*Automatic Aniline Drops.*--In the foregoing experiments the drop was
enlarged until it broke away by feeding it with liquid; but it is
possible to arrange that the formation shall be quite automatic. The
experiment, as we shall see, is extremely simple, and yet it contains an
element of surprise. Into a beaker containing water nearly boiling I
pour a considerable quantity of aniline, which at first breaks up into a
large number of drops. After a short time, however, all the aniline
floats to the surface, having been warmed by contact with the water to a
temperature higher than that of equi-density (147 deg. F., or 64 deg.
C.)--which is exactly what we should expect to happen. There it remains
for a brief period in the form of a large mass with the lower portion
curved in outline. Soon, however, we observe the centre of the mass
sinking in the water, and taking on the now familiar outline of a
falling drop. Gradually, it narrows at the neck and breaks away; but as
aniline is a viscous liquid, the neck in this case is long and therefore
easily seen. The large drop breaks away and falls to the bottom of the
beaker, its upper surface rising and falling for some time owing to the
recoil of its skin after separation, finally becoming permanently
convex. Immediately after the large drop has parted, the upper mass
shrinks upwards, spreading out further on the surface of the water, with
the result that the long neck is severed at the top, its own weight
assisting the breakage. Now follows the resolution of the detached neck
into two or more spheres, usually a large and a small (Fig. 22). And
now, to those who view the experiment for the first time, comes the
surprise. The large drop, which was more or less flattened when it came
to rest at the bottom of the beaker, becomes more and more rounded, and
finally spherical. Then, unaided, it rises to the top and mingles itself
with the aniline which remained on the surface. After a brief interval a
second drop falls, imitating the performance of the first one; and, like
its predecessor, rises to the surface, after remaining for a short time
at the bottom of the vessel. And so long as we keep the temperature a
few degrees above that of equi-density, the process of partition and
reunion goes on indefinitely. The action is automatic and continuous,
and the large size of the drop and of the neck, and the slowness of the
procedure, enables us to follow with ease every stage in the formation
of a parting drop.
[Illustration: __Fig._ 21._]
[Illustration: __Figs._ 21 and 22.--Automatically formed aniline drops,
showing the formation of droplets from the neck._]
And now as to the explanation of this curious performance. When the
aniline reaches the surface, and spreads out, it cools by contact with
the air more rapidly than the water below. As it cools, its density
increases, and soon becomes greater than that of the water, in which it
then attempts to sink. The forces of surface tension prevent the whole
of the aniline from falling--the water surface can sustain a certain
weight of the liquid--but the surplus weight cannot be held, and
therefore breaks away. But when the detached drop reaches the bottom of
the vessel, it is warmed up again; and when its temperature rises above
that of equi-density it floats up to the top. And so the cycle of
operations becomes continuous, owing to cooling taking place at the top
and heating at the bottom.
Perpetual motion, you might suggest. Nothing of the kind. Perpetual
motion means the continuous performance of work without any supply of
energy; it does not mean merely continuous movement. A steam-engine
works so long as it is provided with steam, and an electric motor so
long as it is fed with electricity; but both stop when the supply of
energy is withdrawn. So with our aniline drop, which derives its energy
from the heat of the water, and which comes to rest immediately the
temperature falls below 147 deg. F. or 64 deg. C. But in order that the
process of separation and reunion may continue, the cooling at the top
is quite as necessary as the heating at the bottom. Our aniline drop is
in essence a heat-engine--although it does no external work--and like
all heat-engines possesses a source from which heat is derived, and a
sink into which heat at a lower temperature is rejected. We might, with
certain stipulations, work out an indicator diagram for our liquid
engine, but that would be straying too far from our present subject.
*Automatic Drops of other Liquids.*--Liquids which possess a low
equi-density temperature with water do not form automatic drops like
aniline, as the rate of cooling at the surface is too slow, and hence
the floating mass of liquid does not attain a density in excess of that
of the water beneath. Aceto-acetic ether, however, behaves like aniline,
if the temperature of the water be maintained at about 170 deg. F. (77
deg. C.), but as this liquid is fairly soluble in hot water further
quantities must be added during the progress of the experiment. Results
equal to those obtained with aniline, however, may be secured by using
nitrobenzene in nitric acid of specific gravity 1.2 at 59 deg. F. (15
deg. C.), the acid being heated to 185 deg. F. (85 deg. C.); and here
you see the yellow drop performing its alternate ascents and descents
exactly as in the case of aniline and water. Other examples might be
given; but we may take it as a general rule that when the equi-density
temperature of the liquid and medium is above 125 deg. F. (52 deg. C.),
the phenomenon of the automatic drop may usually be observed when the
temperature is raised by 30 deg. F. (17 deg. C.), above this point.
