The growth of accurate knowledge is continually narrowing, and
often obliterating, the broad lines of distinction that have been
drawn between different classes of things. I well remember when our
best naturalists regarded their “species” of plants and animals as
fundamental and inviolable institutions, separated by well-defined
boundaries that could not be crossed. Darwin has upset all this, and
now we cannot even draw a clear, sharp line between the animal and
vegetable kingdoms. The chemist is even crossing the boundary between
these and the mineral kingdom, by refuting the once positive dictum
that organic substances (_i.e._, the compounds ordinarily formed in the
course of vegetable or animal growth) cannot be produced directly from
dead matter by any chemical device. Many of such organic compounds are
now made in the laboratory from mineral materials.
We all know, broadly, what are the differences between solids, liquids,
and gases, and, until lately, they have been very positively described
as the three distinct states or modes of existence of matter. Mr.
Crookes suggests a fourth. I will not discuss this at present, but
merely consider the three old-established claimants to distinctive
existence.
A solid is usually defined as a body made up of particles which hold
together rigidly or immovably, in contradistinction to a fluid, of
which the particles move freely over each other. “Fluids” is the
general term including both gases and liquids, both being alike as
regards the mobility of their particles. At present, let us confine our
attention to liquids and solids.
The theoretical or perfect fluid which is imagined by the mathematician
as the basis of certain abstract reasonings has no real existence. He
assumes (and the assumption is legitimate and desirable, provided its
imaginary character is always remembered) that the supposed particles
move upon each other with perfect freedom, without any friction or
other impediment; but, as a matter of fact, all liquids exert some
amount of resistance to their own flowing; they are more or less
_viscous_, have more or less of that sluggishness in their obedience to
the law of finding their own level which we see so plainly displayed by
treacle or castor oil.
This viscosity, added to the friction of the liquid against the solid
on which it rests, or in which it is enclosed, may become, even in the
case of water, a formidable obstacle to its flow. Thus, if we make a
hole in the side of a tank at a depth of 16 feet below the surface,
the water will spout from that hole at the rate of 32 feet per second,
but if we connect with this hole a long horizontal pipe of the same
internal diameter as the hole, and then observe the flow from the
outlet of the pipe, we shall find its velocity visibly diminished,
and we shall be greatly deceived if we make arrangements for carrying
swift-flowing water thus to any great distances.
Three or four years ago an attempt was made to supersede the
water-carts of London by laying down on each side of the road a
horizontal pipe, perforated with a row of holes opening towards the
horse-way. The water was to be turned on, and from these holes it was
to jet out to the middle of the road from each side, and thus water it
all. I watched the experiment made near the Bank of England.
Instead of spouting across the road from all these holes, as it would
have done from any _one_ of them, it merely dribbled; the reason being
that, in order to supply them all, the water must run through the whole
of the long pipe with considerable velocity, and the viscosity and
friction to be overcome in doing this nearly exhausted the whole force
of water-head pressure. Many other similar blunders have been made by
those who have sought to convey water-power to a distance by means of a
pipe of such diameter as should demand a rapid flow through a long pipe.
The resistance which water offers to the stroke of the swimmer or the
pull of the rower is partly due to its viscosity, and partly to the
uplifting or displacement of some of the water. If it were perfectly
fluid, our movements within it, and those of fishes, etc., would be
curiously different; the whole face of this globe would be strangely
altered in many respects.
I will not now follow up this idea, but leave it as a suggestion for
the reader to work out for himself, by considering what would remain
undone upon the earth if water flowed perfectly, without any internal
resistance, or friction upon the earth’s surface.
The degrees of approach to perfect fluidity vary greatly with different
liquids.
Is there any such a thing as an absolute solid, or a body that has no
degree of fluidity, the particles or parts of which will admit of no
change of their relative positions, no movement upon each other without
fracture of the mass? This would constitute perfect _rigidity_, or the
opposite to _fluidity_.
Take a piece of copper or soft iron wire, about one eighth of an inch
in diameter, or thereabouts, and bend it backwards and forwards a few
times as rapidly as possible, but without breaking it; then, without
loss of time, feel the portion that has been bent. It is hot—painfully
so—if the experiment is smartly made. How may this be explained?
It is evident that in the act of bending there must have been a
displacement of the relative positions of the particles of the metal,
and the force demanded for the bending indicated their resistance
to this movement upon each other; or, in other words, that there
was friction between them, or something equivalent to such internal
friction, and thus the mechanical force exerted in the bending was
converted into heat-force.
