luminous at ordinary temperatures, and whose emission of light is
increased by heating, cease to be luminous if cooled to -80° C. But
their light energy is merely rendered latent, not destroyed, by such
cold, and they still retain the capacity of taking in light energy at
the low temperature, to be evolved again when they are warmed. At the
temperature of liquid air many bodies become phosphorescent which do not
exhibit the phenomenon at all, or only to a very slight extent, at
ordinary temperatures, e.g. ivory, indiarubber, egg-shells, feathers,
cottonwool, paper, milk, gelatine, white of egg, &c. Of definite
chemical compounds, the platinocyanides among the inorganic bodies seem
to yield the most brilliant effects. Crystals of ammonium
platinocyanide, if stimulated by exposure to the ultra-violet radiation
of the electric arc--or better still of a mercury vapour lamp in
quartz--while kept moistened with liquid air, may be seen in the dark to
glow faintly so long as they are kept cold, but become exceedingly
brilliant when the liquid air evaporates and the temperature rises.
Among organic bodies the phenomenon is particularly well marked with the
ketonic compounds and others of the same type. The chloro-, bromo-,
iodo-, sulpho- and nitro-compounds show very little effect as a rule.
The activity of the alcohols, which is usually considerable, is
destroyed by the addition of a little iodine. Coloured salts, &c., are
mostly inferior in activity to white ones. When the lower temperature of
liquid hydrogen is employed there is a great increase in phosphorescence
under light stimulation as compared with that observed with liquid air.
The radio-active bodies, like radium, which exhibit self-luminosity in
the dark, maintain that luminosity unimpaired when cooled in liquid
hydrogen.
Some crystals become for a time self-luminous when placed in liquid
hydrogen, because the high electric stimulation due to the cooling
causes actual electric discharges between the crystal molecules. This
phenomenon is very pronounced with nitrate of uranium and some
platinocyanides, and cooling such crystals even to the temperature of
liquid air is sufficient to develop marked electrical and luminous
effects, which are again observed, when the crystal is taken out of the
liquid, during its return to normal temperature. Since both liquid
hydrogen and liquid air are good electrical insulators, the fact that
electric discharges take place in them proves that the electric
potential generated by the cooling must be very high. A crystal of
nitrate of uranium indeed gets so highly charged electrically that it
refuses to sink in liquid air, although its density is 2.8 times
greater, but sticks to the side of the vacuum vessel, and requires for
its displacement a distinct pull on the silk thread to which it is
attached. Such a crystal quickly removes cloudiness from liquid air by
attracting all the suspended particles to its surface, just as a fog is
cleared out of air by electrification. It is interesting to observe that
neither fused nitrate of uranium nor its solution in absolute alcohol
shows any of the remarkable effects of the crystalline state on cooling.
_Cohesion._--The physical force known as cohesion is greatly increased
by low temperatures. This fact is of much interest in connexion with two
conflicting theories of matter. Lord Kelvin's view was that the forces
that hold together the ultimate particles of bodies may be accounted for
without assuming any other forces than that of gravitation, or any other
law than the Newtonian. An opposite view is that the phenomena of
cohesion, chemical union, &c., or the general phenomena of the
aggregation of molecules, depend on the molecular vibrations as a
physical cause (Tolver Preston, _Physics of the Ether_, p. 64). Hence at
the zero of absolute temperature, this vibrating energy being in
complete abeyance, the phenomena of cohesion should cease to exist and
matter generally be reduced to an incoherent heap of "cosmic dust." This
second view receives no support from experiment. Atmospheric air, for
instance, frozen at the temperature of liquid hydrogen, is a hard solid,
the strength of which gives no hint that with a further cooling of some
20 degrees it would crumble into powder. On the contrary, the lower the
scale of temperature is descended, the more powerful become the forces
which hold together the particles of matter. A spiral of fusible metal,
which at ordinary temperatures cannot support the weight of an ounce
without being straightened out, will, when cooled to the temperature of
liquid oxygen, and so long as it remains in that cooled condition,
support several pounds and vibrate like a steel spring. Similarly a bell
of fusible metal at -182° C. gives a distinct metallic ring when struck.
Balls of iron, lead, tin, ivory, &c., thus cooled, exhibit an increased
rebound when dropped from a height; an indiarubber ball, on the other
hand, becomes brittle, and is smashed to atoms by a very moderate fall.