*Liquid Jets.*--So far we have been observing the formation of single
drops, growing slowly at the end of a tube, or breaking away from a
large mass of the floating liquid. If, however, we accelerate the speed
at which the liquid escapes, the drop has no time to form at the outlet,
and a jet is then formed. We are all familiar with a jet of water
escaping from a tap; it consists of an unbroken column of the liquid up
to a certain distance, depending upon the pressure, but the lower part
is broken up into a large number of drops, which break away from the
column at a definite distance from the tap. There are many remarkable
features about jets which I do not intend to discuss here, as it is only
intended to consider the manner in which the drops at the end are
formed. To observe this procedure, it is necessary again to resort to
our method of slowing down the rate of formation, by allowing the liquid
to flow into a medium only slightly inferior in density. For this
purpose, orthotoluidine falling into water at the ordinary room
temperature is eminently satisfactory; and we see on the screen the
projection of a pipe, with its end under water, placed so that a jet of
orthotoluidine may be discharged vertically downwards from a stoppered
funnel. I open the tap slightly at first, and we then merely form a
single drop at the end. Now it is opened more widely, and you observe
that the drop breaks away some distance below the outlet, being rapidly
succeeded by another and another (Fig. 23). On still further opening the
tap the drops form at a still greater distance from the end of the pipe,
and succeed each other more rapidly, so that quite a number appear in
view at any given moment (Figs. 24 and 25). Notice how the drop is
distorted by breaking away from the stream of liquid, and how it
gradually recovers its spherical shape during its fall through the
water. Finally, I increase the discharge to such an extent that the
formation of the terminal drops is so rapid as to be no longer visible
to the eye, but appears like the turmoil observed at the end of a jet of
water escaping into air.
[Illustration: __Figs._ 23, 24, 25.--Jets of Orthotoluidine, discharged
under water._]
[Illustration: __Fig._ 26.--Water stretched between a tube and a
plate._]
*Liquid Columns.*--A simple experiment will suffice to illustrate what
is meant by a liquid column. Here is a drop of water hanging from the
end of a glass tube. I place it in the lantern and obtain a magnified
image on the screen, and then bring up a flat plate of glass until it
just touches the suspended drop. As soon as contact is established, the
water spreads outwards over the plate, causing the drop to contract in
diameter at or near its middle part, so that its outline resembles that
of a capstan (Fig. 26). The end of the glass tube is now connected to
the plate by a column of water of curved outline, which is quite stable
if undisturbed. If, however, I gradually raise the tube, or lower the
plate, the narrow part of the column becomes still narrower, and finally
breaks across. In the same way we may produce columns of other liquids;
those obtained with viscous liquids such as glycerine being capable of
stretching to a greater extent than water, but showing the same general
characteristics. A liquid column, then, is in reality a supported drop,
and the severance effected by lowering the support is similar to that
which occurs when a pendent drop breaks away owing to its weight.
In our previous experiments we have seen that in order to produce large
drops of a given liquid, the surroundings should be of nearly the same
density, so as largely to diminish the effective weight of the suspended
mass. We might therefore expect that large columns of liquid could be
produced under similar conditions; and our conjecture is correct. We
may, for example, use the apparatus by means of which large drops of
orthotoluidine were formed (Fig. 13), using a shallow layer of water, so
that the lower end of the drop would come into contact with the bottom
of the vessel before the breaking stage was reached, and thus produce,
on a large scale, the same result as that we have just achieved by
allowing a hanging drop of water to touch a glass plate. This method,
however, restricts the diameter of the top of the column to that of the
delivery tube, and in this respect the shape is strained. The remedy for
this is to hang the drop from the surface of the water, when a degree of
freedom is conferred upon the upper part, which enables the column to
assume a greater variety of shapes. In order to show how this may be
accomplished, I pour a small quantity of water into the rounded end of a
wide test-tube, which is now seen projected on the screen, and then pour
gently down the side a quantity of _ethyl benzoate_, a liquid slightly
denser than water. You observe that the liquid spreads out on the
surface of the water, forming a hanging drop which at first is nearly
hemispherical in shape; but as I continue to add the liquid the drop
grows in size downwards, and finally reaches the bottom of the tube.