Here, then, was fluidity, according to the above definition; not
perfect fluidity, but fluidity attended with resistance to flow, or
what we have agreed to call viscosity. But water also offers such
resistance to flow, or viscosity, therefore the difference between
iron or copper wire and liquid water as regards their fluidity is
only a difference of degree, and not of kind; the demarcation between
solids and liquids is not a broad, clearly-defined line, but a band
of blending shade, the depths of tint representing varying degrees of
viscosity.
Multitudes of examples may be cited illustrating the viscosity of
bodies that we usually regard as types of solidity, such, for example,
as the rocks forming the earth’s crust. In the “Black Country” of South
Staffordshire, which is undermined by the great ten-yard coal-seam,
cottages, chimney-shafts, and other buildings may be seen leaning over
most grotesquely, houses split down the middle by the subsidence or
inclination of one side, great hollows in fields or across roads that
were once flat, and a variety of other distortions, due to the gradual
sinking of the rock-strata that have been undermined by the colliery
workings. In some cases the rocks are split, but usually the subsidence
is a bending or flowing down of the rocks to fill up the vacuity, as
water fills a hollow, or “finds its own level.”
I have seen many cases of the downward curvature of the roof of a
coal-pit, and have been told that in some cases the surrounding
pressure causes the floor to curve upwards, but have not seen this.
Earthquakes afford another example. The so-called solid crust of the
earth is upheaved, and cast into positive billows that wave away on
all sides from the centre of disturbance. The earth-billows of the
great Lisbon earthquake of 1755 traveled to this country, and when they
reached Loch Lomond, were still of sufficient magnitude to raise and
lower its banks through a perpendicular range of two feet four inches.
It is quite possible, or, I may say, probable, that there are tides
of the earth as well as of the waters, and the subject has occupied
much attention and raised some discussion among mathematicians. If the
earth has a fluid centre, and only a comparatively thin crust, as some
suppose, there must be such tides, produced by the gravitation of the
moon and sun.
Ice presents some interesting results of this viscosity. At a certain
height, varying with latitude, aspect, etc., we reach the “snow line”
of mountain slopes, above which the snow of winter remains unmelted
during summer, and, in most cases, goes on accumulating. It soon loses
its flocculent, flaky character, and becomes coherent, clear blue ice
by the pressure of its own weight.
A rather complex theory has been propounded to explain this change—the
theory of _regelation_—_i.e._, re-freezing; a theory which assumes
that the pressure first thaws a film of ice at the surface of contact,
and that presently this re-freezes, and thus effects a healing or
general solidification. Faraday found that two pieces of ice with
moistened surfaces united if pressed together when at just about
the temperature of freezing, but not if much colder. Tyndall has
further illustrated this by taking fragments of ice and squeezing
them in a mould, whereby they became a clear, transparent ball, or
cake. Schoolboys did the like long before, when snowballing with snow
at about the thawing point. Such snow, as we all remember, became
converted into stony lumps when firmly pressed together. We also
remember that in much colder weather no such cohesion occurred, but our
snowballs remained powdery in spite of all our squeezing.
I am a sceptic as regards this theory of regelation. I believe that
the true explanation is much simpler; that the crystals of snow or
fragments of ice in these experiments are simply welded, as the smith
unites two pieces of iron, by merely pressing them together when they
are near their melting point. Other metals and other fusible substances
may be similarly welded, provided they soften or become sufficiently
viscous before fusing.
Platinum is a good example of this. It is infusible in ordinary
furnaces, but becomes pasty before melting, and therefore, one method
adopted in the manufacture of platinum ingots or bars from the ore,
is to precipitate a sort of platinum snow (spongy platinum) from its
solution in acid, and then compress this metallic snow in red-hot steel
moulds by means of pistons driven with great force. The flocculent
metal thus becomes a solid, coherent mass, just as the flocculent ice
became coherent ice in Tyndall’s experiment or in making hard snowballs.
Wax, pitch, resin, and all other solid that fuse _gradually_, cohere,
are weldable, or, in very plain language, “stick together,” when near
their fusing point.
