mous amount of energy, each particle apparently having sufficient energy associated with it to excite phosphorescence visible to the eye. Thus, Crookes has shown that when a fragment of solid radium nitrate is brought near to a screen of "Sidot's hexagonal blende" (zinc sulphide), and the phosphorescent surface of the screen is examined with a pocket lens, it is seen to be dotted all over with brilliant specks of green light. In proportion as the radium salt is brought closer to the screen, these flashes or scintillations become more brilliant and more numerous, following each other with such rapidity that the surface presents the appearance of a "turbulent luminous sea."

It seems probable that we are here actually witnessing the bombardment of the screen by the electrons * hurled off by the radium" (Crookes).

2. The Rays.-These rays are readily deviated by the magnetic field; and also differ from the a rays in their greater penetrating powers. Thus, while the intensity of the latter is reduced to one-half by passing through 0.0005 cm. of aluminium, the B rays are able to traverse a thickness of 0.05 cm. of this metal before their intensity is halved. A sheet of mica 0.01 cm. thick will completely absorb all the a rays, while it transmits the ẞ and also they rays without appreciable diminution. The B rays, like the a rays, also consist of projected particles with a high velocity, but in this case they carry a negative electric charge, and their mass is believed to be greatly less than that of the particles constituting the a rays (Rutherford and Soddy, Phil. Mag., May 1903). B rays are similar in all respects to the "cathode' rays emitted from a vacuum tube, except that the velocity of the particles is greater and consequently they are more penetrative. Their velocity is estimated to be between 2x 1010 and 3 x 1010.

3. The Rays.-These are non-deviable by the magnetic field, and closely resemble the Röntgen or "X" rays. They are far more penetrating than either the a or B rays, being capable of penetrating a thickness of 8.0 cms. of aluminium before their intensity is reduced to one-half. These rays are believed to be a wave motion, and not to consist of projected particles of

matter.

4. "Radioactive Emanation."-The elements thorium and radium † possess the property of emitting something which has the power of imparting radioactivity to any substance in their immediate neighbourhood. The radioactivity thus imparted or excited, is only of a temporary character, its intensity diminishing and dying away when the substance is removed from the influence of the original radioactive body. Experiments seem to prove that these effects are not produced by any of the rays already described, but are due to some other distinct emission, and the term "radioactive emanation," or shortly "emanation," has been adopted to denote this.

The radioactivity which is thus imparted to substances in the proximity of these radioactive elements (usually spoken of as excited radioactivity) is believed to be caused by the deposition upon their surface of radioactive matter, which is transmitted by positively charged carriers; while the radio

These particles of matter, whatever they may ultimately prove to be, are variously spoken of as "electrons," "primary atoms," "corpuscles," meta

bolons.

+ Uranium appears not to share this property.

activity of the emanation itself is believed to be due to the emission from it of a rays only. When a small quantity of thorium oxide* is placed in a tube (the oxide being enveloped in material capable of intercepting the ordinary radiations) and a stream of air is passed over it, the air is found to carry with it the "emanation" which the thorium oxide gives out; and the issuing stream of air, even after being conveyed through many feet of tube, is capable of discharging an electroscope. In the case of radium compounds the amount of this "emanation" was found to be comparatively small when the radium compound is employed in the solid state, but when the radium salt is dissolved in water, the "emanation" appears to be given off in a sudden rush, as it were, and the solution continues to emit this "emanation" in amount many hundred times as great as was produced by the solid salt. A similar enormous increase also takes place when the radium compound is heated. These observations have led to the belief that the "emanation" is actually occluded by the solid compound.†

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In many other respects this "emanation" behaves like an inert gas. Thus if the stream of air carrying the "emanation" is passed through a tube plugged with cotton wool, nothing is arrested or filtered out by the wool and the radioactivity of the air as it issues is not diminished. Neither is it affected by the air being bubbled through strong sulphuric acid, or passed through a red-hot platinum tube. When air conveying "emanation" is slowly passed through a U-tube cooled by liquid air, the "emanation is completely condensed, and the air which passes out is entirely free from all trace of this substance. If a glass tube is employed, and the air current is sufficiently slow, the progress of the condensation can be traced by the fluorescent appearance of the glass, showing that the condensation has all taken place upon the first portions of the tube traversed by the stream of air. If now the tube is closed at both ends and the temperature allowed to rise above a certain point, the condensed emanation appears to vaporise again, and the fluorescence extends throughout the entire length of the tube. The volatilisation point of radium emanation appears to be about - 150°, while that of the thorium emanation is given as about - 120° (Phil. Mag., 1903, P. 575).

