A History of Science — Volume 5

by Henry Smith Williams; Edward Huntington Williams

"Glass in the form of a tube, sealed at both ends; the rubber thermometer-holder; silver tureen; four copper plates; a glass vase or measuring-glass one-quarter of an inch in thickness; three inches of water. There is no previously known substance or agent, whether it be even light or electricity, that possesses such wonderfully penetrative powers."*5*

THE NATURE OF EMANATIONS FROM RADIO-ACTIVE BODIES

What, then, is the nature of these radiations? Are they actually material particles hurled through the ether? Or are they like light—and possibly the Roentgen rays—simply undulations in the ether? As yet this question is an open one, although several of the leading investigators have postulated tentative hypotheses which at least serve as a working basis until they are either confirmed or supplanted. On one point, however, there seems to be unanimity of opinion—there seems to be little question that there are at least three different kinds of rays produced by radio-active substances. According to Sir William Crookes, the first of these are free electrons, or matter in an ultra-gaseous state, as shown in the cathode stream. These particles are extremely minute. They carry a negative charge of electricity, and are identified with the electric corpuscles of Thompson. Rays of the second kind are comparable in size to the hydrogen atom, and are positively electrified. These are easily checked by material obstructions, although they render the air a conductor and affect photographic plates. The third are very penetrating rays, which are not deflected by electricity and which are seemingly identical with Roentgen rays. Professor E. Rutherford has named these rays beta (B), alpha (a), and gamma (v) rays respectively. Of these the beta rays are deviated strongly by the magnetic field, the alpha much less so—very slightly, in fact—while the gamma rays are not affected at all. The action of these three different sets of rays upon certain substances is not the same, the beta and gamma rays acting strongly upon barium platinocyanide, but feebly on Sidot's blende, while the alpha rays act exactly the reverse of this, acting strongly on Sidot's blende.

If a surface is coated with Sidot's blende and held near a piece of radium nitrate, the coated surface begins to glow. If now it is examined with a lens, brilliant sparks or points can be seen. As the radium is brought closer and closer these sparks increase in number, until, as Sir William Crookes says, we seem to be witnessing a bombardment of flying atoms hurled from the radium against the surface of the blende. A little instrument called a spinthariscope, devised by Dr. Crookes and on sale at the instrument and optical-goods shops, may be had for a trifling sum. It is fitted with a lens focused upon a bit of Sidot's blende and radium nitrate, and in a dark room shows these beautiful scintillations "like a shower of stars." A still less expensive but similar device is now made in the form of a microscopic slide, to be used with the ordinary lens.

As we said a moment ago, radium appears to be an elementary substance, as shown by its spark-spectrum being different from that of any other known substance—the determinative test as fixed by the International Chemical Congress. A particle of radium free from impurities should, therefore, according to the conventional conception of an element, remain unchanged and unchangeable. If any such change did actually take place it would mean that the conception of the Daltonian atom as the ultimate particle of matter is definitively challenged from a new direction. This is precisely what has taken place. In July of 1903 Sir William Ramsay and Mr. Soddy, in making some experiments with radium, saw produced, apparently from radium emanations, another quite different and distinct substance, the element helium. The report of such a revolutionary phenomenon was naturally made with scientific caution. Though the observation seemed to prove the actual transformation of one element into another, Professor Ramsay himself was by no means ready to declare the absolute certainty of this. Yet the presumption in favor of this interpretation of the observed phenomena is very strong; and so cautious a reasoner as Professor Rutherford has declared recently that "there can be no doubt that helium is derived from the emanations of radium in consequence of changes of some kind occurring in it."*6*

"In order to explain the presence of helium in radium on ordinary chemical lines," says Professor Rutherford, "it has been suggested that radium is not a true element, but a molecular compound of helium with some substance known or unknown. The helium compound gradually breaks down, giving rise to the helium observed. It is at once obvious that this postulated helium compound is of an entirely different character to any other compound previously observed in chemistry. Weight for weight, it emits during its change an amount of energy at least one million times greater than any molecular compound known. In addition, it must be supposed that the rate of breaking up of the helium compound is independent of great ranges of temperature—a result never before observed in any molecular change. The helium compound in its breaking up must give rise to the peculiar radiations and also pass through the successive radio-active change observed in radium.... On the other hand, radium, as far as it has been examined, has fulfilled every test required of an element. It has a well-marked and characteristic spectrum, and there is no reason to suppose that it is not an element in the ordinarily accepted sense of the term."*7*

THE SOURCE OF ENERGY OF RADIO-ACTIVITY

In 1903 Messrs. Curie and Laborde*8* made the remarkable announcement that a crystal of radium is persistently warmer than its surrounding medium; in other words, that it is perpetually giving out heat without apparently becoming cooler. At first blush this seemed to contradict the great physical law of the conservation of energy, but physicists were soon agreed that a less revolutionary explanation of the phenomenon is perfectly tenable. The giving off of heat is indeed only an additional evidence of the dissipation of energy to which the radio-active atom is subjected. And no one now believes that radio-activity can persist indefinitely without actually exhausting the substance of the atom. Even so, the evidence of so great a capacity to give out energy is startling, and has given rise to various theories (all as yet tentative) in explanation. Thus J. Perrin*9* has suggested that atoms may consist of parts not unlike a miniature planetary system, and in the atoms of the radio-elements the parts more distant from the centre are continually escaping from the central attraction, thus giving rise to the radiations. Monsieur and Madame Curie have suggested that the energy may be borrowed from the surrounding air in some way, the energy lost by the atom being instantly regained. Pilipo Re,*10* in 1903, advanced the theory that the various parts of the atom might at first have been free particles constituting an extremely tenuous nebula.

These parts gradually becoming collected around condensed centres have formed what we know as the atoms of elements, the atom thus becoming like an extinct sun of the solar system. From this point of view the radio-active atoms represent an intermediate stage between nebulae and chemical atoms, the process of contraction giving rise to the heat emissions.

Lord Kelvin has called attention to the fact that when two pieces of paper, one white and the other black, are placed in exactly similar glass vessels of water and exposed to light, the temperature of the vessel containing the black paper is raised slightly higher than the other. This suggests the idea that in a similar manner radium may keep its temperature higher than the surrounding air by the absorption of other radiations as yet unknown.