THE WONDERFUL EFFECTS OF HEAT.
The effects of heat are manifold.
In the first place, an increase of temperature _expands_ or enlarges
almost all bodies, while a decrease causes them to _contract_ or become
diminished in bulk.
Secondly. Heat _changes the form_ of bodies, converting solids into
liquids, and liquids into vapours; while cold, on the other hand,
condenses vapours into liquids, and causes liquids again to solidify or
congeal.
Thirdly. Heat causes _ignition_; that is to say, it changes dark opaque
substances to a bright transparent red, rendering them capable of giving
out light, when their temperatures are raised to a high degree, and, when
increased to the highest point, causing them to become even white in the
fire, and then endowing them with the properties of the solar beams, so
that their rays have the same power of traversing plates of glass, and of
producing chemical changes, even as the rays of the sun itself.
Fourthly. _Combustion_, or the burning of bodies, with the evolution of
flame, is another effect of heat. There is also a species of combustion
called _slow_ (_erema-causis_ is the chemical term for it), which is
unaccompanied with flame—as in the rotting of wood and other organic
tissues, the rusting of metals, and even the breathing of animals and
ourselves. In each of these processes there is the same combination of a
combustible body with the oxygen of the atmosphere—but at a much _slower_
rate—than in the more rapid and energetic forms of combustion; and hence
but slight increase of temperature (if any) is discernible, while no
flames or luminous gases that are perceptible to our senses are evolved
under such conditions.
Fifthly. _Phosphorescence_ is likewise produced by heat. During
combustion and ignition, bodies become _temporarily_ luminous; but in
states of what is called phosphorescence they are _permanently_ so; and
there are many substances—such as the compact phosphate of lime, the
dark-blue kind of Derbyshire spar, several varieties of heavy spar, and
powdered quartz—which acquire the property of shining constantly in the
dark after having been made nearly red hot.
Sixthly. _Electricity_ is induced by heat; for it has been discovered
that if a bar of the metal called antimony be heated at one end, while
the other is kept cool, an electric current ensues.
Lastly. Heat promotes both _vegetable_ and _animal life_. For not only is
intense cold destructive of organic existence, but the increased warmth
of the summer invariably calls into being an infinite number of plants,
flowers, insects, and the many forms of organised nature that give
variety and grace to the earth. Moreover, heat produces in ourselves,
and other sentient animals, a _feeling of warmth_, and the absence of it
a sensation of cold; by which we are enabled to measure—though hardly
with perfect accuracy—the different changes of temperature occurring in
the substances around us, and also, by the agreeable impression which we
derive from warmth, induced to seek that degree of heat which is best
fitted for the promotion of our health and development of our faculties.
* * * * *
Humphry began by studying the laws which regulate the expansion of bodies
under the three different forms in which they exist in nature, viz.
_solids, liquids, and gases or vapours_. To determine the expansion of
different solids, the youth procured short bars of the several substances
upon which he had decided to experiment. The bars were all of the same
length and thickness, and were accompanied with a gauge, which measured
their dimensions at ordinary temperatures.
The following diagram illustrates the apparatus employed. The first
step was to test the length and breadth of each bar that was to be used.
This was performed first, by placing it in the gap at the upper part of
the gauge, and seeing whether it exactly fitted between the notches; and
secondly, ascertaining whether it was precisely of the same diameter
as the hole at the bottom part of the plate. This done, the bars were
successively plunged into hot water, when, on applying them once more
to the gauge, they were found to be so much _enlarged_ in all their
dimensions that it was impossible to make them pass through either of
the apertures. After this they were severally cooled down, by immersion
in a mixture of snow and salt, to the temperature of the freezing-point
of water, when they were discovered to have considerably _contracted_
in bulk; so that they could be passed through both of the openings with
perfect ease.