I have made the following experiment to prove that when this so-called
regelation of snow or ice-fragments occurs, the ice is viscous or
plastic, like wax or pitch. A strong iron squirt, with a cylindrical
bore of half an inch in diameter, is fitted with an iron piston. This
piston is driven forth by a screw working in a collar at one end of the
squirt. Into the other end is screwed a brass nozzle with an aperature
about one twentieth of an inch diameter, tapering or opening inwards
gradually to the half-inch bore.
Into this bore I place snow or fragments of ice, then, holding the body
of the squirt firmly in a vice, I work the lever of the screw, and thus
drive forward the piston and crush down the snow or ice-fragments,
which presently become coherent and form a half-inch solid cylinder of
clear ice. Applying still more pressure, this cylinder is forced like
a liquid through the small orifice of the nozzle of the squirt, and
it jets or spouts out as a thin stick of ice like vermicelli, or the
“leads” of ever-pointed pencils, for the moulding of which the squirt
was originally constructed.
I find that ice at 32° can thus be squirted more easily than beeswax
of the same temperature, and such being the case, I see no reason
for imagining any complex operation of regelation in the case of
the ice, but merely regard the adhesion of two pieces of ice when
pressed together as similar to the sticking together of two pieces of
cobblers’-wax, or softened sealing-wax, or beeswax, or the welding of
iron or glass when heated to their welding temperatures, _i.e._, to a
certain degree of incipient fluidity or viscosity.
If a leaden bullet be cut in half, and the two fresh-cut faces pressed
forcibly together, they cohere at ordinary atmospheric temperatures,
but we have no occasion for a regelation theory here. The viscosity
of the lead accounts for all. At Woolwich Arsenal there is a monster
squirt, similar to my little one. This is charged with lead, and, by
means of hydraulic pressure, the lead is squired out of the nozzle as a
cylindrical jet of any required diameter. This jet or stick of lead is
the material of which the elongated cylindrical rifle bullets are now
made.
But returning to the point at which we started, on the subject of ice,
viz., its Alpine accumulation above the snow-line. If the snow-fall
there exceeds the amount that is thawed and evaporated, it must either
go on growing upward until it reaches the highest atmospheric region
from which it falls, or is formed, or it must descend somehow.
If ice can be squirted through a syringe by mere hand-pressure, we are
justified in expecting that it would be forced down a hill slope, or
through a gully, or across a plain, by the pressure of its own weight
when the accumulation is great. Such is the case, and thus are glaciers
formed.
They are, strictly speaking, rivers or torrents of ice; they flow as
liquid water does, and down the same channels as would carry the liquid
surface drainage of the hills, were rain to take the place of snow.
Like rivers, they flow with varying speed, according to the slope; like
rivers, their current is more rapid in the middle than the sides; like
rivers, they exert their greatest tearing force when squeezed narrow
through gullies; and, like rivers, they spread out into lakes when they
come upon an open basin-like valley, with narrow outlet.
The Justedalsbrae of Norway is a great ice-lake of this character,
covering a surface of about 500 square miles, and pouring down its
ice-torrents on every side, wherever there is a notch or valley
descending from the table-land it covers. The rate of flow of such
downpouring glaciers varies from two or three inches to as many feet
per day, and they present magnificent examples of the actual fluidity
or viscosity of an apparently solid mass. This viscosity has been
disputed, and attempts have been made to otherwise explain the motion
of glaciers; but while it is possible that it may be assisted by
varying expansion and contraction, the downflow due to viscosity is now
recognized as unquestionably the main factor of glacier motion.
Cascades of ice may be sometimes seen. In the course of my first visit
to Norway, I wandered alone over a very desolate mountain region
towards the head of the Justedal, and unexpectedly came upon a gloomy
lake, the Styggevand, which lies at the foot of a precipice-boundary
of the great ice-field above named. Here, the ice having no sloping
valley-trough by which to descend, poured over the edge of the
precipice as a great overhanging sheet or cornice, which bent down
as it was pushed forward, and presented on the convex side of the
sheet some fine blue cracks, or “crevasses” as they are called. These
gradually widened and deepened, until the overhanging mass broke off
and fell into the lake, on the surface of which I saw the result, in
the form of several floating icebergs that had previously fallen.
Something like this, on a small scale, may be seen at home on the edge
of a house roof, on which there has been an accumulation of snow; but,
in this case, it is rather sliding than flowing that has made the
cornice; but its _down-bending_ is a result of viscosity.
These and a multitude of other facts that might be stated, many of
which will occur to the reader, prove clearly enough that the solid and
liquid states of matter are not distinctly and broadly separable, but
are connected by an intermediate condition of viscosity.
