CHAPTER XV

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.

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.