PROPERTIES--_Continued._ RADIO-ACTIVITY
Properties most evident are studied first . . . Then those hidden
from cursory view . . . Radio-activity revealed by the electrician
. . . A property which may be universal and of the highest import
. . . Its study brings us near to ultimate explanations . . .
Faraday’s prophetic views.
Properties age after age have become more and more intimately known. At
first the savage took account solely of the obvious strength of an oak,
the sharpness of a flint, the pliability of a sinew. With the first
kindling of fire he discovered a new round of properties in things long
familiar. All kinds of wood, especially when dry, were found
combustible, so were straw and twigs, as well as the fat of birds, the
oil of fish. Then it was noticed that the ground beneath a fire remained
unburnt and grew firm and hard, so that its clay or mud might be used
for rude furnaces and ovens. Soon come experiments as to the coverings
which maintain coals at red heat, ashes proving the readiest and best.
A century ago the mastery of electricity began to unfold a new knowledge
of properties, so wide and intimate as to recall the immense expansion
of such knowledge that long before had followed upon the kindling of
fire. The successors of Volta, as they reproduced his crown of cups,
asked, What metals dissolved in what liquids will give us an electric
current at least outlay? Then followed the further question, What metals
drawn into wire will bear currents afar with least loss? With the
invention of the electro-magnet came another query, What kinds of iron
are most swiftly and largely magnetized by a current; and when the
current ceases, which of them loses its magnetism in the shortest time?
Plainly enough the electrician regards copper, zinc, iron, steel, acids,
alkalis from a new point of view; he discovers in them properties which
until his advent had been utterly ignored.
Among the properties of matter revealed by electricity none are more
striking than those displayed in tubes containing highly rarified gases.
The study of their phenomena has led to discoveries which bring us
within view of an ultimate explanation of properties, an understanding
of how matter is atomically built. All this began simply enough as
Plucker, in 1859, sent an electric discharge through a tube fairly well
exhausted, producing singular bands of color. Geissler, afterward using
tubes more exhausted, produced bands of still higher variegation. In
1875 Professor William Crookes devised the all but vacuous tube which
bears his name, through which he sent electric pulses from a cathode
pole, revealing what he called “radiant matter,” as borne in a beam of
cathode rays, as much more tenuous than ordinary gases as these are more
rare than liquids. In 1894 Professor Philipp Lenard observed that
cathode rays passed through a thin plate of aluminium, much as daylight
takes its way through a film of translucent marble. Next year came the
epoch-making discovery of Professor Conrad Wilhelm Röntgen that cathode
rays consist in part of X-rays which readily pass through human flesh,
so as to cast shadows of bones upon a photographic plate. Cathode rays
make air a fairly good conductor of electricity, while ordinary air is
non-conducting in an extreme degree. This singular power is also
possessed by the ultra-violet rays of sunshine, as readily shown by an
electroscope. In 1897 Professor Joseph J. Thomson, of Cambridge
University, demonstrated that cathode rays are made up of corpuscles, or
electrons, about one-thousandth part the size of a hydrogen atom, and
bearing a charge of negative electricity. Such electrons form a small
part of every chemical atom, the remainder of which is, of course,
positively electrified. All electrons are alike, however various the
“elements” whence they are derived; as the most minute masses known to
science they may be among the primal units of all matter.
France, as well as Germany and England, was to take a leading part in
furthering the study of radio-activity. In Paris the famous Becquerel
family had for three generations devoted themselves to studying
phosphorescence. Henri Becquerel, third of the line, said, “I wonder if
a phosphorescent substance, such as zinc sulphide, would be excited by
X-rays.” He tried the experiment, causing the sulphide to glow with new
vigor. From that moment proofs have accumulated that the rays of common
phosphorescence such as are emitted by matches, decaying wood and fish,
are of kin to the cathode rays which the electrician evokes from any
substance whatever when he employs a high-tension current. One day M.
Becquerel came upon a remarkable discovery. He noticed that compounds of
uranium, whether phosphorescent or not, affected a photographic plate
through an opaque covering of black paper, and rendered the adjacent air
an electric conductor. Compounds of thorium, similar to those used for
incandescent mantles, were found to have the same properties. And here
was detected the cause of an annoyance and loss which had long perplexed
photographers. Often they had bestowed sensitive paper or plates within
wrappers of stout paper, or card, or thick wood, secluded in dark
cupboards or drawers. All in vain. At the end of a few weeks or months
these carefully guarded surfaces were as much discolored as if they had
been for a few minutes exposed, here and there, to daylight itself. All
the while each material relied upon as a safeguard had been sending
forth a feeble but constant beam; treachery had lurked in the trusted
guardian.
At the suggestion of M. Becquerel, M. and Madame Pierre Curie undertook
a thorough quest for these effects in a wide diversity of substances.