[Illustration]
It was by such means Humphry ascertained that different solids possessed
different degrees of expansibility, and that metals are more susceptible
of change of bulk than other solid bodies. Each solid, however, was found
to have a rate of dilatation peculiar to itself. _Lead_, for instance,
when heated, from the freezing to the boiling-point of water, was
discovered by measurement to have expanded one-350th; _iron_, one-800th;
and _glass_, one-1000th.
_Platinum_, however, was found to be less expansible than iron, and
_copper_ more so. _Silver_, on the other hand, was more expansible than
copper, while _tin_ was more so than silver; _lead_, again, more than
tin, and _zinc_ even more than lead: so that glass was proved to be less
capable of being increased in bulk by heat than the metals; whilst,
among the metals themselves, platinum was ascertained to be the least
expansible, and zinc the most so.
On talking over these matters with Mr. Borlase, the doctor told Humphry
that the expansion of metals was a matter of considerable importance in
many arts. “For instance,” said the gentleman, “coopers put the iron
hoops upon their casks in a heated state, so that they may gradually
contract on cooling, and firmly bind the staves together. With the same
view the wheelwright heats the tire of his wheel, in order that it may,
as it cools, press strongly upon the ‘felly,’ or circumference; and, for
the same reason, the plates of large boilers are united with red-hot
rivets, which, during their contraction on cooling, draw the sheets of
metal closely and securely together.
“In the iron bridge,” continued the doctor, “which was constructed over
the Severn in Shropshire, when I was a lad, it has been found that the
arches are nearly one inch longer in summer than they are in winter; so
that, if due allowance had not been made for the expansion of the metal,
the stone piers, on which the arches rest, must have given way to the
pressure long before this. The same allowance for expansion, Humphry,
has to be made in the clamping together of stones in the construction
of church steeples; for the changes of bulk which occur in metals at
different temperatures, though comparatively small in amount, take place
with irresistible force.”
The perpendicularity of the walls of the Museum of Arts and Manufactures,
in Paris, it may be added, were restored by Molard, upon the same
principle. In consequence of the weight of the roof, the walls were
bulging outward, and, in order to straighten them, iron rods were laid
across the interior of the building, their ends being made to project
through the brickwork outside. These rods were then heated, and, when in
an expanded state, a strong iron plate was passed over each end of them,
and screwed firmly up against the exterior of the walls. As the rods
cooled, they naturally contracted, and drew the walls somewhat nearer
together. The bars were afterwards again elongated by heat, and again
screwed up previous to their contraction; and so, by a repetition of the
process, the walls were gradually brought to a perpendicular position.
Humphry was delighted with the ingenious applications of the expansion
and contraction of metals by heat and cold, and Mr. Borlase, observing
the interest he took in the subject, proceeded to explain to him how, by
the same principle, the alterations in the length of the pendulum of a
clock were “compensated,” and the instrument so made to vibrate seconds
at all seasons. For a pendulum to beat exactly sixty times a minute, he
told the boy, it was necessary that it should be a fraction more than 39
inches long, in the latitude of London. “If, however, the pendulum be
made of metal,” he said, “it will be liable to be _longer_ in summer and
_shorter_ in winter; so that the clock will be slow in the warm weather
season and fast in the cold: for when the bob is let down the one-100th
part of an inch the clock loses 10 seconds in 24 hours, and a change of
temperature equal to 30° (which is nearly the difference between summer
and winter, in our climate), will alter the length of the pendulum-rod
about one-5000th part, and so occasion an error in the rate of going of 8
seconds a day.
