latent_
in the steam. Consequently the total quantity of heat required to
evaporate boiling water would be sufficient to raise the water—provided
it remained all the time in the liquid state, instead of being converted
into steam—as much as 810° above the boiling-point, or altogether to
1022°.
To verify this conclusion, Humphry heated some water under pressure in
a “Papin’s digester,” so that the liquid was prevented evaporating, and
raised the temperature of it to 400°. Then the lad opened the valve,
and part of the water suddenly rushed out in the form of steam, when
the temperature of that remaining in the digester _sank immediately_ to
212°. Consequently 188° (400° - 212°) of heat had suddenly disappeared,
having been carried off by the steam. It was afterwards found that only
⅕th of the water had gone off in vapour, so that this vapour must have
contained not only its own 188°, but the 188° lost by the 4 other parts
remaining in the digester; that is to say, the steam must have contained
188° × 5, or 940° of heat altogether. This experiment therefore showed,
that steam is water _combined_ with nearly 1000° of heat; and in the
same manner as the water had been previously observed to _give out_ its
heat of liquidity, and to make the thermometer _rise_ at the moment of
its conversion into _ice_, so was it now seen to _absorb_ a considerable
amount of heat, and to make the thermometer _fall_ at the moment of its
conversion into _vapour_.
Evaporation, therefore, should be a means of producing cold, in the same
manner as liquefaction had been previously proved to be; for if it be
necessary, in order to convert liquids into vapours, that a certain
amount of heat be absorbed, it is plain that such heat must be drawn from
surrounding substances, and thus the vaporization of one body will be a
cooling process to others near it.
To test this, Humphry spread out a wet cloth in a keen wind when the
atmosphere was a few degrees above the freezing-point, and found that the
particles of water, as they passed into the form of vapour,[35] carried
off so much heat from the liquid in the cloth (in the same manner as the
steam did in the Papin’s digester) that the remainder became frozen,
while the cloth itself was rendered hard and stiff by the formation of
ice in its pores.
Humphry, however, knew that ether was much more vaporizable than water at
ordinary temperatures. Accordingly he availed himself of this substance
in the production of cold by spontaneous evaporation.
First, he folded a strip of cambric round the bulb of a small
thermometer, and allowed some ether to dribble over it, while he
increased the evaporation by projecting a current of air upon it by
means of a bellows. The quicksilver was immediately seen to fall several
degrees below the freezing-point, and on substituting a thin glass tube
containing a small quantity of water for the thermometer previously
used, the boy was enabled to produce ice by the same means.
Next, the lad made use of the air-pump which he had previously
constructed out of the rudest materials,[36] in order to facilitate
the spontaneous evaporation of water, by removing the pressure of the
atmosphere from the surface of it.
Upon the plate of the apparatus he placed a soup-plate, and this he half
filled with oil of vitriol; above the soup-plate he stood a tin basin,
supported on three pieces of tobacco-pipe, and three parts filled with
water, in which a small thermometer was immersed; while over the whole
he put the glass receiver. The arrangement is shown in the annexed
illustration:
[Illustration]
The pump was now set to work, and the air gradually drawn out from the
glass receiver, whereupon the thermometer was observed to sink; for as
the pressure of the atmosphere was removed the evaporation from the water
was increased, while the oil of vitriol at the bottom served to absorb
the vapour as fast as it was produced: so that, though the temperature
was considerably lowered, the evaporation from the water ultimately
became so rapid that it had all the appearance of boiling, and in the
course of 5 or 10 minutes the liquid was converted into ice.
Before the solidification took place, however, the thermometer was
observed to fall several degrees below the freezing-point, whilst at the
moment of its freezing it rose to 32°, in consequence of the escape of
the heat which had previously served to keep the water liquid.
The explanation of the process is almost obvious. As in the case of the
steam issuing from the Papin’s digester, that part of the water passing
off in the form of vapour abstracted heat from the remainder of the fluid
portion, which, thus losing the caloric that served to keep it liquid,
became solid, or froze.
Humphry was now anxious to see what effect the simultaneous evaporation
of ether and water would produce under the air-pump. Accordingly he
procured a thin glass flask, and this he inserted in a tumbler, so that
it fitted almost close. Having poured a little ether into the flask, and
some cold water into the tumbler, he placed the whole apparatus under the
receiver of the air-pump, as here represented:
[Illustration]
On exhausting the receiver the ether was observed to _boil_, from the
rapidity of its evaporation, while the water in which it was immersed
soon became solidified, or converted into _ice_.
