will be seen to choose the long path ef in preference to the short path ab, thus showing that an electrical discharge takes place more readily through a partial vacuum than through air at ordinary pressures.

When the spark first begins to pass between e and ƒ it will have the appearance of a long ribbon of crimson light. As the pumping is continued this ribbon will spread out until the crimson glow fills the whole tube. Ordinary so-called Geissler tubes are tubes precisely like the above except that they are usually twisted into fantastic shapes and are sometimes surrounded with jackets containing colored liquids, which produce pretty color effects.

497. Cathode rays. When a tube like the above is exhausted to a very high degree, say, to a pressure of about .001 millimeter of mercury, the character of the discharge changes completely. The glow almost entirely disappears from the residual gas in the tube, and certain invisible radiations called cathode rays are found to be emitted by the cathode (the terminal of the tube which is connected to the negative terminal of the coil or static machine). These rays manifest themselves, first, by the brilliant fluorescent effects which they produce in the glass walls of the tube, or in other substances within the tube upon which they fall; second, by powerful heating effects; and third, by the sharp shadows which they cast.

FIG. 473. Heating effect of cathode

rays

Thus, if the negative electrode is concave, as in Fig. 473, and a piece of platinum foil is placed at the center of the sphere of which the cathode is a portion, the rays will come to a focus upon a small part of the foil and will heat it white-hot, thus showing that the rays, whatever they are, travel out in straight lines at right angles to the surface of the cathode. This may also be shown nicely by an ordinary bulb of the shape shown in Fig. 475. If the electrode A is made the cathode and B the anode, a sharp shadow of the piece of platinum

in the middle of the tube will be cast on the wall opposite to A, thus showing that the cathode rays, unlike the ordinary electric spark, do not pass between the terminals of the tube, but pass out in a straight line from the cathode surface.

498. Nature of the cathode rays. The nature of the cathode rays was a subject of much dispute between the years 1875, when they first began to be carefully studied, and 1898. Some thought them to be streams of negatively charged particles shot off with great speed from the surface of the cathode, while others thought they were waves in the ether,some sort of invisible light. The following experiment furnishes very convincing evidence that the first view is correct.

m

P

N

n

M

NP (Fig. 474) is an exhausted tube within which has been placed a screen sf coated with some substance like zinc sulphide, which fluoresces brilliantly when the cathode rays fall upon it; mn is a mica strip containing a slit s. This mica strip absorbs all the cathode rays which strike it; but those which pass through the slit s travel the full length of the tube, and although they are themselves invisible, their path is completely traced out by the fluorescence which they excite upon sf as they graze along rays by a magnet it. If a magnet M is held in the position shown, the

FIG. 474. Deflection of cathode

cathode rays will be seen to be deflected, and in exactly the direction to be expected if they consisted of negatively charged particles. For we learned in § 298, p. 244, that a moving charge constitutes an electric current, and in § 350, p. 293, that an electric current tends to move in an electric field in the direction given by the motor rule. On the other hand, a magnetic field is not known to exert any influence whatever on the direction of a beam of light or on any other form of ether waves.

When, in 1895, J. J. Thomson (see opposite p. 440), of Cambridge, England, proved that the cathode rays were also deflected by electric charges, as was to be expected if they consist of negatively charged particles, and when Perrin in

Paris had proved that they actually impart negative charges to bodies on which they fall, all opposition to the projectedparticle theory was abandoned. The mass and speed of these particles are computed from their deflectibility in magnetic and electric fields.

Cathode rays are then to-day universally recognized as streams of electrons shot off from the surface of the cathode with speeds which may reach the stupendous value of 100,000 miles per second.

499. X rays. It was in 1895 that Röntgen (see opposite p. 446) first discovered that wherever the cathode rays impinge upon the walls of a tube, or upon any obstacles placed inside the tube, they give rise to another type of invisible radiation which is now

known under the name of X rays or Röntgen rays. In the ordinary X-ray tube (Fig. 475) a thick piece of platinum P is placed in the

center to serve as a tar

A

FIG. 475. An X-ray bulb

get for the cathode rays, which, being emitted at right angles to the concave surface of the cathode C, come to a focus at a point on the surface of this plate. This is the point at which the X rays are generated and from which they radiate in all directions. The target P is sometimes made of a heavy piece of tungsten.

In order to convince one's self of the truth of this statement it is only necessary to observe an X-ray tube in action. It will be seen to be divided into two hemispheres by the plane which contains the platinum plate (see Fig. 475). The hemisphere which is facing the source of the X rays will be aglow with a greenish fluorescent light, while the other hemisphere, being screened from the rays, is darker. By moving a fluoroscope (a zinc-sulphide screen) about the tube it will be made evident that the rays which render the bones visible come from P.

500. Nature of X rays. While X rays are like cathode rays in producing fluorescence, and also in that neither of them can be refracted or polarized, as light is, they nevertheless differ from cathode rays in several important respects. First, X rays penetrate many substances which are quite impervious to cathode rays; for example, they pass through the walls of the glass tube, while cathode rays ordinarily do not. Again, X rays are not deflected either by a magnet or by an electro2 static charge, nor do they carry electrical charges of any sort. Hence it is certain that they do not consist, like cathode rays, of streams of electrically charged particles.

It has recently been shown that X rays are extremely short waves similar to but very much shorter than light waves, and of a variety of lengths. They are so short that the smoothest mirror we can manufacture is so rough in comparison that it diffuses them. By taking advantage of the regular arrangement of the molecules in the faces of crystals (mica, for example) a kind of reflection known as interference reflection is obtained when the X rays strike at certain favorable angles (see opposite p. 447 for X-ray spectra). Many of the X rays from an ordinary X-ray tube are so short that it would require 250,000,000 of them to make an inch. This represents a rate of vibration of 3,000,000,000,000,000,000 per second.

501. X rays render gases conducting. One of the notable properties which X rays possess in common with cathode rays is the property of causing any electrified body on which they fall to slowly lose its charge.

To demonstrate the existence of this property let any X-ray bulb be set in operation within 5 or 10 feet of a charged gold-leaf electroscope. The leaves at once begin to fall together.

The reason for this is that the X rays shake loose electrons from the atoms of the gas and thus fill it with positively and negatively charged particles, each negative particle being at the instant of separation an electron, and each positive particle an

Most conspicuous figure in the development of the "physics of the electron"; born in Manchester, England; educated at Cambridge University; Cavendish professor of experimental physics in Cambridge since 1884; author of a number of books, the most important of which is the "Conduction of Electricity through Gases," 1903; author or inspirer of much of the recent work, both experimental and theoretical, which has thrown light upon the connection between electricity and matter; worthy representative of twentieth-century physics