X-rays are of very

great help to the sur

geon in setting a

broken bone or in lo

FIG. 552.-Radiograph of a Child's Hand
Note the spaces between the ends of the bones

cating a foreign body, as a bullet.

Radiographs are usually made as follows: A photographic plate is inclosed in a plate holder, the object to be radiographed is placed upon the cover of the holder, and the X-rays are directed upon it, from a tube only a few inches

away.

Several forms of Crookes tubes are

capable of giving off the X-rays. In the forms called focus tubes (Fig. 553), the cathode rays are brought to a focus on a platinum plate turned at such an angle that the rays are sent out radially through the glass. Devices are also employed to regulate the vacuum automatically, for otherwise it would become higher with use, until the current would no longer pass.

FIG. 553 X-ray Field

573. The Fluoroscope, a form of fluorescent screen, was devised by Edison in order that he might study the effect

of the Röntgen rays without

the use of photographic plates. It consists of a screen covered with crystals of calcium tungstate and fixed to a boxlike support with opaque sides which, opposite the screen, fit so closely around the eyes that they cut off all outside light. By means of the flu

oroscope a person may examine, for instance, the bones of his own hand by putting it before the screen and looking through the screen toward a tube that is giving off X-rays.

574. Radio-activity.

The study of the X-ray and its effects gave rise to the idea that there was a relation between phosphorescence and the X-rays. In 1896 Becquerel found

that all the salts of uranium, both those that are phosphorescent and those that are not, emit radiations which affect the photographic plate like the X-ray and pass through thin sheets of metal. Any substance that spontaneously emits radiations like uranium is called radioactive, and is said to possess radio-activity. Besides uranium, thorium and its compounds are remarkably radio-active. Figure 555 shows the effect produced upon a photographic

FIG. 555

plate by the radiation from a piece of gas mantle which was placed upon the plate and left in the dark for five days. The gas mantle contains thorium.

Madame Curie examined many substances to determine their radio-activity, and she found that pitchblende, the mineral from which uranium is obtained, is much more radioactive than uranium. She concluded that pitchblende must contain some element more radio-active than uranium, and finally succeeded in separating from it a minute quantity, a few milligrams per ton, of salts of a new element which she called radium. Radium chloride has been obtained with a radio-activity one million times as great as that of the mineral from which it came.

Rutherford has found in the Becquerel rays three classes of rays: The a-rays (alpha-rays) consist of positively charged particles, having about twice the mass of the hydrogen atom, and a velocity about one tenth that of light. The B-rays (beta-rays) are negatively charged particles, having about

the mass of the hydrogen atom, and a velocity of from 0.6 to 0.96 that of light. They apparently differ from cathode rays in velocity only. The y-rays (gamma-rays) are believed to be pulses in the ether similar to the X-rays produced in a tube having a high vacuum. Of the three classes of

a-rays

Y-rays

B-rays

FIG. 556

rays the y-rays have the greatest penetrating power and the a-rays the least. Figure 556 represents rays sent out by a radio-active material, showing the change in direction caused by subjecting them to a strong magnetic field.

The a-rays are deflected little, the B-rays much, the y-rays

not at all.

575. Electrons; Ionization. When a charged gold-leaf electroscope is surrounded with air in normal condition, it will retain its charge. As soon, however, as X-rays or the radiations from radium fall upon it, it loses its charge and the leaves fall together. To explain this change in the conductivity of the air it is necessary to consider the modern theory of the atom. This is that the atom is a complex structure consisting of minute negatively charged particles in rapid motion, connected with positively charged particles which are themselves in motion. The negatively charged particles are called electrons. The velocity with which the a-rays and B-rays leave a radio-active material is probably due to the velocity which the charged particles have within the atom, and which they retain as they leave it. It is supposed that the X-ray pulses separate electrons from the atoms in air. This leaves the air a mixture of negatively charged electrons and positively charged particles, thus making it a conductor, or ionizing it.

576. Conclusion. Since electrons separate from the atoms of radio-active substances it is evident that, however slow the process, there is going on a disintegration or breaking down of the atom. This expulsion from the atom is accompanied by the production of such an amount of heat that the temperature of radium bromide is sometimes as much as five degrees Centigrade above that of the surrounding air. In order to understand the significance of this atomic disintegration it must be followed a step farther. Certain radioactive substances, as radium, actinium, and thorium, emit a substance like a heavy gas that is endowed with temporary radio-active properties. This is called the emanation. If kept for a number of days, it loses a large portion of its radio

activity, and on being examined with the spectroscope, shows the well-defined lines of the element helium, which were not present when the emanation was first formed. This seems to indicate a change from one element into another. Perhaps it is rather an indication that both these elements are but forms of a third and possibly unknown element.

To Dalton belongs the credit of giving the atom its place of honor as the fundamental unit of the chemist. It has taken the combined research of many physicists since Dalton's time to establish the complicated structure of the atom and to show us at least a part of its function in the formation of matter.