We now come to the question whether there is any similar continuity
between liquids and gases. Ordinary experience decidedly suggests a
negative answer. We can point to nothing within easy reach that has the
properties of a liquid and gaseous half-and-half; that stands between
gases and liquids as pitch and treacle stand between solids and liquids.
Some, perhaps, may suggest that cloud-matter—London fog, for
example—is in such an intermediate state. This, however, is not the
case. White country fog, ordinary clouds, or the so-called “steam”
that is seen assuming cloud forms as it issues from the spout of a
tea-kettle or funnel of a locomotive, consists of minute particles of
water suspended in air, as solid particles of dust are also suspended.
It has been called “vesicular vapor,” on the supposition that it
has the form of minute vesicles, like soap-bubbles on a very small
scale, but this hypothesis remains unproven. London fog consists of
similar particles, varnished with a delicate film of coal-tar, and
intersprinkled with particles of soot.
In order to clearly comprehend the above-stated question, we must
define the difference between liquids and gases. In the first place,
they are both fluids, as already agreed. What, then, is the essential
difference between liquid fluidity and gaseous fluidity? The expert
in molecular mathematics, discoursing to his kinematical brethren,
would produce a tremendous reply to this question. He would describe
the oscillations, gyrations, collisions, mean free paths, and mutual
obstructions of atoms and molecules, and, by the aid of a maddening
array of symbols, arrive at the conclusion that gases, unless
restrained, expand of their own accord, while liquids retain definite
limits or dimensions.
The matter-of-fact experimentalist demonstrates the same by methods
that are easily understood by anybody. I shall, therefore, both for my
own sake and my readers’, describe some of the latter.
In the first place, we all see plainly that liquids have a surface,
_i.e._, a well-defined boundary, and also that gases, unless enclosed,
have not. But as this may be due to the invisibility of the gas, we
must question it further. The air we breathe may be taken as a type
of gases, as water may of liquids. It has weight, as we may prove by
weighing a bottle full of air, then pumping out the contents, weighing
the empty bottle, and noting the difference.
Having weight, it presses towards the earth, and is squeezed by all
that rests above it; thus the air around us is constrained air. It is
very compressible, and is accordingly compressed by the weight of all
the air above it.
This being understood, let us take a bottle full of water and another
full of air, and carry them both to the summit of Mont Blanc, or to a
similar height in a balloon. We shall then have left nearly half of the
atmosphere below, and thus both liquid and gas will be under little
more than half of the ordinary pressure. What will happen if we uncork
them both? The liquid will still display its definite surface, and
remain in the bottle, but not so the gas. It will overflow upwards,
downwards, or sideways, no matter how the bottle is held, and if we
had tied an empty bladder over the neck before uncorking, we should
find this overflow or expansion of the gas exactly proportionate to
the removal of pressure, provided the temperature remained unaltered.
Thus, at just half the pressure under which a pint bottle was corked,
the air would measure exactly one quart, at one-eighth of the pressure
one gallon, and so on.
We cannot get high enough for the latter expansion, but can easily
imitate the effect of further elevation by means of an air-pump. Thus,
we may put one cubic inch of air into a bladder of 100 cubic inches
capacity, then place this under the receiver of an air-pump, and reduce
the pressure outside the bladder to 1/100th of its original amount.
With such atmospheric surrounding, the one cubic inch of air will plump
out the flaccid bladder, and completely fill it. The pumpability of the
air from the receiver shows that it goes on overflowing from it into
the piston of the pump as fast as its own elastic pressure on itself is
diminished.
Numberless other experiments may be made, all proving that all
gases are composed of matter which is not merely incohesive, but is
energetically self-repulsive; so much so, that it can only be retained
within any bounds whatever by means of some external pressure or
constraint. For aught we know _experimentally_, the gaseous contents
of one of Mr. Glaisher’s baloons would outstretch itself sufficiently
to occupy the whole sphere of space that is spanned by the earth’s
orbit, provided that space were perfectly vacuous, and the baloon were
burst in the midst of it, the temperature of the expanding gas being
maintained.
Here, then, in this self-repulsiveness, instead of self-cohesion, this
absence of self-imposed boundary or dimensions, we have a very broad
and well-marked distinction between gases and liquids, so broad that
there seems no bridge that can possibly cross it. This was believed to
be the case until recently. Such a bridge has, however, been built, and
rendered visible, by the experimental researches of Dr. Andrews; but
further explanation is required to render this generally intelligible.