Tables XII. and XIII., which give the mean results of a large number of
experiments, show the increased breaking stress gained by metals while
they are cooled to the temperature of liquid oxygen.
TABLE XII.--_Breaking Stress in Pounds of Metallic Wires 0.098 inch in
diameter._
+15° C. -182° C.
Steel (soft) 420 700
Iron 320 670
Copper 200 300
Brass 310 440
German silver 470 600
Gold 255 340
Silver 330 420
TABLE XIII.--_Breaking Stress in Pounds of Cast Metallic Testpieces;
diameter of rod 0.2 inch._
+15° C. -182° C.
Tin 200 390
Lead 77 170
Zinc 35 26
Mercury 0 31
Bismuth 60 30
Antimony 61 30
Solder 300 645
Fusible, metal (Wood) 140 450
In the second series of experiments the test-pieces were 2 in. long and
were all cast in the same mould. It will be noticed that in the cases of
zinc, bismuth and antimony the results appear to be abnormal, but it may
be pointed out that it is difficult to get uniform castings of
crystalline bodies, and it is probable that by cooling such stresses are
set up in some set of cleavage planes as to render rupture comparatively
easy. In the case of strong steel springs the rigidity modulus does not
appear to be greatly affected by cold, for although a number were
examined, no measurable differences could be detected in their
elongation under repeated additions of the same load. No quantitative
experiments have been made on the cohesive properties of the metals at
the temperature of boiling hydrogen (-252°), owing to the serious cost
that would be involved. A lead wire cooled in liquid hydrogen did not
become brittle, as it could be bent backwards and forwards in the
liquid.
_Electrical Resistivity._--The first experiments on the conductivity of
metals at low temperatures appear to have been made by Wroblewski
(_Comptes rendus_, ci. 160), and by Cailletet and Bouty (_Journ. de
phys._ 1885, p. 297). The former's experiments were undertaken to test
the suggestion made by Clausius that the resistivity of pure metals is
sensibly proportional to the absolute temperature; he worked with copper
having a conductibility of 98%, and carried out measurements at various
temperatures, the lowest of which was that given by liquid nitrogen
boiling under reduced pressure. His general conclusion was that the
resistivity decreases much more quickly than the absolute temperature,
so as to approach zero at a point not far below the temperature of
nitrogen evaporating _in vacuo_. Cailletet and Bouty, using ethylene as
the refrigerant, and experimenting at temperatures ranging from 0° C. to
-100° C. and -123° C., constructed formulae intended to give the
coefficients of variation in electrical resistance for mercury, tin,
silver, magnesium, aluminium, copper, iron and platinum. Between 1892
and 1896 Dewar and Fleming carried out a large number of experiments to
ascertain the changes of conductivity that occur in metals and alloys
cooled in liquid air or oxygen to -200° C. The method employed was to
obtain the material under investigation in the form of a fine regular
wire and to wind it in a small coil; this was then plunged in the liquid
and its resistance determined. The accompanying chart (fig. 14) gives
the results in a compendious form, the temperatures being expressed not
in degrees of the ordinary air-thermometer scale, but in platinum
degrees as given by one particular platinum resistance thermometer which
was used throughout the investigation. A table showing the value of
these degrees in degrees centigrade according to Dickson will be found
in the _Phil. Mag._ for June 1898, p. 527; to give some idea of the
relationship, it may be stated here that -100° of the platinum
thermometer = -94°.2 C., -150° plat. = -140°.78 C., and -200° plat. =
-185°.53 C. In general, the resistance of perfectly pure metals was
greatly decreased by cold--so much so that, to judge by the course of
the curves on the chart, it appeared probable that at the zero of
absolute temperature resistance would vanish altogether and all pure
metals become perfect conductors of electricity. This conclusion,
however, has been rendered very doubtful by subsequent observations by
Dewar, who found that with the still lower temperatures attainable with
liquid hydrogen the increases of conductivity became less for each
decrease of temperature, until a point was reached where the curves bent
sharply round and any further diminution of resistance became very
small; that is, the conductivity remained finite. The reduction in
resistance of some of the metals at the boiling point of hydrogen is
very remarkable. Thus copper has only (1/105)th, gold (1/30)th, platinum
(1/35)th to (1/17)th, silver (1/24)th the resistance at melting ice, but
iron is only reduced to (1/8)th part of the same initial resistance.
