The marvels of radium may be said to have been more or less foreshadowed by the discovery of the Roentgen rays. It was immediately determined that the emanations of a Crookes tube were not ethereal undulations such as ordinary light, but that they consisted of actual material particles of matter highly charged with electricity. Naturally the attempt was made to discover whether the phenomena of phosphorescent substances were not akin to those of the Crookes tube. The leading spirit in this movement was Professor Henri Becquerel, who selected the metal uranium as the subject of his experiments. He accidentally discovered that the so-called phosphorescent attributes of uranium were not due to the absorption of sunlight, but that the substance was spontaneously active, and that the light which came from radium was a new kind of emanation entirely different from the X-rays. To these new radiations the name "Becquerel Rays" was given.

Uranium is obtained from pitchblende, an ore more or less widely distributed about the world, but found chiefly in Bohemia and in Cornwall. Madame Curié, who, at the time Becquerel was making his investigations, was a senior student at the Municipal School of Physics and Technical Chemistry in Paris, had selected "RadioActivity"-a name which she coinedas the subject of her Doctor's thesis. Naturally it was necessary for her to study uranium and similar minerals with some care. She found that, after having extracted all the uranium contained in her specimen of pitchblende, there still remained in the residue a substance far more active than uranium. After isolating this unknown radiant substance and analyzing it, she found that it contained two new elements. The one she christened "polonium," after Poland, the land of her birth; the other she named "radium."

Several tons of pitchblende must be treated and concentrated before a few grains of radium are obtained. But those few grains are worth more than any precious gem or metal in the world. Indeed they have almost any value which their fortunate possessor may choose to give them. There are probably not two pounds of pure radium in existence; but at the present market price they would be worth each about three and one-half million dollars. There is more gold in sea water

than radium in pitchblende; and that is why its price is so high.

The properties of radium will probably necessitate a decided revision in some time-honored chemical theories; for radium refuses to conform to our long-established atomic theories, and behaves in a most inexplicable fashion. In the first place the radio-activity of the element has been found to consist of three distinct sets of emanations, which have been respectively christened the Alpha, the Beta, and the Gamma rays, for want of better

names.

The Alpha rays are not, like ordinary light, ethereal pulsations, but actual material particles hurled off at a speed of about 20,000 miles per second from the parent mass. They are highly charged with positive electricity. Their speed is about 40,000 times greater than that of a rifle bullet.

The Beta rays, which consist of particles of matter, corpuscles of electricity or "electrons" as the modern physicist calls them, move still more swiftly. Each of the Beta particles (very much smaller in size than the Alpha particles) travels at the rate of about 100,000 miles a second. They are the fastest moving objects known in the universe; for their speed is three hundred times faster than that of the swiftest star. Such is their velocity that it takes a foot of solid iron to stop them.

The Gamma rays are probably Roentgen rays, if one may judge by the similarity of the properties of the two. Like the Beta rays, the Gamma emanations have remarkable penetrating properties. But of the three kinds of rays discharged by radium, the Gamma rays are the most difficult to detect and the least perfectly understood.

Professor Curié, Madame Curie's husband, has discovered that radium constantly maintains a temperature of about five or six degrees above the surrounding atmosphere. For some time this startling phenomenon baffled physicists. Here was a substance constantly giving off heat without being apparently consumed, and without anything to make it hot. It is now thought that this strange property can be explained by assuming that the particles collide with one another, and that the heat generated by the impact (a heat that must be very marked when it is considered how enormous

is the energy of a particle moving at the rate of many thousand miles a second) is sufficient to explain the heat generated by radium.

The fact that radium is a spontaneous source of thermal energy is in itself a fact sufficiently startling. Sir William Ramsay, however, has discovered still other startling properties of this startling substance. He col

lected the material particles which are shot from the substance, analyzed them, and found that after a few days they changed into helium, a gas which was first discovered burning in the sun. This seems dangerously like the transmutation of one element into another, the problem on the solution of which the medieval alchemist had worked for centuries. After ages of labor seventy-odd bits of primordial matter had been wrung from the earth, so simple and so unchangeable in their nature that they were deemed elements. And now one of them proves to be nothing but the product of another. Can we ever be certain again that the rest are not also likely to change? Is it any wonder that our chemistry needs revision?

The atomic weight of radium has been ascertained by Madame Curié to be 225; that of helium is 2.2. In other words, every atom of radium breaks up into about 100 parts of helium. What becomes of the old teaching that atoms are indivisible particles of mat

ter? Some of the more advanced thinkers have abandoned the atom and adopted the "electron" as the ultimate unit. The atom is certainly quite inadequate to account for the properties of radium. Atoms may be said to be composed of electrons moving, like miniature solar systems, with inconceivable rapidity in well-defined orbits. Sometimes a little planet of that system becomes unstable, darts off with terrific speed like a comet, and thus gives rise to the phenomena of radium, of uranium, and of every other radioactive substance.

