tube about 2 inches long. In one end is a screen coated with a compound of sulphur and zinc. Close to this is fixed a little copper arrow carrying a trace of radium. In the other end of the brass tube is a lens for better observation. If, after resting the eyes in perfect darkness, you look into the lens, what you see resembles a starry sky, but with the difference that the stars are constantly in motion. Each a-particle, as it strikes the screen, produces a flash which asts only for an instant, then the eye is attracted to some, new place, and so on. The sight is an impressive one, and rightly so, for each flash registers the death of an atom and this is as sublime a spectacle in its way as the death of a universe, though at the other end of the scale of magnitudes.

Since the a-particle is large and not very speedy it is easily arrested. If a radium preparation is wrapped in a sheet of writing paper or a thin layer of aluminium foil, the a-rays are stopped completely, and the ẞ- and y-rays alone pass through and can be studied separately. Even a few centimeters of air have the same effect. It must be remembered, however, that the term "slow" is only applied to the a-particles by comparison with the more swiftly-moving B-particles. Compared with that of any projectile, the speed of the a-particle is enormous. Weight for weight, for instance, it has 600,000,000 times the energy of motion of a rifle bullet.

(b) Brays.-The B-rays have been shown to consist of material particles of about 10 the mass of the hydrogen atom. Their speed approaches but does not quite equal that of light. On account of their small size and great speed they penetrate matter about 100 times as readily as the a-rays. We may consider that ordinary matter bears much the same relation to them as a very coarse net to an extremely small and agile fish. The ẞ-rays are identical with the cathode rays, which are formed at the negative pole when electric discharges pass through a vacuum tube. They are chiefly responsible for the effect of radium preparations upon the photographic plate.

Both the a- and ẞ-rays are turned out of their course

by an electric or magnetic field. This is a proof that they are composed of material particles. In the case of the a-rays the deflection is slight, and its direction shows that each particle carries a positive charge. The B-particles are deflected to a much greater extent and in the opposite direction, showing that each particle is charged negatively. Calculation shows that, taking the a- and B-rays together, three trillions of particles escape from an ounce of radium per second.

(c) The T-rays.-The 7-rays are the most penetrating. They can readily be detected after passing through the human body or a foot of solid iron. A plate of aluminium, 3 inches thick, cuts them down to one-half their value. There is still some question as to the nature of the 7-rays. They are not at all deflected by an electric or magnetic field, so there is no proof that they are material. They may consist of streams of uncharged particles shot off with immense speed. Uncharged particles would not be deflected, but it is more likely that they consist of an ethereal vibration identical with the Röntgen rays. It seems reasonable to suppose that when a particle is pitched off, the atomic convulsion would set up a shiver in the contiguous ether.

(d) Effects of the Rays.-Under this head we shall discuss the effects of the three kinds of rays together, just as they escape from the preparation of radium. It must be remembered, however, that if the radium is sealed up in a glass tube, which is commonly the case, the a-rays produce very little result, since they are almost entirely stopped by the glass.

Many substances exposed to the rays become luminous. Zinc sulphide and the diamond are examples. The latter lights up brilliantly, and this has been proposed as a method of distinguishing the gem from its imitations, which remain unaffected. Strong radium preparations are themselves luminous, and may give a light strong enough to read by. But, aside from the rarity of the substance, the presence of large quantities of high-grade radium preparations in the household would be attended with such serious danger that it will never be used as a source of light.

Radium compounds also continuously give off heat. A piece of pure radium would in an hour produce enough heat to raise its own weight of water from the freezing to the boiling point. This means that it would boil 8,760 times its weight of freezing water per year.

The glass in which a radium preparation is kept becomes light violet and finally black. Salt and similar substances are colored blue or green. Dry air, which is ordinarily an insulator, acquires the ability to conduct the electric current under the influence of radium rays, and other gases behave similarly. This effect has been used extensively in measuring the strength of radium preparations, and in investigating the nature of the rays, since it is much more convenient and exact than the photographic action.

