Man's physical universe

xanabras

ELECTRICAL CONDUCTION IN GASES 551

these X-ray photographs. X-ray machines have also been used to

solve paint problems, to determine the cause of differences in physical

properties of different samples of graphite, and in the examination of

porcelains, papers, electrolytic deposits, patent leather, rayon, asbestos,

grease, rubbers, soaps, resins, ice

cream, and a host of other materials.

Instead of using photographs,

the fluoroscope may be used with

X rays in observing the body.

The fluoroscope depends on the

fact that X rays cause a fluorescence

in certain substances. An

excellent fluorescent screen is produced

by coating it with barium

platinocyanide.

A recent development is the use

of 35-mm. miniature films for X-ray

photographs that greatly reduces

the cost of X-ray examinations.

X rays cannot be bent as light

rays can by lenses, and for that

reason small pictures cannot be

Fig. 274. X-ray photographs locate

weak spots in automobile parts.

taken by the direct use of X rays.

In this new development the subject

to be examined is placed be-

Company.)

(Courtesy of the General Motors

tween the X-ray machine and a

fluorescent screen, and a photograph is made of the resulting image

produced on the screen. The camera and film-holder are made of

lead to keep out X rays.

The nature of X rays remained a mystery for several years after

their discovery. They are not deflected by electric or magnetic fields

and are therefore different from cathode rays in that they have no

charge. Since they cannot be reflected or diffracted by ordinary means,

it appeared at first that they were not electromagnetic radiations similar

to light waves. However, it was later concluded that X rays must

be electromagnetic radiations of very small wave length — so small,

in fact, that no diffraction gratings or prisms would diffract them.

In 1912 Max von Laue conceived the idea that the distances between

atoms in a crystal were just about of the right order to enable crystals

to act as diffraction gratings for electromagnetic waves of such a small

wave length. The experiment was tried by many workers, and it was

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