The apparent anomaly of two liquids made to _boil_ and _freeze_ at one
and the same time puzzled the lad for a while. At length, however, he
divined the reason. The ether, in passing into the form of vapour,
required a certain amount of heat to sustain it in that state, and this
it absorbed principally from the water that surrounded it, which soon
became congealed owing to the loss of that portion of heat which was
requisite for its maintenance in a fluid form.
Humphry had now discovered that, by removing the pressure of the
atmosphere, the evaporation of liquids proceeded at a much greater rate,
and he was anxious to learn whether they could be made to boil at a lower
temperature by the same means.
Accordingly, he fitted a stop-cock into the neck of a Florence flask, and
then turning the cock on, so that the vapour might escape, he proceeded
to heat the water, over the flame of a spirit-lamp, till it boiled;
whereupon he removed the flask from the flame and closed the cock. The
liquid then soon ceased to boil; on plunging the flask, however, into a
vessel of _cold_ water, he found the ebullition instantly to recommence,
but to cease again directly the vessel was held near the _fire_ or over
the _lamp_. Now during the boiling, in the first instance, all the air
above the liquid had been driven out of the flask, and replaced by an
atmosphere of steam; this, upon plunging the vessel into cold water, had
become condensed into a liquid form, so that a vacuum being formed above
the water, the fluid boiled at a lower temperature under the diminished
pressure. On removing the flask, however, from the cold medium, a new
atmosphere of steam was generated, and the pressure of this on the
surface of the liquid prevented its boiling any longer; and thus the
water was made to _boil by being cooled, and to cease boiling by being
heated_.
Humphry afterwards ascertained, that if a glass of water of the
temperature of 90° or 100° be placed under the receiver of an air-pump,
and the pressure of the atmosphere removed by exhausting the air, the
water boils violently at that temperature, and continues to do so until
the whole receiver becomes filled with the vapour; which then, pressing
upon the surface of the liquid, again prevents its ebullition. By
continuing to pump out the vapour, however, the boy was enabled to keep
the water boiling at no less than 112° below its boiling-point in the
open air.
With alcohol and ether, however, it was not even necessary to warm them,
for these fluids boil under the air-pump at all ordinary temperatures.
It has likewise been found that water boils at less than 212° upon the
summits of hills and mountains, where the pressure of the atmosphere is
considerably diminished. At the top of Mont Blanc water has been made
to boil at 187°, and even when the air is lighter at the surface of
the earth the boiling-point of all liquids is reduced; so that in this
country, where the density of the atmosphere fluctuates considerably,
water boils sometimes at 2° lower than 212°, whilst on heavy days it
requires to be raised to 214° in order to produce ebullition. Even
the cleanliness of the vessels in which the liquid is heated has been
ascertained to alter the boiling-point. In glass vessels, from which all
chemical and mechanical impurities have been removed by perfect cleaning,
water may have its temperature raised as high as 220° without being made
to boil; whereas a few metallic filings, or other finely-divided or
insoluble materials, have the effect of causing it to boil at a lower
temperature than 212°.
Humphry would now have sought to learn how much water becomes expanded
in passing into the form of steam. But, though he made several rude
experiments on the subject, he was unable, from the want of proper
apparatus, to arrive at any definite result, and so was obliged to rest
contented with the knowledge which his books afforded him; viz. that
a cubic inch of water becomes converted, at 212°, into very nearly a
cubic foot of steam, the expansion being about 1700 times the bulk of the
original fluid.
Spirits of wine, on the other hand, expand only 493 times, ether about
212 times, and oil of turpentine 192 times; each at the temperature of
212°. Steam, however, is lighter than air—whereas the vapours of spirits
of wine, ether, and turpentine, are much heavier than it, the last being
nearly 5 times the density of our atmosphere at the same temperature.
Before quitting this part of the subject, there is one striking anomaly
connected with the production of vapour that deserves mention here,
though it is but a recent discovery.
If a silver, or other metal spoon, be heated to redness in the flame
of a lamp, and some water be dropped into it while red hot, it will be
found that the liquid, instead of passing off at once into steam, will
instantly assume a globular, or spheroidal form, and float about the
heated metal, revolving with rapidity, and evaporating very slowly; while
the _temperature of the liquid will remain constantly below the boiling
point_—so long as the red heat is maintained. If, however, the lamp be
withdrawn, the water, as the spoon cools down, will suddenly be made to
boil with violence, and be dissipated in vapour with almost explosive
energy.
The cause of this singular phenomenon is, that the water is separated
from the red-hot metal by an atmosphere of highly-elastic steam, which
is generated immediately the liquid is projected on the heated surface,
and which, encircling the water, serves to keep it in a spheroidal,
or globular state; whilst the vapour, being a bad conductor of heat,
prevents the temperature of the hot metal being communicated to the fluid
in connexion with it.