There is our liquid column (Fig. 27), which has formed itself in its own
way, free from the restriction imposed by a delivery tube. Notice the
graceful curved outline, produced by a beautiful balance between the
forces of surface tension and gravitation; and notice also how the
outline may be varied by the gradual addition of water, which causes the
upper surface to rise, and thus stretches the column (Fig. 28). The
middle becomes more and more narrow (Fig. 29), and finally breaks
across, leaving a portion of the former column hanging from the surface,
and the remainder, in rounded form (Fig. 30), at the bottom of the tube.
And, as usual, the partition was accompanied by the formation of a small
droplet.
[Illustration: __Figs._ 27, 28, 29, 30.--A liquid column stretched
upwards until broken by addition of water. Four stages._]
It is possible, by using other liquids, and different diameters of
vessels, to produce columns of a large variety of outlines. Some liquids
spread over a greater area on the surface of water than others, and
therefore produce columns with wider tops. Here we see a column of
orthotoluidine, which has a top diameter of 2 inches; and here again, in
contrast, is a column of aceto-acetic ether, the surface diameter of
which is only 1/2 inch (Fig. 31). Other liquids, such as aniline, give
an intermediate result. The lower diameter is determined by the width of
the vessel; and hence we are able to produce an almost endless number of
shapes. It is interesting to note how workers in glass and pottery have
unconsciously imitated these shapes; and I have here a variety of
articles which simulate the outlines of one or other of the liquid
columns you have just seen. It is possible that designers in these
branches of industry might obtain useful ideas from a study of liquid
columns, which present an almost limitless field for the practical
observation of curved forms.
[Illustration: __Fig._ 31.--A column of aceto-acetic ether in water._]
*Communicating Drops.*--There is a well-known experiment, which some of
you may have seen, in which two soap-bubbles are blown on separate
tubes, and are then placed in communication internally. If the bubbles
are exactly equal in size, no alteration takes place in either; but if
unequal, the smaller bubble shrinks, and forces the air in its interior
into the larger one, which therefore increases in size. Finally, the
small bubble is resolved into a slightly-curved skin which covers the
end of the tube on which it was originally blown. It is evident from
this experiment that the pressure per unit area exerted by the surface
of a bubble on the air inside is greater in a small than in a large
bubble. The internal pressure may be proved to vary inversely as the
radius of the bubble; thus by halving the radius we double the pressure
due to the elastic surface, and so on. The reciprocal of the radius of a
sphere is called its _curvature_, and we may therefore state that the
pressure exerted by the walls of the bubble on the interior vary
directly as the curvature.
[Illustration: __Fig._ 32.--Apparatus for communicating drops, with
extensions of unequal length attached._]
We have already seen that a drop of liquid possesses an elastic surface,
and is practically the same thing as a soap-bubble filled with liquid
instead of air. We might therefore expect the same results if two
suspended drops of liquid were placed in communication as those observed
in the case of soap-bubbles. And our reasoning is correct, as we may now
demonstrate. The apparatus consists (Fig. 32) of two parallel tubes,
each provided with a tap, and communicating with a cross-branch at the
top, which contains a reservoir to hold the liquid used. About half-way
down the parallel tubes a cross-piece, provided with a tap, is placed.
We commence by filling the whole of the system with the liquid under
trial, and the parallel tubes equal in length. Drops are then formed at
the ends of each vertical tube by opening the taps on these in turn, and
closing after suitable drops have been formed. Then, by opening the tap
on the horizontal cross-piece, we place the drops in communication and
watch the result.
