stituted for the fluorescent screen, a photograph is obtained on development such as is shown in figure 467.

778. The Nature of Röntgen Rays.-There is considerable uncertainty as to the nature of the Röntgen radiation, though Stokes has advanced the theory that it consists of very short ether waves not sent out in consecutive trains, as in ordinary light, but at irregular intervals, just as might be expected if each of the electrons in the cathode rays sent out a solitary pulse or wave in the ether when it was suddenly stopped by

striking the oblique platinum plate in the tube. And J. J. Thomson has shown that the sudden stoppage or change in the motion of an electron may be expected to produce exactly this effect.

Stokes has shown that radiation of such a character would probably not be refracted nor absorbed like ordinary light nor would it show interference effects by which the wave length could be determined, and this is quite in accordance with what is observed in case of Röntgen rays. The fact that these rays are not deviated by a magnet shows conclusively that they cannot consist of a stream of charged particles like the cathode

rays.

779. Canal Rays. Still another kind of rays, known as canal rays, was discovered by Goldstein in 1886. When the cathode in a highly exhausted vacuum tube is pierced full of small holes, the canal rays may be observed as streams of light

coming through the holes and out on the back of the cathode, and are thus directed away from the positive electrode and are in the opposite direction to the cathode rays. They derive their name from the small openings or canals through which they pass. These rays are found to consist of streams of positively charged particles having masses more than a thousand times as great as the masses of the electrons in the cathode rays, but their velocity is very much less than that of the cathoderay particles.

780. Gaseous Conduction.-At ordinary temperatures and atmospheric pressure gases are almost complete non-conductors of electricity. Careful experiments have shown, however, that there is a slight loss of charge through the air, which is believed to be due to the presence in the air of a few ionized molecules, or molecules which are broken up each into a positive ion and a negative ion or electron. Positive ions coming in contact with a negatively charged body receive negative charge and become neutral.

The ionization of a gas may be brought about by intense chemical action and high temperature, and so flame gases readily conduct electricity. Also it may result from the passage of an electric spark through the gas, or from a very intense electric force as in case of discharge from electrified points. A mass of gas may also be ionized and made conducting by exposing it to Röntgen rays. When a charged electroscope is exposed to Röntgen rays it quickly loses its charge.

It seems probable that in Geissler tubes the initial ionization is caused by electrons driven off with great velocity from the cathode and striking against gaseous molecules.

In the process of conduction a gas soon loses its ionization partly because the ions are neutralized in the act of conduction and partly because the recombination of positive and negative ions is constantly going on.

RADIOACTIVITY.

781. Radioactivity. In 1896 the French physicist Becquerel discovered that minerals containing uranium gave out a radiation which resembled Röntgen rays in acting on a photographic plate through an envelope of black paper. He also showed that

the uranium or Becquerel rays, as they were called, had the power to discharge electrified bodies, so that by using a sensitive electroscope they could be easily and accurately detected and their intensity measured. The photographic action was very slow, an exposure of several days being required to produce a distinct impression.

Following out this discovery, Mme. Curie made a systematic search for other active substances and found that thorium possessed a similar power of radioactivity, as it now came to be called, a discovery which was also independently made by Schmidt.

It also appeared from these investigations that radioactivity is an atomic phenomenon. For the radioactivity of any given compound of uranium was found to be simply proportional to the amount of uranium in the substance, and in no way dependent on its physical or chemical condition.

But pitchblende, a mineral ore of uranium, was found to be several times more active than pure uranium or thorium, and hence Mme. Curie concluded that it must contain some unknown and highly radioactive substance, and resolutely set out to isolate it. As a result of the laborious treatment of several tons of pitchblende and uranium residues, a few hundredths of a gram of a new and astonishingly active element were obtained. (in 1898) to which the name radium was given. This substance is obtained usually as a chloride or bromide, and is estimated to have in the pure state about two million times the activity of uranium.

Some radium compounds glow with a faint luminosity in in the dark, though pure radium bromide is only feebly luminous. Crookes showed that if a minute particle of radium bromide is supported about a millimeter in front of a surface coated with phosphorescent zinc sulphide, the latter lights up with flashes of light, probably due to its bombardment by alpha particles from the radium.

782. Complex Character of the Radiation. The radiation from uranium, thorium, or radium, is found to contain three distinct kinds of rays known as alpha, beta, and gamma rays.

These different rays are distinguished from each other in two ways: by their penetrating power and by their deflection in a magnetic or electric field.

The alpha rays are completely stopped by a few centimeters of air or a layer of aluminum foil .05 mm. in thickness. They consist of positively charged particles having a mass about four times that of the hydrogen atom and are projected with a velocity of about twenty thousand miles per second.

The beta rays have about 100 times the penetrating power of the alpha rays. They consist of negatively charged particles having only about one-thousandth the mass of the hydrogen atom and seem to be of exactly the same nature as the cathode `rays in a vacuum tube except that the velocity of projection of the particles in the beta rays is much greater than is usual in cathode rays, and in case of radium is found to be from 0.3 to 0.9 the velocity of light.

The gamma rays are the most penetrating of all, these rays from 30 mgs. of radium bromide having been detected by their effect on an electroscope after passing through 30 centimeters of iron; they are not deviated in a magnetic or electric field and seem to be of the same nature as the Röntgen rays from a "hard" X-ray tube. They probably originate in the collisions of beta particles with the substance itself in its interior, just as Röntgen rays arise from the impacts of cathode-ray particles against some obstacle.

783. Energy and Heating Effect of Rays.-Although the velocity of the alpha particles is less than that of the beta particles, yet in consequence of their greater mass the energy of motion of each alpha particle is something like 120 times that of a beta particle, consequently the heating and ionizing effect of the rays is mainly due to the alpha particles.

It was found by Curie and Laborde, in 1903, that radium was a constant source of heat, one gram of radium bromide giving out about 100 gram-calories of heat per hour, or more than enough to melt its own weight of ice in an hour. This extraordinary development of energy is believed to be mainly due to the escaping alpha particles which are absorbed within the mass of radium itself and in the enclosing vessel, their energy of motion being transformed into heat. Originally the energy must have existed in the radium atoms before the alpha particles were given off.

784. Radioactive Transformations.-Evidently an atom of

radium cannot give out alpha and beta particles with a corresponding expenditure of energy and remain the same as before. It is therefore believed that the atoms of the radioactive substances have such slight stability that in every instant some of them are reaching a condition of instability and breaking up or exploding with the expulsion of alpha or beta particles and coming to a new state of equilibrium. In case of radium, about half the particles in a given mass will have broken up in this way in the course of 2000 years. When such a change has taken place, and an alpha particle has been expelled, the new state of equilibrium is apt to be even less stable than the original one, and consequently another alpha particle is soon expelled and another state of equilibrium reached which itself may also be short-lived, and so there takes place a series of changes in the course of which the various kinds of rays are given off, and finally a condition of such stability is reached that no further change is detected.

The diagram indicates the series of changes of the radium atom as followed out by Rutherford, and under each step is given the time in which that product is half transformed. Thus, while radium itself is half transformed in 2000 years, the instability of the emanation is so great that any given portion of it will be half transformed in 3.75 days; while radium A is the most unstable of all, its period of half transformation being only 3 minutes.

The sizes of the spheres in the diagram represent the relative amounts of the various products that are present in a mass of radium which is undergoing change, and has come to such a