Part 2 (1/2)

3. By exposure to magnetic and electric fields, noting extent and direction of deflection.

4. By their relative absorption by solids and gases.

5. By the scintillations on a zinc sulphide screen.

Identification of the Rays

The alpha rays have been identified as similar to the so-called ca.n.a.l rays. These were first observed in the study of the _X_ rays. When an electrical discharge is pa.s.sed through a vacuum tube with a cathode having holes in it, luminous streams pa.s.s through the holes toward the side away from the anode and the general direction of the stream. They travel in straight lines and render certain substances phosph.o.r.escent.

These rays are slightly deflected by a magnetic field and in an opposite direction from that taken by the cathode rays in their deflection. The rays seem to be positive ions with ma.s.ses never less than that of the hydrogen atom. Their source is uncertain, but they may be derived from the electrodes.

The beta rays are identical in type with the cathode rays and are negative electrons.

The gamma rays are a.n.a.logous to the _X_ rays and are of the order of light. They are in general considerably more penetrating than _X_ rays. For example, the gamma rays sent out by 30 milligrams of radium can be detected by an electroscope after pa.s.sing through 30 centimeters of iron, a much greater thickness than can be penetrated by the ordinary _X_ rays.

CHAPTER III

CHANGES IN RADIO-ACTIVE BODIES

Is Radio-activity a Permanent Property?

Is this power of emitting radiations a permanent property or is it lost with the pa.s.sage of time? The first investigations of the activity of uranium and thorium showed no loss of intensity at the end of several years, and radium also seemed to show no decrease in its enormous activity. Polonium, however, was found to lose most of its activity in a year, and later it appeared that some radio-active substances lost most of their activity in the course of a few minutes or hours.

Induced Activity

A phenomenon called induced or secondary radio-activity was also observed. Thus a metal plate or wire exposed to the action of thorium oxide for some hours became itself active. This induced activity was not permanent but decreased to half its value in about eleven hours and practically disappeared within a week. Similar phenomena were observed when radium was subst.i.tuted for thorium.

Discovery of Uranium X

In 1900 Crookes precipitated a solution of an active uranium salt with ammonium carbonate. The precipitate was dissolved so far as possible in an excess of the reagent, leaving an insoluble residue. This residue was many hundred times more active, weight for weight, than the original salt, and the solution containing the salt was practically inactive. At the end of a year the uranium salt had regained its activity while the residue had become inactive.

Another method of obtaining the same result is to dissolve crystallized uranium nitrate in ether. Two layers of solution are formed, one ether and the other water coming from the water of crystallization. The aqueous layer is active, while the water layer is inactive. Similarly, by adding barium chloride solution to a solution of a salt of uranium and then precipitating the barium as sulphate, the activity is transferred to this precipitate. These experiments give proof of the formation and separation of a radio-active body by ordinary chemical operations.

So, too, in the case of thorium salts a substance can be obtained by means of ammonium hydroxide which is several thousand times more active than an equal weight of the original salt. After standing a month, the separated material has lost its activity and the thorium salt has regained it. Here, again, there is the formation, separation, and loss of a radio-active body.

Conclusions Drawn

Now, these are ordinary chemical processes for the separation of distinct chemical individuals. The results, therefore, lead naturally to the conclusions: (1) it would seem that uranium and thorium are themselves inactive and the activity is due to some other substance formed by these elements; (2) this active substance is produced by some transformation in those elements, for on standing the activity is regained. This latter conclusion is startling, for it indicates a change in the atom which, up to the time of this discovery, was deemed unchangeable under the influence of such physical and chemical changes as were known to us.

Search for New Radio-active Bodies

The search for new radio-active bodies and the study of their characteristics has been systematically and successfully carried on.

The bodies obtained in the above experiments were named uranium _X_ and thorium _X_, respectively. Further, it became clear from the investigation of uranium minerals that radium, polonium, actinium, and ionium originated from uranium. From thorium minerals a body was separated called mesothorium, which was a.n.a.logous to radium. Both thorium and radium were found to give off a radio-active gas. The first lost half of its activity in less than one minute. The second was more stable and lost half of its activity in about four days. The name radium emanation was given to the latter and it was found chemically and physically to belong to the cla.s.s of monatomic or n.o.ble gases, such as helium, argon, neon, etc., which had been discovered by Ramsay. In some cases the chemical action was determined and these new bodies were found a.n.a.logous to well-known elements, as radium to barium, polonium to bis.m.u.th. The physical properties were investigated and, where possible, spectra were mapped and atomic weights determined.