Until quite lately it was customary to divide gases into two
classes—“permanent gases” and “condensable gases,” or “vapors.”
Gaseous water or steam was usually described as typical of the latter;
oxygen, hydrogen, or nitrogen of the former. Earlier than this, many
other gases were included in the permanent list; but Faraday made a
serious inroad upon this classification when he liquefied chlorine
by cooling and compressing it. Long after this, the gaseous elements
of water, and the chief constituents of air, oxygen, hydrogen, and
nitrogen, resisted all efforts to condense them; but now they have
succumbed to great pressure and extreme cooling.
We thus arrive at a very broad generalization, viz., that all gases are
physically similar to steam (I mean, of course, “dry steam,” _i.e._,
true invisible steam, and not the cloudy matter to which the name of
steam is popularly given), that they are all formed by raising liquids
above their boiling point, just as steam is formed when we boil water
and maintain the steam above the boiling-point of the water.
But some liquids boil at temperatures far below that at which others
freeze; liquid chlorine boils at a temperature below that of freezing
water, and liquid carbonic acid below even that of freezing mercury,
and liquid hydrogen far lower still. These are cases of boiling,
nevertheless, though it seems a paradox according to the ideas we
commonly attach to this word. But such ideas are based on our common
experience of the properties of our commonest of liquids, viz., water.
When water boils under the conditions of our ordinary experience, the
passage from the liquid to the gaseous state is a sudden leap, with
no intermediate state of existence that we are able to perceive; and
the conditions upon which water is converted into steam—the liquid
into the gas—while both are at the bottom of our atmospheric ocean,
are such as to render an intermediate condition rationally, as well as
practically, impossible.
We find that the expansive energy by which the steam is enabled to
resist atmospheric pressure is conferred upon it by its taking into
itself, and utilizing for its expansive efforts a large amount of
calorific energy. When any given quantity of water is converted into
steam, under ordinary circumstances, its bulk _suddenly_ becomes
above 1700 times greater—a cubic inch of water forms about a cubic
foot of steam, and nearly 1000 degrees of heat (966·6) disappears
_as temperature_. Otherwise stated, we must give to the cubic inch
of water at 212° as much heat as would raise it to a temperature of
212 plus 966·6, or 1,178·6°, if it remained liquid. This is about
the temperature of the glowing coals of a common fire; but the steam
that has thus taken enough heat to make the water red-hot is still at
212°—no _hotter_ than the water was while boiling.
This heat, which thus ceases to exhibit itself as _temperature_,
is otherwise occupied. Its energy is partly devoted to the work of
increasing the bulk of the water to the above-named extent, and
partly in conferring on the steam its gaseous specialty—that is,
in overcoming liquid cohesion, and substituting for it the opposite
property of internal repulsive energy which is characteristic of gases.
My reasons for thus defining and separating these two functions of the
so-called “latent” heat will be seen when we come to the philosophy of
the interesting researches of Dr. Andrews.
As already explained, all gases are now proved to be analogous to
steam, they are matter expanded and rendered self-repulsive by heat.
All _elementary_ matter may exist in either of the three forms—solid,
liquid, or gas, according to the amount of heat and pressure to which
it is subjected. I limit this wide generalization to _elementary_
substances for the following reasons:
Many compounds are made up of elements so feebly held together
that they become “dissociated” when heated to a temperature below
their boiling-point; or, their condition maybe otherwise defined by
stating that the bonds of chemical energy, which hold their elements
together, are weaker than the cohesion which binds and holds them in
the condition of solid or liquid, and are more easily broken by the
expansive energy of heat.
To illustrate this, let us take two common and well-known oils—olive
oil and turpentine. The first belongs to the class of “fixed oils,” and
second to the “volatile oils.” If we apply heat to liquid turpentine,
it boils, passes into the state of gaseous turpentine, which is easily
condensible by cooling it. If the liquid result of this condensation
is examined, we find it to be turpentine as before. Not so with the
olive oil. Just as this reaches its boiling point, the heat, which
would otherwise convert it into olive-oil vapor, begins to dissociate
its constituents, and if the temperature be raised a little higher, we
obtain some gases, but these are the products of decomposition, not
gaseous olive oil. This is called “destructive” distillation.