Table XIV. shows the progressive decrease of resistance for certain
metals and one alloy as the temperature is lowered from that of boiling
water down to that of liquid hydrogen boiling under reduced pressure; it
also gives the "vanishing temperature," at which the conductivity would
become perfect if the resistance continued to decrease in the same ratio
with still lower temperatures, the values being derived from the
extrapolation curves of the relation between resistance and temperature,
according to Callendar and Dickson. It will be seen that many of the
substances have actually been cooled to a lower temperature than that at
which their resistance ought to vanish.
[Illustration: FIG. 14.--Chart of the Variation of Electrical Resistance
of Pure Metals and Alloys with Temperature. (Dewar and Fleming.)]
In the case of alloys and impure metals, cold brings about a much
smaller decrease in resistivity, and the continuations of the curves at
no time show any sign of passing through the zero point. The influence
of the presence of impurities in minute quantities is strikingly shown
in the case of bismuth. Various specimens of the metal, prepared with
great care by purely chemical methods, gave in the hands of Dewar and
Fleming some very anomalous results, appearing to reach at -80° C. a
maximum of conductivity, and thereafter to increase in resistivity with
decrease of temperature. But when the determinations were carried out on
a sample of really pure bismuth prepared electrolytically, a normal
curve was obtained corresponding to that given by other pure metals. As
to alloys, there is usually some definite mixture of two pure metals
which has a maximum resistivity, often greater than that of either of
the constituents. It appears too that high, if not the highest,
resistivity corresponds to possible chemical compounds of the two metals
employed, e.g. platinum 33 parts with silver 66 parts = PtAg4; iron 80
with nickel 20 = Fe4Ni; platinum 80 with iridium 20 = IrPt4; and copper
70 with manganese 30 = Cu2Mn. The product obtained by adding a small
quantity of one metal to another has a higher specific resistance than
the predominant constituent, but the curve is parallel to, and therefore
the same in shape as, that of the latter (cf. the curves for various
mixtures of Al and Cu on the chart). The behaviour of carbon and of
insulators like gutta-percha, glass, ebonite, &c., is in complete
contrast to the metals, for their resistivity steadily increases with
cold. The thermo-electric properties of metals at low temperatures are
discussed in the article THERMOELECTRICITY.
TABLE XIV.
+-------------------------------------+---------+---------+--------+--------+--------+----------+
| | |Platinum-| | | | |
| Metals. |Platinum.| rhodium | Gold. |Silver. | Copper.| Iron. |
| | | Alloy. | | | | |
+-------------------------------------+---------+---------+--------+--------+--------+----------+
| Resistance at 100° | 39.655 | 36.87 | 16.10 | 8.336 | 11.572 | 4.290 |
| " 0° C | 28.851 | 31.93 | 11.58 | 5.990 | 8.117 | 2.765 |
| " carbonic acid | 19.620 | .. | .. | .. | .. | .. |
| " liquid oxygen | 7.662 | 22.17 | 3.380 | 1.669 | 1.589 | 0.633 |
| " " nitrogen | .. | .. | .. | .. | 1.149 | .. |
| " " oxygen | | | | | | |
| under exhaustion| 4.634 | 20.73 | .. | .. | .. | |
| " " hydrogen | 0.826 | 18.96 | 0.381 | 0.244 | 0.077 | 0.356 |
| " " hydrogen | | | | | | |
| under exhaustion| 0.705 | 18.90 | 0.298 | 0.226 | 0.071 | |
| Resistance coefficients | 0.003745| 0.003607|0.003903|0.003917|0.004257| 0.005515 |
| Vanishing temperatures (Centigrade)/| -244.50°| -543.39°|-257.90°|-252.26°|-225.62°|-258.40°C.|
| \| -244.15°| -530.32°|-257.8° |-252.25°|-226.04°|-246.80°D.|
+-------------------------------------+---------+---------+--------+--------+--------+----------+
_Magnetic Phenomena._--Low temperatures have very marked effects upon
the magnetic properties of various substances. Oxygen, long known to be
slightly magnetic in the gaseous state, is powerfully attracted in the
liquid condition by a magnet, and the same is true, though to a less
extent, of liquid air, owing to the proportion of liquid oxygen it
contains. A magnet of ordinary carbon steel has its magnetic moment
temporarily increased by cooling, that is, after it has been brought to
a permanent magnetic condition ("aged"). The effect of the first
immersion of such a magnet in liquid air is a large diminution in its
magnetic moment, which decreases still further when it is allowed to
warm up to ordinary temperatures. A second cooling, however, increases
the magnetic moment, which is again decreased by warming, and after a
few repetitions of this cycle of cooling and heating the steel is
brought into a condition such that its magnetic moment at the
temperature of liquid air is greater by a constant percentage than it
is at the ordinary temperature of the air. The increase of magnetic
