Man's physical universe



the uranium extracted from it. After laboriously working over about

a ton of the rock, they extracted about 0.1 gram of a material that

gave radiations several thousand times as intense as those emitted by

uranium salts. After the death of her husband, Madame Curie obtained

in 1910 a metallic element from their material and gave the

new element the appropriate name radium. The property of emitting

radiations shown by uranium, radium, and various other elements is

called radioactivity.

In addition to uranium and radium, Madame Marie Curie discovered

another radioactive element, polonium; and since that time several

others have been identified.

It has been found that the radioactivity

of some elements persists for a very long time (millions of years),

while that of other elements lasts for only a few hours, minutes, seconds,

or fractions of seconds.

It has also been found that as the radioactivity

of an element decreases, new elements appear. In 1903 Rutherford

and Soddy announced their disintegration hypothesis, which

supposed that masses composed of large numbers of atoms of radioactive

elements gradually disintegrate to produce atoms of different

atomic characteristics or of a different element, somewhat as popcorn

pops in a popper. There are rays given off in this process w'hich carry

enormous quantities of energy.

It was also discovered that three kinds of rays, the alpha, beta, and

gamma rays, were emitted, although all three kinds were not emitted

by any one element.

Radioactive Rays Resemble Discharge-tube Rays.

Alpha rays are not like true electromagnetic rays such as X rays or

other electromagnetic radiations but consist rather of positively

charged particles of matter, as is shown by their deflection by magnetic

and electrical fields. These alpha particles, as they are called, ionize

gases by dislodging electrons from the molecules with which they

collide. The formation of these ions can be demonstrated by causing

the alpha particles to pass through a space that is saturated with water


When the moist air is expanded under suitable conditions, the

ions produced act as nuclei for the condensation of small droplets of

water, which may then be photographed. When alpha particles strike

a screen painted with zinc sulfide,

tiny flashes of light are produced.

This furnishes a method of counting alpha particles. By means of

such a device, called a spinthariscope, and by means of alpha-particle

tracks in a cloud chamber, we become witnesses of the effects produced

by single individual atoms, and our belief in the atomic theory is thereby

strengthened. The total charge of a number of alpha particles may

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