In olive oil, the boiling-point and dissociation point are near to each
other. In the case of glycerine, these points so nearly approximate
that, although we cannot distil it unbroken under ordinary atmospheric
pressure, we may do so if some of this pressure is removed. Under
such diminished pressure, the boiling-point is brought down below the
dissociation point, and condensible glycerine gas comes over without
decomposition.
Sugar affords a very interesting example of dissociation, commencing
far below the boiling-point, and going on gradually and visibly, with
increasing rapidity as the temperature is raised. Put some white sugar
into a spoon, and heat the spoon gradually over the smokeless gas-flame
or spirit-lamp. At first the sugar melts, then becomes yellow (barley
sugar); this color deepens to orange, then red, then chestnut-brown,
then dark brown, then nearly black (caramel), then quite black, and
finally it becomes a mere cinder. Sugar is composed of carbon and
water; the heat dissociates this compound, separates the water,
which passes off as vapor, and leaves the carbon behind. The gradual
deepening of the color indicates the gradual carbonization, which is
completed when only the dry insoluble cinder remains. An appearance of
boiling is seen, but this is the boiling of the dissociated water, not
of the sugar.
The dissociation temperature of water is far above its boiling-point.
It is 5072° Fahr., under conditions corresponding to those which
make its boiling-point 212°. If we examine the variations of the
boiling-point of water, as the atmospheric pressure on its surface
varies, some curious results follow. To do this the reader must endure
some figures. They are extremely simple, and perfectly intelligible,
but demand just a little attention.
Following are three columns of figures. The first represents
atmospheres of pressure—_i.e._, taking our atmospheric pressure when
it supports 30 inches of mercury in the barometer tube as a unit, that
pressure is doubled, trebled, etc., up to twenty times in the first
column. The second column states the temperature at which water boils
when under the different pressures thus indicated. The third column,
which is the subject for special study just now, shows how much we
must rise the temperature of the water in order to make it boil as we
go on adding atmospheres of pressure; or, in other words, the increase
of temperature due to each increase of one atmosphere of pressure. The
figures are founded on the experiments of Regnault.
Pressure in Temperature, F. Rise of Temperature
Atmospheres ° for each additional
Atmosphere
1 212
2 249·5 37·5
3 273·3 23·8
4 291·2 17·9
5 306·0 14·8
6 318·2 12·2
7 329·6 11·4
8 339·5 9·9
9 348·4 8·9
10 356·6 8·2
11 364·2 7·6
12 371·1 6·9
13 377·8 6·7
14 384·0 6·2
15 390·0 6·0
16 395·4 5·4
17 400·8 5·4
18 405·9 5·1
19 410·8 4·9
20 415·4 4·6
It may be seen from the above that, with the exception of one
irregularity, there is a continual diminution of the additional
temperature which is required to overcome an additional atmosphere of
pressure, and if this goes on as the pressure and temperatures advance,
we may ultimately reach a curious condition—a temperature at which
additional pressure will demand no additional temperature to maintain
the gaseous state; or, in other words, a temperature may be reached at
which no amount of pressure can condense steam into water, or at which
the gaseous and liquid states merge or become indifferent.
But we must not push this mere numerical reasoning too far, seeing
that it is quite possible to be continually approaching a given point,
without ever reaching it, as when we go on continually halving the
remaining distance. The figures in the above do not appear to follow
according to such a law—nor, indeed, any other regularity. This
probably arises from experimental error, as there are discrepancies
in the results of different investigators. They all agree, however,
in the broad fact of the gradation above stated. Dulong and Arago,
who directed the experiments of the French Government Commission for
investigating this subject, state the pressure at 20 atmospheres to
be 418·4, at 21 = 422·9, at 22 = 427·3, at 23 = 431·4, and at 24
atmospheres, their highest _experimental_ limit, 435·5, thus reducing
the rise of temperature between the 23d and 24th atmospheres to 4·1.
If we could go on heating water in a transparent vessel until
this difference became a vanishing quantity, we should probably
recognize a visible physical change coincident with this cessation of
condensibility by pressure; but this is not possible, as glass would
become red-hot and softened, and thus incapable of bearing the great
pressure demanded. Besides this, glass is soluble in water at these
high temperatures.
