MACHINE-MADE COLD
One of the most remarkable adaptations of scientific knowledge is the
"manufacture of cold." At first that phrase seems strange, but it is
really quite legitimate. There are machines at work at this moment which
are turning out cold as if it were any other manufactured article. It is
not that they manufacture cold water or cold air, it is the cold itself
which they produce.
Of course, cold has no real existence, since it is simply a negative
quantity, an absence of heat, yet its effects are so real that we are in
the habit of talking of it as if it were a reality, and in that sense we
can regard it as a product of manufacture.
Moreover, we see in this a conspicuous instance of the interdependence
of invention and science, for scientific principles were first adapted
to produce cold, and then artificial cold was employed in scientific
investigations, whereby the rare gases of the atmosphere have been
discovered, as we shall see presently.
In _Mechanical Inventions of To-day_ I have dealt with the uses which
can be made of heat as a motive power. Here we have in some sense a
reversal of the process. In the heat-engine the expenditure of heat
produces motion. In the refrigerating machine motion produces heat, on
the face of it a strange way of producing cold. Yet it is by the
production of heat in the first instance that we are ultimately able to
obtain the cold.
One way to make a thing cold is to place it in contact with ice. But
that process suffers from severe limitations. In the first place, we may
not be able to procure ice when we want it. And in the second place, we
may want to produce a temperature much lower than that of ice.
Now a machine can produce any degree of coldness, almost down to the
"absolute zero," the point at which a body is absolutely devoid of any
heat whatever, the condition in which its molecules are absolutely
still. That point is 274 deg. C. _below_ freezing-point. Freezing-point on
that scale is "zero," and so this _absolute_ zero is _minus_ 274 deg. Or,
to put it another way, freezing-point is 274 deg. _absolute_ temperature.
The absolute zero has never been reached, and there is reason to believe
that it never can be quite reached, but by methods about to be described
a temperature within a few degrees of it has been attained. And all of
this can be done without any cooling agent colder than water at an
ordinary temperature.
There are several systems, but the one which illustrates the principle
most simply is that in which carbonic acid gas is the "working fluid."
This is a very compressible gas, and so is well fitted for the purpose.
First of all a pump or compressor compresses it. That has the effect of
heating it. Such we might expect from the fact that heat is molecular
activity: when by compressing the gas we force the molecules closer
together, they naturally hit each other and the sides of the containing
vessel harder than they did before, and the increased activity is
manifested as increased heat. So the first effect, as was remarked just
now, is to produce, apparently, increased heat.
But then the hot compressed gas, by being passed through a coil of pipe
surrounded by cold water, can be robbed of that heat. According to the
speed at which it traverses the coil it will be more or less cooled: by
causing it to travel slowly it can be brought down almost to the
temperature of the water. So we start with the gas at atmospheric
pressure and at somewhere about atmospheric temperature too. This we
convert into compressed gas at a high temperature. After cooling it we
have compressed gas at a moderate temperature.
Then, to complete the process, we let the gas expand again. Now just as
compressing a gas heats it, letting it expand cools it. If we compressed
it and then expanded it again we should be just as we were to commence
with. But since, in between the two operations we extract a quantity of
heat by means of the cooling water, we get at the end a very much lower
temperature than that with which we started.
We cannot cool the gas without compressing it, because heat will only
flow from one body into another when the second is cooler than the
first. But by making the gas hot temporarily by compression we enable
the water to draw some heat from it, and then, allowing it to sink back
to its original state, we get practically the old temperature, less what
the water has extracted. The principle is really absurdly simple when
one once gets to understand it. The application is not so simple as far
as the designer of the machine is concerned, for he has to adjust the
various parts to exactly the right shape and dimensions, so that they
may work well with one another and produce the desired result with the
minimum expenditure of power.
To the observer, however, and to the user too, the finished machine is
wonderful in its simplicity. The principle is illustrated
diagrammatically in Fig. 5.
In the centre is the compressor. Its action forces the gas along the
pipe to the right and down into the condenser. As it flows downwards
through the coil there cold water enters at the bottom of the tank,
flows upward past the coil and escapes again at the top. Thus the coil
is kept in contact with _cold_ water.