I have chosen orthotoluidine as the liquid, and by placing the ends of
the vertical tubes under water--which at the temperature of the room is
slightly less dense than orthotoluidine--I am able to form much larger
drops than would be possible in air. You now see a small and a large
drop projected on the screen; and I now open the cross-tap, so that they
may communicate. Notice how the little drop shrinks until it forms
merely a slightly-curved prominence at the end of its tube. It attains a
position of rest when the curvature of this prominence is equal to that
of the now enlarged drop which has swallowed up the contents of the
smaller one. So far the result is identical with that obtained with
soap-bubbles; but we can extend the experiment in such a way as to
reverse the process, and make the little drop absorb the big one. In
order to do this I fasten an extension to one of the tubes, and form a
small drop deep down in the water, and a larger one on the unextended
branch near the top. When I open the communicating top, the system
becomes a kind of siphon, the orthotoluidine tending to flow out of the
end of the longer tube. The tendency of the large drop to siphon over is
opposed by the superior pressure exerted by the skin of the smaller
drop; but the former now prevails, and the big drop gradually shrinks
and the little one is observed to grow larger. It is possible by
regulating the depth at which the smaller drop is placed, to balance the
two tendencies, so that the superior pressure due to the lesser drop is
equalled by the extra downward pressure due to the greater length of the
column of which it forms the terminus. Both pressures are numerically
very small, but are still of sufficient magnitude to cause a flow of
liquid in one or other direction when not exactly in equilibrium. In the
case of communicating soap-bubbles, containing air and surrounded by
air, locating the small bubble at a lower level would not reverse the
direction of flow, which we succeeded in accomplishing with liquid drops
formed in a medium of slightly inferior density.
[Illustration: __Fig._ 33.--Combined drops of vapour and liquid._]
*Combined Vapour and Liquid Drops.*--All liquids when heated give off
vapour, the amount increasing as the temperature rises. The vapour
formed in the lower part of the vessel in which the liquid is heated
rises in the form of bubbles, which may condense again if the upper part
of the liquid be cold. When the liquid becomes hot throughout, however,
the vapour bubbles reach the surface and break, allowing the contents to
escape into the air above. Everyone who has watched a liquid boiling
will be familiar with this process, but it should be remembered that a
liquid may give off large quantities of vapour without actually boiling.
A dish of cold water, if exposed to the air, will gradually evaporate
away; whilst other liquids, such as petrol and alcohol, will disappear
rapidly under the same circumstances--and hence are called "volatile"
liquids.
The formation of vapour and its subsequent escape at the surface of the
liquid, enable us to produce a very novel kind of drop; if, instead of
allowing the bubbles to escape into air, we cause them to enter a second
liquid. Here, for example, is a coloured layer of chloroform[1] at the
bottom of a beaker, with a column of water above. I project the image of
the beaker on the screen, and then heat it below. The chloroform vapour
escapes in bubbles; but notice that each bubble carries with it a
quantity of liquid, torn off, as it were, at the moment of separation.
The vapour bubbles and their liquid appendages vary in size, but some of
them, you observe, have an average density about equal to that of the
water, and float about like weighted balloons. Some rise nearly to the
surface, where the water is coldest; and then the vapour partially
condenses, with the result that its lifting power is diminished, and
hence the drops sink into the lower part of the beaker. But the water is
warmer in this region, and consequently the vapour bubble increases in
size and lifting power until again able to lift its load to the surface.
So the composite drops go up and down, until finally they reach the
surface, when the vapour passes into the air, and the suspended liquid
falls back to the mass at the bottom of the beaker. Notice that the drop
of liquid attached to each bubble is elongated vertically. This is
because chloroform is a much denser liquid than water (Fig. 33). There
is a practical lesson to be drawn from this experiment. Whenever a
bubble of vapour breaks through the surface of a liquid, it tends to
carry with it some of the liquid, which is dragged mechanically into the
space above. In our experiment the space was occupied by water, which
enabled the bubble to detach a much greater weight than would be
possible if the surface of escape had been covered by air, which is far
less buoyant than water. But even when the bubbles escape into air, tiny
quantities of liquid are detached; so that steam from boiling water, for
example, is never entirely free from liquid. All users of steam are well
acquainted with this fact.
[1] Mono-brom-benzene is better than chloroform for this experiment,
but is more costly. It may be coloured with indigo. Chloroform may
be coloured with iodine.