They found that several minerals containing uranium were more
radio-active than that element itself. Pitchblende, for instance,
consisting mainly of an oxide of uranium, was especially energetic as it
approached an electroscope, suggesting the presence of an uncommonly
active constituent, thus far not identified. At the end of a most
laborious series of separations they came at last to a minute quantity
of radium chloride displaying extraordinary properties. Another compound
of radium, a bromide, has since been arrived at: radium by itself has
not yet been obtained. In radio-activity radium chloride surpasses
uranium about one-million-fold. Provided with an electroscope of
exquisite sensibility, Professor Ernest Rutherford of McGill
University, Montreal, has discovered seven distinct radiations from
radium, each with characteristics of its own. Directed upon plates of
aluminium he finds its _gamma_ rays to be 100 times more penetrating
than its _beta_ rays, and _beta_ rays 100 times more penetrating than
its _alpha_ rays. Each radiation has qualities as distinct as those of
an ordinary chemical element. _Beta_ rays behave in all respects like
cathode rays, so that here a bridge is discerned betwixt the qualities
of radium and the long familiar phenomena of the Crookes tube.
The substance ranking next in radio-activity to radium is thorium.
Professor Rutherford has observed it throwing off a substance he calls
Thorium X; this radiates strongly for a time, the parent mass not
radiating at all. Gradually Thorium X ceases to radiate and the original
thorium resumes an emission of Thorium X. From Thorium X emanates what
seems a gas, condensible by extreme cold, which attaches itself to
adjacent bodies so as to make them radio-active. This emanation in its
turn produces successively three new and distinct kinds of radiation.
Professor Charles Baskerville, of the College of the City of New York,
has separated from thorium two substances probably elementary,
carolinium and berzelium.
Other radio-active substances have each several derivatives: actinium
has nine, uranium has four. As researchers broaden their range of
inquiry they steadily lengthen the list of radio-active substances.
Minerals of many kinds, water from springs, especially those of
medicinal value, the leaves of plants, newly fallen snow, and even
common air, are found to be radio-active, although usually in but a
slight degree, so that the doubt may be expressed, Is the observed
effect due to a trace of some highly radio-active material diffused in
something else which is not radio-active at all? Should it be
established that radio-activity is really present in all matter it would
be no other than a parallel to what, at another point in the physical
scale, presents itself as ordinary evaporation.
Solids are not as Solid as They Seem.
In a northern winter we may observe in air almost still, the wasting
away of a large block of ice, so that during a week it loses a
considerable part of its bulk. The giving forth of vapor is evidently
not restricted to high or to ordinary temperatures, but may occur below
the freezing point of water. In 1863, Thomas Graham, the eminent
Scottish physicist, from many experiments with metals expressed the
opinion that what seems to be a solid may be also in a minute degree
both liquid and gaseous as well. Confirmation of this view was afforded
in 1886 by Professor W. Spring, of Liege, who formed alloys by strongly
compressing their constituents as powders at ordinary temperatures. It
is probable that a slight pervasive liquidity gave success to the
experiment. Professor Roberts-Austen once observed that an
electric-deposit of iron on a clean copper plate adhered so firmly that
when they were severed by force, a film was stripped from the copper
plate and remained on the iron, signifying that the two metals had
penetrated each other at an ordinary temperature. This interpenetration
he found to take place through films of electro-deposited nickel. In a
remarkable round of experiments he also found that at 100° C., a
temperature much below the fusing point of lead, gold as leaf is
slightly diffused through a mass of lead; when the lead is fluid at 550°
C., the proportion of diffused gold is increased 160,000 times. This
volatility of the particles of a heavy metal shows us plainly that
virtual evaporation may be always taking place from metallic surfaces at
ordinary temperatures,--a phenomenon which may be the same in kind as
the pouring out of a perceptible stream of corpuscles under strong
electrical excitation. The analogy goes further, at least in the case of
liquids, which exhale a vapor usually different in composition from the
parent body; take, for example, a solution of sugar in water which sends
forth watery vapor only, or observe a mixture of much water and a little
alcohol as it emits a vapor largely alcoholic and but slightly aqueous.
Every Property May be Universal.
Here we are reminded of a striking experiment by Faraday: exciting an
electro-magnet of gigantic proportions he showed that every substance he
brought near to it was affected in a definite degree. He found iron to
be pre-eminently magnetic, much as Madame Curie has shown radium to be
vastly more radio-active than any other substance. From Faraday’s time
to the present hour the whole trend of investigation has built up the
probability that every known property in some degree exists in all
matter whatever. Copper conducts electricity remarkably well, and gutta
percha conducts remarkably ill; but gutta percha has some little
conductivity, or thinner sheets of it than those now used would suffice
to keep within an ocean cable the throbs which pass between America and
Europe. In radio-activity many substances may be as low in the scale as
is gutta percha in the list of electric conductors; in that case no
existing means of detection would make the property manifest.