Has radium any practical value? it may be asked. So far it is more of a scientific curiosity than anything else. Still, it is not without some use. It is an excellent detector of false diamonds; for it causes the real gem to glow with wonderful brilliancy, while the paste imitation is left comparatively lusterless. Then, again, radium kills bacteria and even very small animals. The modern physician has used the substance with some success in treating certain diseases, among them cancer and lupus. Living tissues of the body are strangely affected by short exposures to the substance. Sores are produced, like burns, which heal only after weeks have elapsed. An electroscope has also been invented, the underlying principle of which is dependent upon the properties of radium.

THE TELESCOPE.-Telescopes are of two kinds, namely, refracting and reflecting telescopes. The refracting telescope consists of an object-glass which forms an image of the object, and an eye-glass by which the image is viewed. The reflecting telescope consists of a concave mirror which receives light from the distant object, and reflects it so that the rays converge to a focus and form an image, the image being viewed by an eye-glass. The terrestrial telescope consists of two telescopes like the preceding-which are called astronomical telescopes, and give an inverted image the second inverting the inverted image of the first, and so giving an upright image. Eye-pieces generally have two lenses, and have names according to the position of the focus. Ramsden's eye-piece has two lenses, the focus being just beyond the field lens. It is called a positive eye-piece, and it can be used as a magnifying glass. Huyghens' eye-piece also has two lenses, the focus being between the two. It is called a negative eye-piece, and cannot be used as a magnifying glass. These compound eyepieces enable us to get rid of spherical and chromatic aberration. The achromatic object-glass is made by joining together two lenses, one of flint glass and the other of crown glass. The dispersion is made equal and opposite, but the bending powers are unequal. A lens is equivalent to a number of prisms placed base to base, the outer prisms having a greater angle to cause the rays to bend more, so that all the rays may come to one point, called the focus. The magnifying power of a telescope is found by dividing the focal length of the object-glass by the focal length of the eye-piece.

THE EQUATORIAL TELESCOPE. The equatorial is an ordinary telescope, mounted in such a way that it can easily be directed to any part of the heavens. The polar axis is parallel to the earth's axis, that is to say, it is inclined at an angle equal to the latitude of the place, at Washington about 39°, at London about 51°. The telescope can be moved round the polar axis in a plane which is parallel to the earth's equator, and this motion is said to be motion in right ascension. The telescope can also be moved up and down in a plane at right angles to the earth's equator, and this motion is called motion in declination. Whatever part of the skies an object is in, the equatorial can be directed to it, and the object can be kept constantly in view, because there is a kind of clock which drives the instrument round at the same speed at which the earth is turning round.

THE TRANSIT INSTRUMENT.-The transit instrument is a telescope mounted on a horizontal axis, so as to be capable of moving in the meridian only. It is used to determine the exact moment at which celestial bodies cross the meridian, that is, when they are in a true north or south position. It is also used for determining the declination of celestial objects, that is, how far in angular measures these bodies are from the celestial equator.

THE SIDEREAL CLOCK.-The sidereal clock is similar to an ordinary clock, but it is regulated to keep accurate time with the apparent diurnal movements of the stars, instead of with the mean sun. It shows the same time as clocks and watches only once in a year, namely, at the Vernal Equinox, about the 21st of March. It gains about four minutes each day on the ordinary clock, and in a year it gains a whole day, so that there are 366 sidereal days and only 365 solar days in one year. The sidereal noon occurs when the first point of Aries passes the meridian, and the hours are reckoned from 0 to 24. The time by the sidereal clock at which a celestial body crosses the meridian is equal to the right ascension of that particular object. Conversely, if the exact right ascension of a star be known, the error of the clock can be determined by observing a transit of the star.

THE CHRONOGRAPH.-The chronograph consists of a cylinder covered with paper, and made to rotate uniformly by clockwork. It is connected electrically with the sidereal clock, which, as it ticks, makes dots on the paper at equal distances by means of a recording pen, and these dots represent seconds. Fractions of a second are recorded by the observer touching a key, which causes a second pen to make a dot on the cylinder as it turns round. This dot would come between two second dots, and the distance is measured from these. In this manner the 180 or 1000 of a second can be estimated. The small fractions of a second obtained by the chronograph are necessary in fixing the right ascension and declination by the transit instrument.

THE MICROMETER.-The micrometer is used for measuring small arcs. It consists of two wires, which can be brought together or separated at pleasure by means of a screw. An equatorial star appears to move through about 15° in one hour, 1° in four minutes, 15' in one minute, or 15" of arc in one second of time. The distance that the wire moves for one turn of the screw is found by allowing a star to pass from one wire to