Passing then through the bottom of the tank the gas travels from right
to left through the "regulating valve" and into an arrangement almost
exactly similar to the condenser but called the evaporator. Here the gas
expands and suffers a great fall in temperature. This cold is
communicated to liquid circulating in the tank which forms a part of the
evaporator, and this liquid can be circulated through pipes into any
rooms to be cooled or around vessels of water which it is desired to
freeze. This liquid, which acts as the carrier of the cold, is called
"brine," and is water to which is added calcium chloride to keep it from
freezing.
[Illustration: FIG. 5.--This diagram shows the working of the
Refrigerating Machine. The pump compresses the gas and drives it through
the coil in the condenser, where it is cooled by water. It passes thence
through the coil in the evaporator, where it expands and cools the
surrounding brine.]
Now the observant reader may have noticed that there is no apparent
reason for the name of the left-hand vessel. It will be quite clear,
however, when I explain that although I have spoken of the working fluid
all along as gas, I have only done so to avoid bringing in too many
explanations at once. It is actually liquid for a good part of its
journey. Carbonic acid gas liquefies at a very moderate temperature and
pressure, and so while it leaves the compressor as a gas it becomes
liquid in the condenser and remains so until it has passed the
regulating valve. Then it begins to expand into gas once more, and in
that state it passes back to the compressor.
There is a pressure-gauge on the pipe leaving the compressor and another
on the one entering it. A comparison of the readings on these two tells
how the apparatus is working. The difference between them indicates how
much compression is being given to the gas. Assuming that the compressor
is working at a constant speed, this compression can be regulated to a
nicety by the valve: close it a little and the compression will
increase: open it a little and the compression will decrease. By this
means the degree of cold produced can be varied at will.
This is the way in which many ships are enabled to carry cargoes of
frozen meat. The chambers in which the meat is stowed are
insulated--that is to say, their walls are made as impervious as
possible to heat. Then the brine is carried into the chambers in pipes,
cooling them much as the hot-water pipes heat an ordinary public
building.
Or another method is to carry the pipe which constitutes the evaporator
into the chamber to be cooled. A third way is to dispense with brine and
to blow air through the coils of the evaporator, whereby the air is made
to carry away the cold to wherever it is needed.
Ice can be made easily in moulds of metal or wood around which brine
circulates. If made of ordinary water the ice is likely to be cloudy and
opaque, which is quite good enough for many purposes. In cases where it
is desired that it should be clear, the water is agitated during
freezing, or else distilled water is used. To enable the blocks to be
got out of the moulds it is sometimes arranged to circulate warm brine
for a few moments.
Ice skating rinks are formed by making, first, an insulating layer of
sawdust, slag-wool or something of that sort (those by the way, being
the materials generally used for insulating cold chambers) underneath
the floor. The floor, too, is made waterproof and then upon it is laid
as closely as possible a series of iron pipes. Water is flooded on to
the floor until the pipes are covered to a depth of several inches, and
then brine is pumped through the pipes. In time the water freezes, and
so long as the brine circulates it remains so.
But although the "CO_{2} process" described above is the simplest
illustration of the principle, there are other systems. In one very
popular form ammonia gas is the "working fluid." This is liquefied by
pressure and cooling with water, being subsequently expanded just as
described above.
Another much-used system is the "ammonia-absorption" process, in which
the ammonia is not liquefied, but when under pressure is absorbed by
water, returning to gas again when the pressure is released.
But the degree of cold attained in these commercial machines is as
nothing to the extremely intense cold generated on the same principles
in the liquid-air machine, which is found in every well-equipped
physical laboratory.
Briefly, this consists of a coil of many turns of small tube enclosed in
a small double vessel, the space between the inner and outer skins of
which is packed with insulating material. A compressor pumps air in at
the top of the coil at a pressure of from 150 to 200 atmospheres. An
"atmosphere," it may be remarked, is a unit often used in scientific
matters, meaning the normal pressure of the atmosphere, which is,
roughly speaking, 15 lb. per square inch. Hence 200 atmospheres is about
3000 lb. per square inch.