*Condensation of Drops from Vapour,--Mists, Fogs and Raindrops.*--The
atmosphere is the great laboratory for the manufacture of drops. It is
continually receiving water in the form of vapour from the surface of
the sea, from lakes, from running water, and even from snow and ice. All
this vapour is ultimately turned into drops, and returned again to the
surface, and to this never-ceasing exchange all the phenomena connected
with the precipitation of moisture are due. The atmosphere is only
capable of holding a certain quantity of water in the form of vapour,
and the lower the temperature the less the capacity for invisible
moisture. When fully charged, the atmosphere is said to be
"saturated"--a condition realized on the small scale by air in a corked
bottle containing some water, which evaporates until the air can hold no
more. The maximum weight of vapour that can be held by 1 cubic metre of
air at different temperatures is shown in the table:--
------------------------------------------------------------------
Temperature. Weight of water vapour
(grammes) required to
Deg. C. Deg. F. saturate 1 cubic metre.
------------------------------------------------------------------
0 32 4.8
5 41 6.8
10 50 9.3
15 59 12.7
20 68 17.1
25 77 22.8
30 86 30.0
35 95 39.2
40 104 50.6
------------------------------------------------------------------
It will be seen from the table that air on a warm day in summer, with a
temperature of 77 deg. F., can hold nearly five times as much moisture
as air at the freezing point, or 32 deg. F. The amount actually present,
however, is usually below the maximum, and is recorded for
meteorological purposes as a percentage of the maximum. Thus if the
"relative humidity" at 77 deg. F. were 70 per cent., it would imply that
the weight of moisture in 1 cubic metre was 70 per cent. of 22.8
grammes; that is, nearly 16 grammes. If 1 cubic metre of air at 77 deg.
F., containing 16 grammes of moisture, were cooled to 50 deg. F., a
quantity of water equal to (16-9.3) = 6.7 grammes would separate out, as
the maximum content at the lower temperature is 9.3 grammes.
Precipitation would commence at 66 deg. F., at which temperature 1 cubic
metre is saturated by 16 grammes. And similarly for all states of the
atmosphere with respect to moisture, cooling to a sufficient extent
causes deposition of water to commence immediately below the saturation
temperature, and the colder the air becomes afterwards the greater the
amount which settles out. The temperature at which deposition commences
is called the "dew point."
Whenever atmospheric moisture assumes the liquid form, drops are
invariably formed. These may vary in size, from the tiny spheres which
form a mist to the large raindrops which accompany a thunderstorm. But
in every instance it is necessary that the air shall be cooled below its
saturation point before the separation can commence; and keeping this
fact in mind we can now proceed to demonstrate the production of mists
and fogs. Here is a large flask containing some water, fitted with a
cork through which is passed a glass tube provided with a tap. I pump
some air into it with a bicycle pump, and then close the tap. As excess
of water is present, the enclosed air will be saturated. Now a
compressed gas, on expanding into the atmosphere, does work, and is
therefore cooled; and consequently if I open the tap the air in the
flask will be cooled, and as it was already saturated the result of
cooling will be to cause some of the moisture to liquefy. Accordingly,
when I open the tap, the interior of the flask immediately becomes
filled with mist. If we examine the mist in a strong light by the aid of
a magnifying glass, we observe that it consists of myriads of tiny
spheres of water, which float in the air, and only subside very
gradually, owing to the friction between their surfaces and the
surrounding air preventing a rapid fall. The smaller the sphere, the
greater the area of surface in proportion to mass, and therefore the
slower its fall. And so in nature, the mists are formed by the cooling
of the atmosphere by contact with the surface, until, after the
saturation point is reached, the surplus moisture settles out in the
form of tiny spheres, which float near the surface, and are dissipated
when the sun warms up the ground and the misty air, and thus enables the
water again to be held as vapour.
Fogs, like mists, are composed of small spheres of water condensed from
the atmosphere by cooling; but in these unwelcome visitors the region of
cooling extends to a higher level, and the lowering of temperature is
due to other causes than contact with the cold surface of the earth. In
our populous cities, the density of the fogs is accentuated by the
presence of large quantities of solid particles in the atmosphere, which
arise from the smoke from coal fires, and the abrasion of the roads by
traffic. We can make a city fog in our flask. I blow in some tobacco
smoke, and then pump in air as before. You will notice that the smoke,
which is now disseminated through the air in the flask, is scarcely
visible; but now, on opening the tap, the interior becomes much darker
than was the case in our previous experiment. We have produced a genuine
yellow fog, that is, a dense mist coloured by smoke. When we have
learned how to abolish smoke, and how to prevent dust arising from the
streets, our worst fogs will be reduced to dense mists, such as are now
met with on the sea or on land remote from large centres of habitation.