The property of the rays of causing the air to conduct the current is so important in the study of the subject that we will endeavor to show it to you on the screen. The lantern electroscope which we shall use for that purpose is the property of Dr. John H. Shober, and I am indebted to his kindness for the use of it to-night. It consists essentially of a metal rod, at the upper part of which is attached a strip of aluminium foil. The whole is insulated and supported in a glass box in such a way that the image can be thrown upon the screen by the lantern. You notice that the strip of foil hangs down in contact with the rod, but when the apparatus is charged you observe that the foil stands out at right angles to the rod, because similar electric charges repel each other. This position it will retain for a long time, for since the air is practically an insulator, the charge is lost very slowly. Now a sealed tube containing some radium of 7,000 strength (rather a weak preparation) is introduced.* Immediately the air in the apparatus becomes a conductor and the charge leaks away. You observe that the aluminium leaf gradually falls till it is in contact with the rod. A repetition of the experiment yields the same result. Other things being equal, of course a stronger radium

*It is customary to state the strength of radium preparations in terms of the radio-activity of uranium taken as a unity.

preparation would make the aluminium leaf fall more quickly. With a preparation of 300,000 strength* the fall of the aluminium leaf is instantaneous. It is clear that the method can be used quantitatively. Radium can be identified and determined by it in substances in which the quantity is so small that it would be entirely hopeless to search for it, even by spectroscopic methods. It has been calculated to be about 1,000,000 times as delicate as the spectroscope, and, as you see, it is very rapid and does not entail the loss or destruction of any of the sample tested. Of course, to attain such accuracy requires special elaborate apparatus.

Paper turns brown and falls to pieces under the influence of radium rays. The action on living things is mainly destructive. Seeds lose the power of germination. Some investigators report that bacteria are destroyed, but there is some doubt about this, and it is probable that different organisms differ in their susceptibility. Leavened dough fails to rise if a strong radium tube is buried in it, but when the tube is removed fermentation goes on, so that the yeast is temporarity paralyzed, not killed. Enzymes are destroyed. Perhaps the most remarkable of all the effects of the radium rays is their power to prolong indefinitely the larval stage of certain insects. The strength and distance of the radium tube must be carefully chosen; otherwise the worms are killed. When these conditions are right the worms remain in the larval state, while others in a bottle not containing a radium tube have passed through twenty-four generations. The tadpole stage of the common frog is prolonged in somewhat the same way by a radium tube of proper strength suspended in the water, but in this case eccentricities of growth occur and monsters are produced. When a strong radium tube is suspended over white mice they appear to be unaffected for several days, then the hair falls out and death results after a week or more. Rabbits act similarly. There are indications that weaker radium preparations. have a stimulating effect upon the growth of the hair and the vital functions generally of these animals.

The effect of the rays from a strong preparation upon

the human body is similar to a severe burn, the chief difference being that nothing is noticed until some days after the exposure. To alter slightly a remark of Curie's, a pound of radium would probably destroy the human population of the planet, if each person were subjected to its influence for a time. For several days nothing would be noticed. Then the hair would fall out, the sight would be lost, the skin all over the body would become one continuous burn, and death would result.

IV. THE EMANATION.

What becomes of the radium atom when the particle which has produced the ray has been pitched off? What is the nature of the substance formed of the residual atomic fragments? It can no longer be radium, for a vast amount of chemical experience shows that a difference in atomic or molecular weight is always correlated with a difference in properties, and that the variation in the latter may be profound when the change in the former is relatively small. Compare, for instance, the properties of gold (atomic weight, 1973) with those of mercury (atomic weight, 200), or those of lead (atomic weight, 207) with those of bismuth (atomic weight, 208).

Investigation shows that the residual substance is a gas which is continually escaping from radium compounds. It can be passed through glass tubes and preserved over mercury. When it is passed through a tube sunk in liquid air it is liquefied, producing a bright phosphorescence in the tube. It is dense, as shown by its rate of diffusion. It is somewhat soluble in water. It resembles helium and its allies in refusing absolutely to take part in any chemical reaction -all attempts to induce it to unite with other substances have failed. It is poisonous, for animals made to breathe air which has passed over a strong radium preparation die, although they are screened from the rays, and the air is entirely freed from any dust or powdered radium which might conceivably be carried with it. The lesions-as might be expected-are different from those produced by the rays. Instead of superficial burns we find congested and inflamed lung-tissue.