moment seems then to have reached a limit, because on further cooling to
the temperature of liquid hydrogen hardly any further increase is
observed. The percentage differs with the composition of the steel and
with its physical condition. It is greater, for example, with a specimen
tempered very soft than it is with another specimen of the same steel
tempered glass hard. Aluminium steels show the same kind of phenomena as
carbon ones, and the same may be said of chrome steels in the permanent
condition, though the effect of the first cooling with them is a slight
increase of magnetic moment. Nickel steels present some curious
phenomena. When containing small percentages of nickel (e.g. 0.84 or
3.82), they behave under changes of temperature much like carbon steel.
With a sample containing 7.65%, the phenomena after the permanent state
had been reached were similar, but the first cooling produced a slight
increase in magnetic moment. But steels containing 18.64 and 29% of
nickel behaved very differently. The result of the first cooling was a
reduction of the magnetic moment, to the extent of nearly 50% in the
case of the former. Warming again brought about an increase, and the
final condition was that at the temperature of liquid air the magnetic
moment was always less than at ordinary temperatures. This anomaly is
all the more remarkable in that the behaviour of pure nickel is normal,
as also appears to be generally the case with soft and hard iron.
Silicon, tungsten and manganese steels are also substantially normal in
their behaviour, although there are considerable differences in the
magnitudes of the variations they display (_Proc. Roy. Soc._ lx. 57 et
seq.; also "The Effect of Liquid Air Temperatures on the Mechanical and
other Properties of Iron and its Alloys," by Sir James Dewar and Sir
Robert Hadfield, _Id._ lxxiv. 326-336).
Low temperatures also affect the permeability of iron, i.e. the degree
of magnetization it is capable of acquiring under the influence of a
certain magnetic force. With fine Swedish iron, carefully annealed, the
permeability is slightly reduced by cooling to -185° C. Hard iron,
however, in the same circumstances suffers a large increase of
permeability. Unhardened steel pianoforte wire, again, behaves like soft
annealed iron. As to hysteresis, low temperatures appear to produce no
appreciable effect in soft iron; for hard iron the observations are
undecisive.
_Biological Research._--The effect of cold upon the life of living
organisms is a matter of great intrinsic interest as well as of wide
theoretical importance. Experiment indicates that moderately high
temperatures are much more fatal, at least to the lower forms of life,
than are exceedingly low ones. Professor M'Kendrick froze for an hour at
a temperature of -182° C. samples of meat, milk, &c., in sealed tubes;
when these were opened, after being kept at blood-heat for a few days,
their contents were found to be quite putrid. More recently some more
elaborate tests were carried out at the Jenner (now Lister) Institute of
Preventive Medicine on a series of typical bacteria. These were exposed
to the temperature of liquid air for twenty hours, but their vitality
was not affected, their functional activities remained unimpaired and
the cultures which they yielded were normal in every respect. The same
result was obtained when liquid hydrogen was substituted for air. A
similar persistence of life has been demonstrated in seeds, even at the
lowest temperatures; they were frozen for over 100 hours in liquid air
at the instance of Messrs Brown and Escombe, with no other effect than
to afflict their protoplasm with a certain inertness, from which it
recovered with warmth. Subsequently commercial samples of barley, peas
and vegetable-marrow and mustard seeds were literally steeped for six
hours in liquid hydrogen at the Royal Institution, yet when they were
sown by Sir W. T. Thiselton Dyer at Kew in the ordinary way, the
proportion in which germination occurred was no smaller than with other
batches of the same seeds which had suffered no abnormal treatment. Mr
Harold Swithinbank has found that exposure to liquid air has little or
no effect on the vitality of the tubercle bacillus, although by very
prolonged exposures its virulence is modified to some extent; but
alternate exposures to normal and very cold temperatures do have a
decided effect both upon its vitality and its virulence. The suggestion
once put forward by Lord Kelvin, that life may in the first instance
have been conveyed to this planet on a meteorite, has been objected to
on the ground that any living organism would have been killed before
reaching the earth by its passage through the intense cold of
interstellar space; the above experiments on the resistance to cold
offered by seeds and bacteria show that this objection at least is not
fatal to Lord Kelvin's idea.
