During Long Periods Minute Influences Become Telling.
Qualities of matter, much more simple, may now engage our attention. First, then, let us note how minute influences, acting for long stretches of time, may change the qualities of metals and rocks. Forces, too slight for measurement as yet, are known in the course of a year or two to affect steel at times favorably, at other times unfavorably. The highest grades of tool-steel are improved by being kept in stock for a considerable time, the longer the better. It seems that bayonets, swords, and guns are liable to changes which may account for failure under sudden thrust or strain. Gauges of tool steel, which are required to be hard in the extreme, are finished to their standard sizes a year or two after the hardening process. Slow molecular changes register themselves in altered dimensions. In the Bureau of Standards at Washington are a yard in steel and a yard in brass, at first identical in length; after twenty years they were found to vary by the 1⁄5000 of an inch. Take another case, familiar enough to the railroad engineer: in a mine, or a tunnel, the roof or wall may tumble down a month or more after a blasting. The stone which fell immediately upon the explosion was far from representing all the work done by the dynamite. A stress was set up in large areas of rock and this at last, beginning in slight cracks, overcame the cohesion of masses of huge extent.
Iron tube enclosing marble
before and after deformation.
Marble before deformation
and after.
Properties undergo change during the simple flight of time: a parallel diversity is worthy of remark. A substance exhibits quite diverse qualities according to whether the action upon it is slow or speedy. A paraffine candle protruding horizontally half way out of a box, during a New York summer will at last point directly downward, for all its brittleness. If shoemaker’s wax is struck a sudden blow, it breaks into bits as might a pane of window glass. But place leaden balls on the surface of this same wax and in the course of ten or twelve weeks you will find them sunk to the bottom of the mass. When sharply smitten, the wax is rigid and brittle; to a long continued, moderate pressure the wax proves plastic, semi-fluid almost. All this is repeated when stone is subjected to severe pressure for as long a period as two months. At McGill University, Montreal, a small cylinder of marble thus treated by Professor Frank D. Adams became of bulging form, without fracture, but with a reduction in tensile strength of one-half. When the pressure was applied during but ninety minutes the tensile strength of the resulting mass was but one-third that presented by the original marble; when the experiment occupied but ten minutes the tenacity fell to somewhat less than one-fourth its first degree. These researches shed light on the stratifications of rocks often folded under extreme pressure as if rubber or paste.
Take another and quite different example of how variations in time bring about wide contrasts of result: a rubber ball thrown in play at a wall rebounds; send it forth from a cannon, with a hundred-fold this velocity, and it pierces the wall as might a shot of steel.
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.