Canal Rays and X-Rays

The nature of the resinously charged corpuscles which constitute cathode rays being thus far determined.

Do corresponding bodies carry charges of vitreous electricity?

This was provisionally answered by W. Wien[90] of Aachen in the same year.

More than a decade previously E. Goldstein[91] had shown that when the cathode of a discharge-tube is perforated, a certain radiation passes outward through the perforations into the part of the tube behind the cathode.

He named these ‘canal rays’.

He showed that the canal rays were formed of positively charged particles. It obtained a value of m/e immensely larger than Thomson had obtained for the cathode rays, and of the same order of magnitude as the corresponding ratio in electrolysis.

The disparity thus revealed between the corpuscles of cathode rays and the positive ions of Goldstein’s rays excited great interest. It seemed to offer a prospect of explaining the curious differences between the relations of vitreous and of resinous electricity to ponderable matter.

These phenomena had been studied by many previous investigators; in particular Schuster,[92] in the Bakerian lecture of 1890, had remarked that “if the law of impact is different between the molecules of the gas and the positive and negative ions respectively, it follows that the rate of diffusion of the two sets of ions will in general be different,” and had inferred from his theory of the discharge that “the negative ions diffuse more rapidly.”

This inference was confirmed in 1898 by John Zeleny,[93] who showed that of the ions produced in air by exposure to X-rays, the positive are decidedly less mobile than the negative.

The magnitude of the electric charge on the ions of gases was not known with certainty until 1898, when a plan for determining it was successfully executed by J. J. Thomson[94] The principles on which this celebrated investigation was based are very ingenious.

By measuring the current in a gas which is exposed to Röntgen rays and subjected to a known electromotive force, it is possible to determine the value of the product nev, where n denotes the number of ions in unit volume of the gas, e the charge on an ion, and v the mean velocity of the positive and negative ions under the electromotive force. As v had been already determined,[95] the experiment led to a determination of ne; so if n could be found, the value of e might be deduced.

The method employed by Thomson to determine n was founded on the discovery, to which we have already referred, that when X-rays pass through dust-free air, saturated with aqueous vapour, the ions act as nuclei around which the water condenses, so that a cloud is produced by such a degree of saturation as would ordinarily be incapable of producing condensation.

The size of the drops was calculated from measurements of the rate at which the cloud sank; and, by comparing this estimate with the measurement of the mass of water deposited, the number of drops was determined, and hence the number n of ions. The value of e consequently deduced was found to be independent of the nature of the gas in which the ions were produced, being approximately the same in hydrogen as in air, and being apparently in both cases the same as for the charge carried by the hydrogen ion in electrolysis.

Since the publication of Thomson’s papers his general conclusions regarding the magnitudes of e and m/e for gaseous ions have been abundantly confirmed. It appears certain that electric charge exists in discrete units, vitreous and resinous, each of magnitude 1·5 x 10-19 coulombs approximately.

Each ion, whether in an electrolytic liquid or in a gas, carries one (or an integral number) of these charges. An electrolytic ion also contains one or more atoms of matter, and a positive gaseous ion has a mass of the same order of magnitude as that of an atom of matter.

But it is possible in many ways to produce in a gas negative ions which are not attached to atoms of matter; for these the inertia is only about one-thousandth of the inertia of an atom; and there is reason for believing that even this apparent mass is in its origin purely electrical.[96]

The closing years of the 19th century saw the foundation of another branch of experimental science which is closely related to the study of conduction in gases.

When Röntgen announced his discovery of the X-rays, and described their power of exciting phosphorescence, a number of other workers commenced to investigate this property more completely. In particular, Henri Becquerel resolved to examine the radiations which are emitted by the phosphorescent double sulphate of uranium and potassium after exposure to the sun.

The result was communicated to the French Academy on February 24, 1896.[97]

“Let a photographic plate,” he said, " be wrapped in two sheets of very thick black paper, such that the plate is not affected by exposure to the sun for a day. Outside the paper place a quantity of the phosphorescent substance, and expose the whole to the sun for several hours. When the plate is developed, it displays a silhouette of the phosphorescent substance. So the latter must emit radiations which are capable of passing through paper opaque to ordinary light, and of reducing salts of silver."

At this time Becquerel supposed the radiation to have been excited by the exposure of the phosphorescent substance to the sun; but a week later he announced[98] that it persisted for an indefinite time after the substance had been removed from the sunlight, and after the luminosity which properly constitutes phosphorescence had died away; and he was thus led to conclude that the activity was spontaneous and permanent.

It was soon found that those salts of uranium which do not phosphoresce—e.g., the uranous salts,—and the metal itself, all emit the rays; and it became evident that what Becquerel had discovered was a radically new physical property, possessed by the element uranium in all its chemical compounds.

Attempts were now made to trace this activity in other substances.

In 1898, it was recognized in thorium and its compounds;[99]

P. Curie and Madame Sklodowska Curie announced to the French Academy the separation from the mineral pitchblende of two new highly active elements, to which they gave the names of polonium[100] and radium.[101]

A host of workers was soon engaged in studying the properties of the Becquerel rays. The discoverer himself had shown[102] in 1896 that these rays, like the X- and cathode rays, impart conductivity to gases.

It was found in 1899 by Rutherford[103] that the rays from uranium are not all of the same kind, but that at least two distinct types are present; one of these, to which he gave the name α-rays, is readily absorbed; while another, which he named β-radiation, has a greater penetrating power. It was then shown by Giesel, Becquerel, and others, that part of the radiation is deflected by a magnetic field,[104] and part is not.[105]

After this, Monsieur and Madame Curie[106] found that the deviable rays carry negative electric charges, and Becquerel[107] succeeded in deviating them by an electrostatic field. The deviable or β-rays were thus clearly of the same nature as cathode rays; and when measurements of the electric and magnetic deviations gave for the ratio m/e a value of the order 10-7, the identity of the β-particles with the cathode-ray corpuscles was fully established.