Of course air so highly compressed as that is hot, but after it has
passed down the coil and has escaped from the valve which liberates it
at the bottom it is much cooler. But that is only the beginning of the
operation. The expanded, and therefore cooled, air finds its way upward
through the turns of the coil down which the following air is coming.
That, expanding in its turn, is colder still, because of the cooling
action of the first air, and so the process goes on.
[Illustration: _By permission of Messrs. J. and E. Hall, Ltd., London
and Dartford_
MACHINE-MADE ICE
Here we see a huge block of ice being lifted (it may be on a hot summer
day) from the mould in which it has been made]
This is perhaps easier to understand if we imagine that the air comes
through the coil in gusts and we notice what happens to each succeeding
gust. The first comes down, expands, cools and ascends, thereby cooling
the second gust as it comes down. The second then, after expansion, will
be cooler than the first was. That in its turn will cool the third, and
so the third after expansion will be cooler than the second. And that
will go on, each succeeding gust being cooler than the one before. And
although the flow of air is continuous, and not in gusts, the result is
just the same: it goes on getting cooler and cooler until at last the
air comes out in its liquid form. This liquid collects in a little
chamber formed at the bottom of the vessel which contains the coil and
can be drawn off when desired.
Air in its liquid state looks very much like water. In fact it is
difficult to get chance observers to believe that it is not water. It
boils at a temperature far below the freezing-point of water, so that
liquid air if placed in a cup made of ice will boil furiously. Ice is so
much the hotter that it behaves towards liquid air as a very hot fire
does to water.
The feature of the above machine, it will be noticed, is that no cooling
water is required, as in the refrigerating machine, although the
principle of the two is the same. The coil is the "condenser" and the
vessel in which it is enclosed is the "evaporator," and so the cold air
produced by the process in the evaporator cools the coil of the
condenser. Thus it is "self-intensive," as the makers call it.
Hydrogen can be liquefied in a similar machine, except that it needs a
little preliminary cooling with liquid air. Liquid hydrogen is the
coolest thing known approaching the region of absolute zero.
And now we can turn to the wonderful discoveries which have followed
upon the manufacture of liquid air.
To make the story complete we need to go back to the time of Priestly
and Cavendish, early in last century. They investigated the atmosphere
and showed that it consisted of oxygen and nitrogen in certain
invariable proportions, with under certain conditions a small proportion
of carbonic acid. These facts were so well authenticated, and they
seemed to explain everything so satisfactorily, that it was quite
thought almost up to the end of the nineteenth century that there was
nothing more to learn about the atmosphere.
Nevertheless there was an idea in the minds of some scientists that
there must be another group of elements somewhere, the existence of
which was then undiscovered, but it was never dreamed that these were in
the air.
Soon after the weights of the atoms had been found a medical student
named Prout in an anonymous essay called attention to the fact that
there were curious numerical relationships between them. Speculation on
the subject went on for many years, until in 1865 the great Russian
chemist Mendeleeff published his conclusions. He had arranged the
elements in the form of a table _in the order of their atomic weights_.
The table consisted of twelve rows of names forming eight vertical
columns, and the remarkable thing was that all those elements which fell
into any particular column, although their atomic weights were very
widely different, had similar properties. This enabled him to _predict_
the discovery of certain new elements, for the table contained a number
of blank spaces. Three elements _have been found_ since, and their
atomic weights and properties are just such as to fill three of the
blank spaces. One blank space, it is thought, may be filled some day by
the gas coronium, which like helium has been discovered in the sun, but
unlike it has not yet been detected here. When it is, there is the place
in the table which it may fill. The table then commenced with what is
still called Group 1, but for reasons too complicated to explain here it
appeared as if there must be a group before that, a group the chief
characteristic of which would be the inactivity of the elements included
in it. These were expected to be of various atomic weights, but these
weights, it was anticipated, would so occur in the intervals between the
others that they would all fall into a new column to the left of "Group
1."