This is the reason why water, when accidentally dropped upon the heated
bars, or hobs of a grate, is occasionally observed to run along them like
globules of quicksilver. If these, however, be smartly struck with a
hammer, so as to bring them suddenly into contact with the hot metal, the
globules will be instantaneously converted into steam, and the change of
form attended with a slight explosion.
Another form of the same singular phenomenon consists in plunging a mass
of white-hot metal into a vessel of cold water, when the incandescence
will be found to continue, rather than to be quenched, in the liquid, the
metal still shining with a bright white light, while the water may be
seen to circulate around, though at some distance from, the glowing mass,
being separated from it by an atmosphere of non-conducting vapour, which,
for a time, prevents its heat being communicated to, and so reduced by,
the surrounding fluid. At length, as the metal cools, the water around
it is brought into contact with the heated surface, when it is made
suddenly to boil with energy.
Moreover, if an iron shell containing water be made red hot, and a hole
then drilled in it, no water will be found to flow through the orifice
until the iron has been considerably cooled, when it will suddenly issue
forth with great violence, in the form of steam.
So, again, if water be poured upon an iron sieve, the wires of which have
been heated to redness, it will not pass through the interstices. As the
sieve cools down, however, it will be found to run through rapidly.
Further, if a red-hot cinder be let fall into a pan of water, it will be
seen to swim upon the surface, and then to sink with a hissing sound,
accompanied with a sudden irruption of steam.
But a far more striking illustration of this strange property consists
in heating to redness a silver or platinum capsule (or small crucible),
and filling it while red hot with a freezing mixture, when the whole mass
will instantly be thrown into the spheroidal state, and on introducing
a thermometer therein the temperature of the liquid will be found to be
scarcely increased, so that a small tube filled with water soon becomes
frozen when immersed in it; and _thus ice may thus be produced even in a
vessel, the heat of which is no less than 1000°_. Indeed, by introducing
some ether and solid carbonic acid into an incandescent crucible, even
quicksilver itself has been made to freeze in it, though this requires
a temperature of 82° below the freezing-point of water; and yet this
extreme cold (equal to that of a Polar winter) has been produced, _and
mercury frozen inside a red-hot vessel_.
* * * * *
Humphry, however, was anxious not to conclude his investigations
concerning the changes of form produced by heat without ascertaining
whether all bodies, in passing from a lower to a higher temperature—or,
on the other hand, from a higher to a lower one—absorbed the same
quantities of caloric; that is to say, did one body require a _greater
amount of heat_ to raise it to a given temperature than another, and did
some bodies give off more heat than others in cooling?
First, the lad dealt with equal quantities of the _same_ fluid at
different temperatures, in order to determine whether, on mixing the
two together, the resulting temperature amounted to the _mean_ of both.
He added ½ pint of cold water at 50° to ½ pint of warm water at 100°
and found that the two together gave a _mean_ temperature of 75°
(⁽¹⁰⁰⁺⁵⁰⁾⁄₂); so that the hot water had lost 25°, whilst the cold
had gained precisely the same amount.
Then Humphry proceeded to try whether the result was the same with
equal quantities of _different_ fluids. Accordingly, he took the same
amount of water at 50° as he had previously employed for one portion,
but, instead of the water at 100° for the other, he substituted a like
quantity of _quicksilver_ at the same temperature, and found to his
astonishment, on pouring the one to the other, that the heat of both
together was no longer the mean of the two (or 75° as before), but only
66⅔°. In this case, therefore, the quicksilver had lost as much as 33⅓°
(100 - 66⅔), whilst the water had gained only 16⅔° (66⅔ - 50); so that
_the quicksilver had parted with twice as much heat as the water had
absorbed_. Consequently it was evident that, in order to raise a certain
_measure_ of water to a given temperature, it required just _double_ the
quantity of heat to be added to it that _an equal measure_ of quicksilver
did; or, in other words, the capacity of water for heat was twice that of
quicksilver.
This referred, however, only to equal _measures_ of the two fluids; so
Humphry wished to ascertain whether the effect would be the same with
equal _weights_ of them. He mixed, therefore, 1 pound of water at 50°
with 1 pound of quicksilver at 100°, and discovered that the resulting
temperature was not quite 52°. Here, then, the quicksilver had lost
rather more than 48° of heat, while this amount had served to increase
the warmth of the water only about 2°. There was but one conclusion to
be arrived at, therefore; namely, that the capacity of a given _weight_
of water for heat is about 30 times greater than an _equal weight_ of
quicksilver, whereas the capacity of a given _measure_ of the former is
only twice that of an equal measure of the latter.