There is one feature common to the spheres which compose a mist or fog,
or indeed to any kind of drop resulting from the condensation of
moisture in the atmosphere. As shown by the deeply interesting
researches of Aitken and others, each separate sphere forms round a core
or nucleus, which is usually a small speck of dust, and hence an
atmosphere charged with solid particles lends itself to the formation of
dense fogs immediately the temperature falls below the dew-point. But
dust particles are not indispensable to the production of condensed
spheres, for it has been shown that the extremely small bodies we call
"ions," which are electrically charged atoms, can act as centres round
which the water will collect; and much atmospheric condensation at high
elevations is probably due to the aid of ions.[2] Near the surface,
however, where dust is ever present, condensation round the innumerable
specks or motes is the rule. Here, for example, is a jet of steam
escaping into air, forming a white cloud composed of a multitude of
small spheres of condensed water. If now I allow the steam to enter a
large flask containing air from which the dust has been removed by
filtration through cotton wool, no cloud is formed in the interior, but
instead condensation takes place at the end of the jet, from which large
drops fall, and on the cold sides of the flask. The cloud we see in
dusty air is entirely absent, and the effect of solid particles in the
process of condensation is thus shown in a striking manner. Clouds are
masses of thick mist floating at varying heights in the atmosphere. On
sinking into a warmer layer of dry air the particles of which clouds are
composed will evaporate and vanish from sight. If the condensation
continue, however, the spheres will grow in size until the friction of
the atmosphere is unable to arrest their fall; and then we have rain.
And whether the precipitation be very gentle, and composed of small
drops falling slowly, as in a "Scotch mist," or in the form of
rapid-falling large drops such as accompany a thunderstorm, the
processes at work are identical. Every particle of a mist or cloud, and
every raindrop, is formed round a nucleus, and owes its spherical shape
to the tension at the surface.
[2] Mr. C. T. R. Wilson has recently devised an apparatus for making
visible the tracks of ionizing rays, by the condensation of water
vapour round the freshly liberated ions.
*Liquid Clouds in Liquid Media.*--Just as the excess of moisture is
precipitated from saturated air when the temperature falls, so is the
excess of one liquid dissolved in another thrown down by cooling below
the saturation temperature. Moreover, a liquid when precipitated in a
second liquid appears in the form of myriads of small spheres, which
have the appearance of a dense cloud. Here is some boiling water to
which an excess of aniline has been added, so that the water has
dissolved as much aniline as it can hold. Aniline dissolves more freely
in hot water than in cold, so that if I remove the flame, and allow the
beaker to cool, the surplus of dissolved aniline will settle out.
Cooling takes place most rapidly at the surface, and you observe white
streaks falling from the top into the interior, where they are warmed up
and disappear. Soon, however, the cooling spreads throughout; and now
the streaks become permanent, and the water becomes opaque, owing to the
thick white cloud of precipitated aniline. The absence of the red colour
characteristic of aniline is due to the extremely fine state of division
assumed in the process. If left for some hours, the white cloud sinks
through the water to the bottom of the beaker, where the small particles
coalesce and form large drops, leaving the overlying water quite
transparent. The process is quite analogous to the precipitation of
moisture from the atmosphere in the form of small spheres, which, if
undisturbed, would gradually sink to the ground and leave the air clear.