At the Lister Institute of Preventive Medicine liquid air has been
brought into use as an agent in biological research. An inquiry into the
intracellular constituents of the typhoid bacillus, initiated under the
direction of Dr Allan Macfadyen, necessitated the separation of the
cell-plasma of the organism. The method at first adopted for the
disintegration of the bacteria was to mix them with silver-sand and
churn the whole up in a closed vessel in which a series of horizontal
vanes revolved at a high speed. But certain disadvantages attached to
this procedure, and accordingly some means was sought to do away with
the sand and triturate the bacilli _per se_. This was found in liquid
air, which, as had long before been shown at the Royal Institution, has
the power of reducing materials like grass or the leaves of plants to
such a state of brittleness that they can easily be powdered in a
mortar. By its aid a complete trituration of the typhoid bacilli has
been accomplished at the Jenner Institute, and the same process, already
applied with success also to yeast cells and animal cells, is being
extended in other directions.
_Industrial Applications._--While liquid air and liquid hydrogen are
being used in scientific research to an extent which increases every
day, their applications to industrial purposes are not so numerous. The
temperatures they give used as simple refrigerants are much lower than
are generally required industrially, and such cooling as is needed can
be obtained quite satisfactorily, and far more cheaply, by refrigerating
machinery employing more easily condensable gases. Their use as a source
of motive power, again, is impracticable for any ordinary purposes, on
the score of inconvenience and expense. Cases may be conceived of in
which for special reasons it might prove advantageous to use liquid air,
vaporized by heat derived from the surrounding atmosphere, to drive
compressed-air engines, but any advantage so gained would certainly not
be one of cheapness. No doubt the power of a waterfall running to waste
might be temporarily conserved in the shape of liquid air, and thereby
turned to useful effect. But the reduction of air to the liquid state is
a process which involves the expenditure of a very large amount of
energy, and it is not possible even to recover all that expended energy
during the transition of the material back to the gaseous state. Hence
to suggest that by using liquid air in a motor more power can be
developed than was expended in producing the liquid air by which the
motor is worked, is to propound a fallacy worse than perpetual motion,
since such a process would have an efficiency of more than 100%. Still,
in conditions where economy is of no account, liquid air might perhaps,
with effectively isolated storage, be utilized as a motive power, e.g.
to drive the engines of submarine boats and at the same time provide a
supply of oxygen for the crew; even without being used in the engines,
liquid air or oxygen might be found a convenient form in which to store
the air necessary for respiration in such vessels. But a use to which
liquid air machines have already been put to a large extent is for
obtaining oxygen from the atmosphere. Although when air is liquefied the
oxygen and nitrogen are condensed simultaneously, yet owing to its
greater volatility the latter boils off the more quickly of the two, so
that the remaining liquid becomes gradually richer and richer in
oxygen. The fractional distillation of liquid air is the method now
universally adopted for the preparation of oxygen on a commercial scale,
while the nitrogen simultaneously obtained is used for the production of
cyanamide, by its action on carbide of calcium. An interesting though
minor application of liquid oxygen, or liquid air from which most of the
nitrogen has evaporated, depends on the fact that if it be mixed with
powdered charcoal, or finely divided organic bodies, it can be made by
the aid of a detonator to explode with a violence comparable to that of
dynamite. This explosive, which might properly be called an emergency
one, has the disadvantage that it must be prepared on the spot where it
is to be used and must be fired without delay, since the liquid
evaporates in a short time and the explosive power is lost; but, on the
other hand, if a charge fails to go off it has only to be left a few
minutes, when it can be withdrawn without any danger of accidental
explosion.