Radium Reveals Properties Unknown Till Now.
While radio-activity may be a universal property of matter, to be
disclosed more and more as means of detection are refined and improved,
radium compounds are to-day in a class quite by themselves. Radium
bromide constantly maintains itself at a temperature of 3° to 5° C.
higher than that of its surroundings, so that every hour it could boil
its own weight of water. Professor Rutherford estimates the life of
radium as 1,800 years, its emanations in breaking up through their
successive stages emitting about three million times as much energy as
is given out by the union of an equal volume of hydrogen and oxygen,
mixed in the proportions which form water, a union accompanied by more
heat than that evolved in any other chemical change. Whence this amazing
stream of energy? It is probable that each radium atom may break into
minute parts, or corpuscles, which, moving at a velocity of 120,000
miles a second or so, collide so as to cause the observed heat.
From another side the compounds of radium bid us revise the laws of
chemical change as taught up to the close of the nineteenth century. In
the pores of many radio-active minerals may be found that remarkable
element, helium, first detected in the sun by means of the spectroscope,
then afterward discovered in the pores of cleveite, a mineral unearthed
in Norway. Sir William Ramsay and Mr. Frederick Soddy have found helium
in the gases evolved from radium chloride kept as a solid for some
months. The spectrum of helium was at first invisible; it soon appeared
and steadily grew more intense with the lapse of time. “It appears not
unlikely,” says Professor Rutherford, “that many of the so-called
chemical elements may prove to be compounds of helium, or, in other
words, that the helium atom is one of the secondary units with which the
heavier atoms are built up.”[21]
[21] Ernest Rutherford “Radio-activity.” Second edition. New York:
Macmillan Co.; Cambridge, England, University Press, 1905.
[Illustration: Photograph by Rice, Montreal.
PROFESSOR ERNEST RUTHERFORD,
MCGILL UNIVERSITY, MONTREAL.]
Already the phenomena of radio-activity, although of puzzling intricacy,
have greatly broadened our conceptions of matter. Where we were wont to
deem it of simple structure, it displays a baffling complexity, as
indeed has long been suggested in so highly diversified a spectrum as
that of iron. We find that radiations from an “element” may consist not
only in the undulations of an ether, but also in an emission of matter
as real as the projection of steam from a boiling pot. Newton believed
sunshine to be a stream of corpuscles: he was wrong with respect to
sunlight, his conception is true of many other kinds of radiation. Until
quite lately we looked upon atoms as indivisible bodies; to-day we have
learned that at least some of them may on occasion divide into many
parts, each part moving with a speed approaching that of light, with
energy far exceeding that of any chemical action we know. In the field
of ray-transmission our knowledge has undergone a like gain in width.
Twenty years ago we spoke of the opacity of lead, the transparency of
flint glass, as absolute properties. To-day we learn that given its
accordant ray any substance whatever affords that ray free passage, as
when oak an inch thick transmits pulses from radium. Yet more: ordinary
chemical changes require us to bring one substance into contact with
another; usually we must also apply heat or electricity to the bodies
thus joined; they are always responsive to changes of temperature.
Within the past six years we have become acquainted with changes
incomparably more energetic than those of the most violent chemical
action; many of them proceed with apparent spontaneity from a substance
all by itself. In the case of radium neither extreme cold nor extreme
heat has any perceptible effect upon the radiant stream.
One of the results of investigation in radio-activity is that it shows
the alchemists in their attempts at transmutation to have stood on solid
ground. Says Professor Rutherford: “There can be no doubt that in the
radio-elements we are witnessing the spontaneous transformation of
matter, and that the different products which arise mark the stages or
halting places in the process of transformation, where the atoms are
able to exist for a short time before breaking up into new systems.”
History of the Universe Rewritten in the Light of Radio-Activity.
Radio-activity has a vivid interest far beyond the laboratories of
chemists and physicians. One of the long standing puzzles of geology has
been to explain why the temperature of the earth has remained fairly
constant ever since organic life made its appearance. A sister problem
has been the maintenance by the sun of its vast output of heat and
light, age after age, with little or no diminution of intensity.
Professor Rutherford and Mr. Soddy believe that the phenomena of
radio-activity may solve both these problems: an element like helium may
furnish a store of energy vastly greater than that of ordinary chemical
action, and much lengthen the cooling process due to radiation from
either the sun or the earth.
Radio-activity, furthermore, throws new light upon evolution regarded in
its broadest aspects. The corpuscles discovered in 1897 by Professor J.