After this the boy added 1 pound of water at 50° to an equal _weight_ of
spermaceti oil at 100°, when the temperature of the mixture was found to
be 66⅔°; so that the oil had parted with twice as much heat as the water
had gained.
Thus it was evident that different substances required _different_
quantities of heat to raise them to the same temperature; and that in
order to warm a certain weight of water to the same degree as an equal
_weight_ of oil and quicksilver, twice as much heat must be given to the
water as to the oil, and 30 times as much as to the quicksilver.
Still the cautious boy was anxious to test the truth of this result by
another experiment.
Accordingly, he took 1 pound weight of each of the three substances
above mentioned, and having brought them severally to a temperature of
50°, he placed the flasks in which they were respectively contained in
a large bath of warm water, the heat of which he kept constantly at
100°. This done, he proceeded to note the time and manner in which each
of the fluids was heated, and found that when the thermometer in the
quicksilver had reached 80°, that in the oil stood at 52°, while the one
in the flask of water marked only 51°; and, though the three liquids
ultimately attained the same temperature as the water-bath in which they
were immersed, _the water took 30 times longer to acquire that heat than
the quicksilver, and twice as long as the oil_.
Now it was manifest that each of the liquids in this experiment must have
been receiving heat alike, so that the only feasible explanation was that
the water, in order to have its temperature raised to a given degree,
required 30 times the quantity of heat that the metallic fluid did, and
double the quantity of the oleaginous one.
Nevertheless, to avoid all possible chance of error, Humphry repeated the
experiment in another form, so as to see whether, in cooling, the water
would part with more caloric than either the oil or the quicksilver; and
just as much more, too, as it had been found to imbibe while being heated.
With this view Humphry filled three Florence flasks—one with a pound of
water, another with a pound of oil, and the third with an equal weight
of quicksilver—all at the temperature of 212°, and having placed each of
the flasks in a large funnel, that rested on a graduated glass jar, he
surrounded them one after another with pounded ice, as here shown:
[Illustration]
When the fluids had been severally cooled down to the same temperature as
the ice around them, the lad proceeded to ascertain, by the quantity of
water produced by the thawing of the ice surrounding each flask, how much
heat had been given out by the three liquids respectively, in sinking
from 212° to 32°; and he ultimately found that the hot water, in cooling,
had thawed twice as much ice as the hot oil, and 30 times as much as the
equally hot quicksilver. Hence it was beyond doubt that water, at a given
temperature, _contained considerably more heat than either of the other
fluids_, and hence the reason why it took a longer time than they to be
warmed or cooled to the same extent.
These experiments naturally led the boy to think how great a magazine
of heat the sea must be, and what a beneficial influence its slow rate
of heating and cooling must have in equalizing the temperature of the
atmosphere. Quicksilver, on the other hand, however, having a small
capacity for heat, and, consequently, being quickly warmed and cooled,
becomes of great value as a liquid for the thermometer, since it is this
property that gives great sensibility to the instrument.
It now only remained for Humphry to ascertain the relative capacities
for heat among solids. This he did by cooling down equal weights of the
metals and other bodies under the exhausted receiver of his air-pump, and
noting how long they took to pass each from a like higher to a like lower
temperature.
By such means the youth ascertained that _Lead_ had the smallest capacity
for heat among the metals, cooling more rapidly than even quicksilver
itself, and 34½ times quicker than water. Next in order came _Platinum_,
which again had less capacity than quicksilver, and cooled 32¼ times
quicker than water. After this, _Silver_ was found to cool 18 times
quicker than water; _Zinc_, 10¾ times; _Copper_, 10½ times; and, lastly,
_Iron_, which was ascertained to part with its heat only 9 times quicker
than water. Glass, however, was found to occupy a longer time in cooling
than any of the metals, giving off its caloric but 8½ times quicker than
water; while sulphur, on the other hand, retained its warmth longer even
than glass, but still cooled 5⅓ times quicker than water.
The capacities for heat, therefore, among the above-mentioned substances,
were inversely as their rates of cooling: that is to say, lead, which
cooled the quickest, contained the least quantity of heat, and,
therefore, required less caloric to raise it to a given temperature;
while sulphur, on the other hand, which took nearly 7 times as long to
cool as lead, contained 7 times more heat, and required to be warmed for
just so much longer a period.
The relative capacity for heat among substances is generally termed their
“SPECIFIC HEAT;” for as different bodies are found to possess _unequal
quantities of heat at equal temperatures_, and as this exists in them in
a _latent_ or _insensible_ state, the term specific heat has therefore
been adopted to express _the relative amount of latent caloric existing
in different substances at the same temperatures_.