*Overheated Drops.*--The temperature at which a liquid boils, under
normal conditions, depends only upon the pressure on its surface. Thus
water boiling in air, when the pressure is 76 centimetres or 29.92
inches of mercury, corresponding to 14.7 pounds per square inch,
possesses a temperature of 100 deg. C. or 212 deg. F. At higher
elevations, where the pressure is less, the boiling point is lower; thus
Tyndall observed that on the summit of the Finsteraarhorn (14,000 feet)
water boiled at 86 deg. C. or 187 deg. F. Conversely, under increased
pressure, the boiling point rises; so that at a pressure of 35 pounds
per square inch water does not boil until the temperature reaches 125
deg. C. or 257 deg. F. There are certain abnormal conditions, however,
under which the boiling point of a liquid may be raised considerably
without any increase in the pressure at the surface; and it is then said
to be "over-heated." Dufour showed that when drops of water are floating
in another liquid of the same density, they may become greatly
overheated, and if very small in size may attain a temperature of 150
deg. C. or 302 deg. F., or even higher, before bursting into steam. In
order to provide a medium in which water drops would float at these
temperatures, Dufour made a mixture of linseed oil and oil of cloves,
which possessed the necessary equi-density temperature with water. To
demonstrate this curious phenomenon, I take a mixture of 4 volumes of
ethyl benzoate and 1 volume of aniline, which at 125 deg. C. or 257 deg.
F. is exactly equal in density to water at the same temperature. I add
to the mixture two or three cubic centimetres of freshly-boiled water,
the temperature being maintained at 125 deg. C. by surrounding the
vessel with glycerine heated by a flame. At first the water sinks, but
on attaining the temperature of the mixture it breaks up with some
violence, forming spheres of various sizes which remain suspended in the
mixture. Any portion of the water which has reached the surface boils
vigorously, and escapes in the form of steam; and some of the larger
spheres may be observed to be giving off steam, which rises to the
surface. Most of the spheres, however, remain perfectly tranquil, in
spite of the fact that the water of which they are composed is many
degrees above its normal boiling point. If I penetrate one of these
spheres with a wire, you notice that it breaks up immediately, with a
rapid generation of steam. A complete explanation of this abnormal
condition of water is difficult to follow, as a number of factors are
involved. One of the contributory causes--though possibly a minor
one--is the opposition offered to the liberation of steam by the tension
at the surface of the spheres.
[Illustration: __Fig._ 34.--Spheroid of water on a hot plate._]
*Floating Drops on Hot Surfaces.*--If a liquid be allowed to fall in
small quantity on to a very hot solid, it does not spread out over the
surface, but forms into drops which run about and gradually evaporate.
By careful procedure, we may form a very large, flattened drop on a hot
surface, and on investigation we shall notice some remarkable facts. I
take a plate of aluminium, with a dimple in the centre, and make it very
hot by means of a burner. You see the upper surface of this plate
projected on the screen. I now allow water to fall on the plate drop by
drop, and you hear a hissing noise produced when each drop strikes the
plate. The separate drops gather together in the depression at the
centre of the plate, forming a very large flattened globule. You might
have expected the water to boil vigorously, but no signs of ebullition
are visible; and what is more remarkable, the temperature of the drop,
in spite of its surroundings, is actually less than the ordinary boiling
point. Notice now how the drop has commenced to rotate, and has been set
into vibration, causing the edges to become scalloped (Fig. 34). The
drop, although not actually boiling, is giving off vapour rapidly, and
therefore gradually diminishes in size. And now I want to prove that the
drop is not really touching the plate, but floating above it. To do this
I make an electric circuit containing a cell and galvanometer, and
connect one terminal to the plate and place the other in the drop. No
movement is shown on the galvanometer, as would be the case if the drop
touched the plate and thus completed the electric circuit. And at close
range we can actually see a gap between the drop and the plate, so that
the evidence is conclusive. If now I remove the flame--leaving the
electric circuit intact--and allow the plate to cool, we notice after a
time that the globule flattens out suddenly and touches the plate, as
shown by the deflection of the galvanometer; and simultaneously a large
cloud of steam arises, due to the rapid boiling which occurs immediately
contact is made.
What we have seen in the case of water is shown by most liquids when
presented to a surface possessing a temperature much higher than the
boiling point of the liquid. A liquid held up in this manner above a hot
surface is said to be in the _spheroidal state_, to distinguish it from
the flat state usually assumed by spreading when contact occurs between
the liquid and the surface. It is doubtful whether any satisfactory
explanation of the spheroidal state has ever been given. Evidently, the
layer of vapour between the plate and the drop must exert a considerable
upward pressure in order to sustain the drop, but the exact origin of
this pressure is difficult to trace.