“To counteract the expansions of the metal rod of the pendulum,”
continued his preceptor, “there are many ingenious contrivances. The
simplest of these, perhaps, is as follows: A compound bar of two
differently expansive metals, such as steel and brass, is formed by
rivetting or soldering the two metals together; for if such a bar, with
the brass _uppermost_, be placed upon a heated plate, it will be found to
warp or curve _downwards_, in consequence of the expansion of the brass
being greater than that of the steel. If, however, on the other hand,
the compound bar be placed on a plate cooled down by a mixture of snow
and salt, it will be found to warp or curve _upwards_, because the brass
will contract the more with the cold. Now, if two such compound bars,
with the most expansible metal at top, be placed at the upper part of a
pendulum-rod, one on either side of it, and firmly fixed at one end, they
will, as they warp upwards or downwards, tend to shorten the pendulum-rod
when it becomes lengthened by the heat, and to lengthen it when it
becomes contracted by the cold.”
To make this more readily intelligible to Humphry, the doctor exhibited
to him the following engravings:
[Illustration]
“Let us now,” said Mr. Borlase, as he placed his finger on the centre
drawing, “suppose the pendulum, with the compensation-bars perfectly
horizontal, to be vibrating seconds at a temperature of 60°, and that,
some few months afterwards, the heat rises to 80°; in such a case, of
course, the pendulum-rod would be _elongated by the heat_, and the
longer the rod the slower the vibrations, so that it would then vibrate
_less_ than sixty times in the minute. The effect of the increase of
temperature, however, on the compensation-bars (the most expansible
metal being uppermost), would be to warp them _downwards_ (as shown in
the left-hand drawing),” said the doctor, pointing to the illustration,
“and thus they would _shorten_ the pendulum-rod as much as the heat had
lengthened it. In cold weather, however, Humphry, the metal rod of the
pendulum would be _diminished in length_; but then the compensation-bars
would warp _upwards_, and so tend to _elongate_ it, to the same extent as
it had been contracted by the cold (as may be seen on reference to the
picture on the right hand).”
The next day Humphry was busy making experiments concerning the expansion
of _liquids_. He first took a large thermometer tube and poured into it
a sufficient quantity of spirits of wine to fill the bulb at the bottom
and make the fluid rise some few inches in the stem above. Then, having
marked upon the glass with a file the level at which the spirit stood at
an ordinary temperature, the boy plunged the instrument into a vessel of
boiling water, and immediately beheld the liquid rise in the tube till
it stood several inches above its former level. After this he immersed
the tube in a mixture of snow and salt, and found the liquid contract,
so that it fell in the stem almost down to the bulb. On removing the
instrument, however, the fluid immediately commenced rising again, and so
pleased was the youth with the motions that he repeated the experiment
over and over again, being not a little delighted to perceive that each
time, as he plunged the instrument into the hot or cold bath, the spirit
invariably rose or fell to precisely the same place in the tube.
To measure the different rates of expansion among different liquids,
the young chemist provided himself with a long and narrow glass tube,
which was graduated into cubic inches, and into this he poured a certain
quantity of the liquid he wished to experiment upon. Then plunging the
graduated tube into the snow-and-salt mixture, he noted the precise
volume of the fluid at that temperature; after which he immersed it in a
vessel of boiling water, and then noted again how many cubic inches it
occupied in the tube at the higher temperature; so that the difference
told him how much the liquid had been expanded between 32° and 212°, or
the freezing and boiling point of water.
It was thus the lad ascertained that 9 measures of spirits of wine at
32° become expanded into 10 measures at 212°, and 9 measures of strong
aquafortis also become 10, between the same extremes of temperature.
Again, 12 measures of olive oil are increased into 13, while 14 of
ether and the same quantity of oil of turpentine swell each into 15,
with the like increase of heat. Then 17 measures of oil of vitriol, at
the freezing-point of water, are dilated into 18 at the boiling-point,
and 22¾ measures of water are increased to 23¾ within the same range of
temperature, while 55½ of quicksilver become 56½ when similarly treated;
consequently spirits of wine is no less than 6 times more expansible than
quicksilver, so that in the depth of winter 100 pints of spirits of wine
are dilated into 105 in the height of summer.