If, however, we can find some liquid with a lower boiling-point, we may
go on piling atmosphere upon atmosphere of elastic expansive pressure,
as the temperature is raised, without reaching an unmanageable degree
of heat. Liquid carbonic acid, which, under a single atmosphere of
pressure, boils at 112° below the zero of our thermometer, may thus be
raised to a temperature having the same relation to its boiling-point
that a red-heat has to that of water, and may be still confined within
a glass vessel, provided the walls of the vessel are sufficiently thick
to bear the strain of the elastic outstriving pressure. In spite of its
brittleness glass is capable of bearing an enormous strain _steadily
applied_, as may be proved by trying to break even a mere thread of
glass by direct pull.
Dr. Andrews thus treated carbonic acid, and the experiment, as I have
witnessed its repetition, is very curious. A liquid occupies the lower
part of a very strong glass tube, which appears empty above. But this
apparent void is occupied by invisible carbonic acid gas, evolved by
the previous boiling of the liquid carbonic acid below. We start at
a low temperature—say 40° Fahr. Then the temperature is raised; the
liquid boils until it has given off sufficient gas or vapor to exert
the full expansive pressure or tension due to that temperature. This
pressure stops the boiling, and again the surface of the liquid is
becalmed.
This is repeated at a higher temperature, and thus continued until we
approach nearly to 88° Fahr., when the surface of the liquid loses
some of its sharp outline. Then 88° is reached, and the boundary
between liquid and gas vanishes; liquid and gas have blended into one
mysterious intermediate fluid; an indefinite fluctuating something
is there filling the whole of the tube—an etherealized liquid or a
visible gas. Hold a red-hot poker between your eye and the light; you
will see an upflowing wavy movement of what appears like liquid air.
The appearance of the hybrid fluid in the tube resembles this, but is
sensibly denser, and evidently stands between the liquid and gaseous
states of matter, as pitch or treacle stands between solid and liquid.
The temperature at which this occurs has been named by Dr. Andrews
the “_critical temperature_”; here the gaseous and liquid states are
“_continuous_,” and it is probable that all other substances capable of
existing in both states have their own particular critical temperatures.
Having thus stated the facts in popular outline, I shall conclude the
subject by indulging in some speculations of my own on the philosophy
of these general facts or natural laws, and on some of their possible
consequences.
As already stated, the conversion of water into steam under ordinary
atmospheric pressure demands 966·6° of heat over and above that which
does the work of raising the water to 212°, or, otherwise stated, as
much heat is at work in a given weight of steam at 212°, as would raise
the same quantity of water to 1178·6° if it remained liquid.
James Watt concluded from his experiments that a given weight of
steam, whatever may be its density, or, in other words, under whatever
pressure it may exist, contains the same quantity of heat. According
to this, if we reduced the pressure sufficiently to bring down the
boiling-point to 112°, instead of 212°, the latent heat of the steam
thus formed would be 1066·6° instead of 966·6°, or if, on the other
hand, we placed it under sufficient pressure to raise the boiling-point
to 312°, the latent heat of the steam would be reduced to 866·6°,
_i.e._, only 866·6° more would be required to convert the water into
steam. If the boiling-point were 412°, as it is between 19 and 20
atmospheres of pressure, only 766·6° more heat would be required, and
so on, till we reached a pressure which raised the boiling-point to
1178·6°; the water would then become steam without further heating,
_i.e._, the critical point would be reached, and thus, if Watt is
right, we can easily determine, theoretically, the critical temperature
of water.[36]
Mr. Perkins, who made some remarkable experiments upon very high
pressure steam many years ago, and exhibited a steam gun at the
Adelaide Gallery, stated that red-hot water does not boil; that if the
generator be sufficiently strong to stand a pressure of 60,000 lbs.
load on the safety-valve, the water may be made to exert a pressure of
56,000 lbs. on the square inch at a cherry-red heat without boiling.
He made a number of rather dangerous experiments in thus raising water
to a red-heat, and his assertion that red-hot water does not boil is
curious when viewed in connection with Dr. Andrews’ experiments.
I cannot tell how he arrived at this conclusion, having been unable
to obtain the original record of his experiments, and only quote the
above second hand. It is worthy of remark that the temperature he names
is about 1170°, or that which, if Watt is right, must be the critical
temperature of the water. Perkins’ red-hot water would not boil, being
then in the intermediate condition.
So far, we have a nice little theory, which not only shows how
the critical state of water must be reached, but also its precise
temperature; but all this is based on the assumption that Watt made no
mistake.