J. Thomson, as he severed atoms in pieces, are all alike whatever
chemical element may be the parent body. Hence it is argued that we may
have here the primal units of all matter whatever. Sir Norman Lockyer
long ago pointed out that helium and hydrogen predominate in the hottest
stars, while in stars less hot more complex types of matter appear. He
argues that these stars as they successively lose heat show a
development of what chemists call elements. His views are parallel with
the suggestion that in the radio-active corpuscle we make acquaintance
with an ultimate element of all matter, whether observed in a laboratory
tube or in the squadrons bright of the midnight heavens.[22]
[22] Radio-activity and other physical phenomena recently discovered
are set forth in “The New Knowledge,” by Professor Robert Kennedy
Duncan, published by A. S. Barnes & Co., New York, 1905; and “The
Recent Development of Physical Science,” by W. C. D. Whetham,
published by John Murray, London, and P. Blakiston, Son & Co.,
Phila., 1906.
The phenomena of radio-activity revive interest in the prophetic views
of Michael Faraday. In 1816, when he was but twenty-four years of age,
he delivered a lecture at the Royal Institution in London on Radiant
Matter. In the course of his remarks there occurs this passage:--
Faraday’s Prophetic Views.
“If we now conceive a change as far beyond vaporization as that is above
fluidity, and then take into account the proportional increased extent
of alteration as the changes arise, we shall perhaps, if we can form any
conception at all, not fall short of radiant matter; and as in the last
conversion many qualities were lost, so here also many more would
disappear.
“It was the opinion of Newton, and of many other distinguished
philosophers, that this conversion was possible, and continually going
on in the processes of nature, and they found that the idea would bear
without injury the applications of mathematical reasoning--as regards
heat, for instance. If assumed, we must also assume the simplicity of
matter; for it would follow that all the variety of substances with
which we are acquainted could be converted into one of three kinds of
radiant matter, which again may differ from each other only in the size
of their particles or their form. The properties of known bodies would
then be supposed to arise from the varied arrangements of their ultimate
atoms, and belong to substances only as long as their compound nature
existed; and thus variety of matter and variety of properties would be
found co-essential.”[23]
[23] “Life and Letters of Faraday,” by Bence Jones. Vol. I, p. 216.
Three years later he returned to this theme in another lecture:--
“By the power of heat all solid bodies have been fused into fluids, and
there are very few the conversion of which into gaseous forms is at all
doubtful. In inverting the method, attempts have not been so successful.
Many gases refuse to resign their form, and some fluids have not been
frozen. If, however, we adopt means which depend on the rearrangement of
particles, then these refractory instances disappear, and by combining
substances together we can make them take the solid, fluid, or gaseous
form at pleasure.
“In these observations on the changes of state, I have purposely avoided
mentioning the radiant state of matter, being purely hypothetical, it
would not have been just to the demonstrated parts of the science to
weaken the force of their laws by connecting them with what is
undecided. I may now, however, notice a progression in physical
properties accompanying changes of form, and which is perhaps sufficient
to induce, in the inventive and sanguine philosopher, a considerable
belief in the association of the radiant form with the others in the set
of changes I have mentioned.
“As we ascend from the solid to the fluid and gaseous states, physical
properties diminish in number and variety, each state having some of
those which belong to the preceding state. When solids are converted
into fluids, all varieties of hardness and softness are necessarily
lost. Crystalline and other shapes are destroyed. Opacity and color
frequently give way to a colorless transparency, and a general mobility
of particles is conferred.
“Passing onward to the gaseous state, still more of the evident
characters of bodies are annihilated. The immense differences in their
weights almost disappear; the remains of difference in color that were
left, are lost. Transparency becomes universal, and they are all
elastic. They now form but one set of substances, and the varieties of
density, hardness, opacity, color, elasticity and form, which render the
number of solids and fluids almost infinite, are now supplied by a few
slight variations in weight, and some unimportant shades of color.
“To those, therefore, who admit the radiant form of matter, no
difficulty exists in the simplicity of the properties it possesses, but
rather an argument in their favor. These persons show you a gradual
resignation of properties in the matter we can appreciate as the matter
ascends in the scale of forms, and they would be surprised if that
effect were to cease at the gaseous state. They point out the greater
exertions which nature makes at each step of the change, and think that,
consistently, it ought to be greatest at the passage from the gaseous to
the radiant form.”[24]
[24] “Life and Letters of Faraday,” by Bence Jones. Vol. I, p. 307.
This remarkable deliverance recalls what another great experimental
philosopher, Count Rumford, deduced as by dint of mechanical motion he
melted ice in a closed and insulated receiver. He inferred that the
heat thus generated was not a material substance, as then generally
supposed, but must be in essence motion, for only motion had brought it
into existence. As we follow Faraday’s recital of the successive changes
in properties which follow upon additions of heat, in other words, of
mechanical motion, the inference is irresistible that properties consist
in the distinct motions of masses of definite form and size, these very
motions, perhaps, deciding both the form and size of each mass.