While making his experiments, however, as to the rate of expansion in
liquids, the boy had been astonished to perceive, when the tube contained
water, that, on placing it in the mixture of snow and salt, the liquid,
as it was cooled down, continued to shrink till it had attained the
temperature of about 40°; and then, _instead of contracting any farther_
(as was the case with other liquids till they froze), _it began to expand
slowly, and kept rising in the tube until it congealed_. He noticed, too,
that, when the water was at its freezing point, or 32°, it was of the
same bulk as it was at 48°; so that it expanded just as much for the 8°
below 40° as it had contracted in the 8° above that point.
Humphry then tried another experiment illustrative of this remarkable
property of water. Having produced two cylindrical glass vessels, he
surrounded one of them at the bottom with a circular tin tray, that
fitted closely to the exterior of the cylinder, and affixed to the other
a similar tray, but this he placed at the upper, instead of the lower
part of the cylinder as before—in the manner represented in the subjoined
engraving:
[Illustration]
A thermometer then having been placed in each of the glass vessels, they
were respectively filled with water at 50°, while a freezing mixture of
pounded ice and salt was placed in each of the trays.
After the temperature of the whole of the water in both vessels had been
reduced to 40°, it was found by the thermometer in the vessel, with
the freezing mixture _at the top_, that the cooling effect would not
proceed downwards, _but was limited to the surface_, where the water
ultimately froze; for the ice-cold water being _lighter_ than the water
below at 40°, necessarily _floated_ like oil upon the surface. In the
other cylinder, however, where the cold was applied at the _lower_ rather
than the upper part of the water, the effect was very different; for
there, the liquid becoming lighter, as its temperature sank below 40°,
_ascended_, whilst the warmer and heavier water at the top _descended_,
until it was cooled, and so expanded in its turn; and thus the _whole_
of the liquid was ultimately reduced to the freezing-point; whereas
in the other cylinder this effect was limited to the _surface_ only.
Humphry now could see the reason why lakes and ponds froze only on the
surface, and why, on breaking the ice (as he had repeatedly done when out
snipe-shooting with his uncle, Leonard Millett), the water underneath was
always found to be warmer than the air above.
* * * * *
The lad had now but to investigate the rate of expansion among _aëriform
bodies_, or _gases_, to complete this part of the subject.
Accordingly, he took the thermometer tube he had before used, and placed
it, with its open end, downwards, in a glass of water, thus:
[Illustration]
The tube was of course filled with air, so he applied his palm to the
bulb, and found the heat of the hand sufficient to expand the air within,
and drive a stream of bubbles up through the water. On removing the
source of heat, however, the volume of air began to contract, and the
liquid to mount in the tube, so that he could see by the height the water
rose in the stem the amount of expansion which the air had undergone.
Humphry then proceeded to ascertain the amount of expansion produced in
a given quantity of air, when heated from the freezing to the boiling
point of water, and discovered that 100 cubic inches of air at 32°
become dilated to 137½ cubic inches at 212°. Air, therefore, at the
freezing-point, expands one-480th for every degree of heat that is added
to it; so that 480 cubic inches at 32° become 481 at 33°, and 482 at
34°, and so on, the volume expanding one cubic inch with each additional
degree of heat. A volume of air, therefore, at 32° would be doubled
at 480°, and tripled at 960°, the latter temperature being that of a
dull-red heat.
Steam, and other vapours, when heated by themselves, are subject to the
same law of expansion as air.
But although the expansion produced in aëriform bodies by heat is great
in amount, the actual force which is thus developed is small when
compared with that of solids and liquids under the same circumstances.
This is owing to the extreme elasticity of aëriform bodies; so that,
although air becomes tripled in volume at a red heat, vessels are easily
found capable of sustaining the pressure of the expanded fluid. It is
only when a portion of liquid is present, so that _volume after volume_
of vapour is added to those already generated—as in the production of
steam—that, on resisting the expansion, the pressure becomes enormous,
and mounts up to a dangerous point.