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In the case of thorium, the "emanation" loses its radioactivity, or decays, much more rapidly than the radium emanation. Thus, while the activity of thorium emanation falls to half its intensity in the space of one minute, the intensity of the radium emanation only sinks to half its value in the space of four days, while still retaining sufficient activity to be detected after the lapse of one month. This rate of decay of the radioactivity of the "emanation" is the same even at the low temperature of liquid air, and it is considered probable that the marked difference in the rates of decay of the "emanation" from thorium and radium may account for the difference observed in their vaporisation temperatures.

It was at one time supposed that the radioactivity of these radioactive elements was not a property intrinsic to the elements themselves, but was due

* Most of the earlier work by Rutherford and Soddy (Phil. Mag., 1902) in this connection was done with thorium compounds.

† Rutherford and Soddy, Phil. Mag., 1903, p. 449.

to the presence in small and varying quantity of some unknown substance, Crookes found (Proc. Royal Soc., 1900) that by processes of a purely chemical nature he was able to separate from uranium nitrate small quantities of material which seemed to possess all the radioactivity, leaving the bulk of the uranium compound inactive. He applied the name Uranium X to this "unknown uranium." Similarly in the case of thorium; when the hydroxide was precipitated by ammonia, and the filtrate (which chemically should contain no thorium) was evaporated to dryness and ignited to expel ammonia salts, minute residues were obtained which were many hundred times more active than an equal weight of thorium oxide (Rutherford and Soddy, Phil. Mag., September 1902). The precipitated hydroxide, although not entirely robbed of radioactivity, was found to have its activity greatly reduced. This supposed "active constituent was therefore called Th X. Later investigations, however, revealed the remarkable fact that the thorium compound which had thus been partially deprived of its radioactivity gradually regained it when left to itself; while the separated Th X gradually lost it. Moreover, it was found that the two processes went on exactly at the same rate, that the rate of decay of the activity of Th X was the same as the rate of recovery of activity of original thorium compound. From this it would appear that two opposing processes are simultaneously going forward in a radioactive substance, namely, the continual production of fresh radioactive material and the constant decay of the radiating power of the active material. In other words, what may be called the normal radioactivity is a condition of equilibrium, where the rate of increase of activity due to the production of fresh active material balances the rate of the decay of the activity in the radioactive material already formed.

The views now generally held are that the phenomena of radioactivity are due to atomic changes, but changes of a character altogether different from any that have previously been dealt with in chemistry. It is believed that the atoms of these radioactive elements (which, it will be noted, are possessed of the highest atomic weights of all the elements) are undergoing a process of disintegration or degradation: that in the course of their movements the kinetic energy of some of the atoms reaches a point beyond which the stability of the atom is no longer possible. Under these circumstances the atom breaks up, throwing off some matter from itself ("primary atoms," "metabolons," electrons,") and assumes a more stable configuration. The ejected particles or fragments of the original atoms themselves undergo further changes, giving off other "metabolons," which it is believed give rise to the various phenomena of radioactivity.

The energy which is liberated during this process of atomic disintegration is enormous, taking into account the minute quantities of matter concerned. M. and Mme. Curie have shown that a sample of a radium salt gave out energy sufficient to melt half its own weight of ice per hour. This energy, which is stored up in the atoms of these elements, the "internal energy of the chemical atom as it has been termed, and which is set free during radioactive change, is of an entirely different order of magnitude from that which is disengaged during any processes of ordinary chemical change. It has been calculated, indeed, that the energy of radioactive change is many thousand times, or even a million times, as great as that of any known chemical

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change, when equal weights of matter are concerned.

The idea of an atom

as a system, and, moreover, one capable of undergoing changes into simpler systems, is a view which, to the chemist, may at first seem strangely heterodox, and one altogether opposed to fundamental doctrines of chemistry. In reality, however, this new view as to the constitution of an atom does not touch the question of the indivisibility of the atom in the purely chemical sense. From this point of view the chemical atom still retains its position as the lowest stage in the complexity of matter. The chemical atoms of these radioactive elements are not divisible into what may be called " chemical fragments." If the atom is a system, then in all chemical reactions and changes the system in its entirety takes part. No chemical compounds have yet been produced in which the disintegrated fragments of these atoms form an integral part. When it is borne in mind that the weight of matter which the atom, regarded as a changing system, throws off in the form of " metabolons" is infinitely minute, that it has been estimated that it would require many hundreds, if not thousands of years before enough of it could be collected to be detected by the most delicate balance, it will be evident that we are dealing with phenomena of a totally different order from those in which the relative weights of matter entering into chemical combination are concerned.