Unfortunately for the simplicity of this theory, Regnault states
that _his_ experiments contradict those of Watt, and prove that the
latent heat of steam does not diminish just in the same degree as
the boiling-point is raised, but that instead of this the diminution
of the latent heat progresses 30½ per cent more slowly than the rise
of temperature, so that, instead of the latent heat of steam between
boiling-points of 212° and 312° falling from 966·6° to 866·6° it
would only fall to 895·1° or 69·5° of latent heat for every 100° of
temperature.
If this is correct, the temperature at which the latent heat of
steam is reduced to zero is much higher than 1178·6°, and is, in
fact, a continually receding quantity never absolutely reached; but
I am not prepared to accept these figures of Regnault as implicitly
as is now done in text-books (I was nearly saying “as is now the
fashion”), seeing that they are not the actual figures obtained by his
experiments, but those of his “empirical formulæ” based upon them.
His actual experimental figures are very irregular; thus, between
steam temperature of 171·6° and 183·2° a difference of 11·6°, the
experimental difference in the latent heat came out as 4·7°; between
steam temperature of 183·2° and 194·8°, or 11·6° again, the latent heat
difference is tabulated as 8·0°.
Regnault’s experiments were not carried to very high temperatures and
pressures, and indicate that as these advance the deviation from Watt’s
law diminishes, and may finally vanish at about 1500° or 1600°, where
the latent heat would reach zero, and there, according to the above,
the critical temperature would be reached. Any additional heat applied
after this will have but one function to perform, viz., the ordinary
work of increasing the bulk of the heated body without doing anything
further in the way of conferring upon it any new self-repulsive
properties.
Our notions of solids, liquids, and gases are derived from our
experiences of the state of matter here upon this earth. Could we be
removed to another planet, they would be curiously changed. On Mercury
water would rank as one of the condensible gases; on Mars, as a fusible
solid; but what on Jupiter?
Recent observations justify us in regarding this as a miniature sun,
with an external envelope of cloudy matter, apparently of partially
condensed water, but red-hot, or probably still hotter within. His
vaporous atmosphere is evidently of enormous depth, and the force of
gravitation being on his visible outer surface two and a half times
greater than that on our earth’s surface, the atmospheric pressure in
descending below this visible surface must soon reach that at which the
vapor of water would be brought to its critical condition. Therefore we
may infer that the oceans of Jupiter are neither of frozen liquid nor
gaseous water, but are oceans or atmospheres of critical water. If any
fish-birds swim or fly therein they must be very critically organized.
As the whole mass of Jupiter is three hundred times greater than
that of the earth, and its compressing energy towards the centre
proportional to this, its materials, if similar to those of the earth
and no hotter, would be considerably more dense, and the whole planet
would have a higher specific gravity; but we know by the movement of
its satellites that, instead of this, its specific gravity is less than
a fourth of that of the earth. This justifies the conclusion that it
is intensely hot, for even hydrogen, if cold, would become denser than
Jupiter under such pressure.
As all elementary substances may exist as solids, liquids, or gases, or
critically, according to the conditions of temperature and pressure,
I am justified in hypothetically concluding that Jupiter is neither a
solid, a liquid, nor a gaseous planet, _but a critical planet_, or an
orb composed internally of dissociated elements in the critical state,
and surrounded by a dense atmosphere of their vapors, and those of some
of their compounds, such as water. The same reasoning applies to Saturn
and the other large and rarefied planets.
The critical temperature of the dissociated elements of the sun is
probably reached at the base of the photosphere, or that region
revealed to us by the sun-spots. When I wrote “The Fuel of the Sun,”
thirteen or fourteen years ago, I suggested, on the above grounds,
the then heretical idea of the red-heat of Jupiter, Saturn, Uranus,
and Neptune, and showed that all such compounds as water must be
dissociated at the base of the sun’s atmosphere; but being then
unacquainted with the existence of this critical state of matter, I
supposed the dissociated elements to exist as gases with a small solid
nucleus or kernel in the centre.
Applying now the researches of Dr. Andrews to the conditions of
solar existence, as I formerly applied the dissociation researches
of Deville, I conclude that the sun has no nucleus, either solid,
liquid, or gaseous, but is composed of dissociated matter in the
critical state, surrounded, first, by a flaming envelope due to the
re-combination of the dissociated matter, and outside of this another
envelope of vapors due to this combination.