In the year 1892 Lord Rayleigh was investigating the question of the
density of a number of different gases, including, so it happened,
nitrogen. Now there are several ways of procuring nitrogen. One is to
get it from the atmosphere by ridding it of the oxygen with which it is
normally mixed. Another way is to split up some compound, such as
ammonia, of which it forms a part, in such a way as to catch the
nitrogen and leave the other elements with which it was combined
elsewhere.
Lord Rayleigh tried both ways, and he found that the nitrogen from the
atmosphere was denser than that derived from ammonia. Sir William Ramsey
then carried the matter a step further. He heated atmospheric nitrogen
in the presence of magnesium, under which conditions some of the
nitrogen combines with the latter element to form nitride of magnesium.
That, it was found, made the remaining nitrogen denser still. The
explanation then seemed obvious. Suppose we imagine a mixture of sawdust
and iron filings: it will be heavier than an equal quantity of pure
sawdust. And if we contrive to take away some of the sawdust from the
mixture we shall find that what is left is heavier still, when compared
with an equal bulk of pure sawdust. For it is clear that as we take away
sawdust we thereby increase the proportion of the heavier iron filings
and so we make the mixture heavier.
Applying a similar process of reasoning to these discoveries, the
conviction grew that the nitrogen of the air was not pure, but that it
had mixed with it a small proportion of some other gas of greater
density. They soon succeeded in isolating this denser gas, to which they
gave the name of argon. Its atomic weight was found, and, wonderful to
relate, it was such that argon fell into a new column to the left of
Group 1, as had been anticipated.
The discovery of argon was announced in 1894. The next year Sir William
Ramsey, investigating a gas which had been discovered locked up in the
interstices of a mineral called clevite, was able to state that it was
helium, the element which had been previously noticed by the
spectroscope in the sun. Like argon, it was found to be extremely
inactive, and its atomic weight turned out to be such that it too fell
into the "Zero Group."
In 1898 Professors Ramsey and Travers found two more gases in the air,
krypton and neon, and a little later still, there was found mixed with
the krypton a further new gas, xenon. All of these had their atomic
weights found, and fell into that new column in the periodic table.
But what has all this got to do with liquid air? The two subjects are
closely related, for it is by liquid-air machines that these rare gases
are now obtained, and it was from liquid air that the last three were
first discovered.
For air, as we well know, is a mixture of gases, and when extreme cold
and pressure are applied these gases liquefy, each behaving according to
its own nature. They do not all liquefy at the same time, nor on being
relieved from the pressure and heated do all evaporate again at the same
temperature. Although they emerge from the liquid-air machine in the
form of a single liquid, it is really a mixture of liquids, each with
its own boiling-point.
In an earlier chapter we saw how petroleum can be separated into its
various constituents, such as petrol, by fractional distillation,
advantage being taken of the difference in the "boiling-point" of the
various "fractions." The boiling-point of a liquid is, of course, the
temperature at which it turns freely into vapour, and just as petroleum
when heated gives off first cymogene, next rhigolene, then petrol,
benzine, kerosene and so on, in the order named, so liquid air, when it
is evaporated, gives off its different constituents in order. Nitrogen,
oxygen, argon, helium, krypton, neon and xenon can all be separated each
from the others in this way, by "fractional distillation." The heat from
the surrounding objects is allowed to get at the liquid, and the gases
are then given off in the order of their boiling-points.
And thus we see how the mechanical production of cold has assisted in
the pursuit of pure science. The newly-found gases are not of any great
use at present. They are so inactive that possibly they never will be,
with one exception, and that is neon. If an electric discharge be made
to pass through a tube filled with this gas, a beautiful glow is the
result, and it is just possible that neon tubes may become the electric
light of the future. That is only a prediction, however, and a
hesitating one at that.
The inactive elements may become of value in explosives. We have seen
how important nitrogen is in these dangerous substances, the chief
feature of which is their instability--their readiness, that is, to
change into something else--which instability is due to the reluctance
with which nitrogen enters into them. Now nitrogen, though inactive, is
much less so than these others, and if a way should ever be found of
inducing them to enter into a compound, that compound will probably be
an extremely powerful explosive.