The phenomena of electrical discharges in partial and high vacuums have been investigated by many distinguished physicists; but the name of Sir William Crookes is associated with some of the most remarkable of the discoveries that have been made, and many of the tubes that have been prepared for illustrating the properties of high vacuums were designed by him and are, therefore, known by his name. Crookes discovered, for example, that when the exhaustion of a tube is pushed considerably be yond the point at which the appearances de scribed above are observed, the character of the electrical discharge is again changed, and mechanical, electrical and luminous effects are to be observed, which are not manifested in the lower vacuums. Exceedingly interesting me chanical effects were discovered in tubes which had been exhausted until the pressure of the residual gas was reduced to about the 250,000th of an atmosphere, and Crookes invented the radiometer for the purpose of demonstrating some of these. The radiometer consists of a glass sphere, two inches or so in diameter, and containing a light flier or wheel provided with four horizontal arms, each arm carrying at its extremity a light, vertical vane made of mica and blackened on one side. The wheel is poised delicately upon the point of a fine needle, so that the smallest force will cause it to rotate. When the glass bulb has been exhausted so that the residual pressure is only about the 250,000th of an atmosphere, and the instrument is placed in the sunlight or exposed to any source of radiant heat, the little flier or wheel at once begins to rotate in such a direction that the unblackened sides of the vanes move for ward. Numerous explanations of the operation of this little instrument have been given, but that which is based upon the kinetic theory of appears to be the most satisfactory. (See GASES, KINETIC THEORY or). According to this view a gas consists of an enormous number of little particles, or molecules, flying about in a space that is otherwise devoid of matter. In a gas of ordinary density these particles are so numerous that they do not travel on an average more than a few millionths of an inch without colliding with one another. But when, by re moving nearly all of the gaseous molecules, we make it possible for the molecules that remain to travel on an average something like an inch or two between successive collisions with one another, these residual molecules act like tiny projectiles and are capable of producing me chanical effects that are not observable when the motions of the molecules are interfered with by the incessant collisions among the molecules themselves. The full theory of the radiometer is not yet in an entirely satisfactory condition; but in a general way we may say that when the vanes of the little wheel are exposed to a source of radiant heat, the black sides absorb heat more readily than the light ones, and hence be come warmer. This means that the molecules composing the vanes are vibrating more ener getically on the black sides, so that on this side they strike more energetic blows against such gas molecules as chance to come in contact with them. From the equality of action and reaction it follows that the vanes of the little flier will experience a reactionary force tending to make them revolve with the light side foremost. In order that the radiometer may work satisfac torily it appears to be necessary to have the vacuum sufficiently high to ensure that, on an average, a gas molecule which has collided with one of the vanes will strike the glass wall of the enclosing bulb before encountering another gas molecule.
The behavior of a high vacuum under the influence of electrical discharges is especially interesting. When the perfection of the vacuum is such that the average distance that a mole cule of the residual gas travels between suc cessive collisions with its fellow molecules is comparable with the dimensions of the tube itself (that is, when the pressure in the tube is only about 1,000,000th of an atmosphere), the position and shape of the positive electrode, or anode, appear to have very little influence upon the character of the discharge, In such a case the nature of the visible discharge appears to be determined almost absolutely by the cathode, or negative electrode, and the discharge makes it self manifest in the form of a shaft of pale bluish or purplish light, extending outward into the tube in a direction approximately perpendic ular to the surface of the negative electrode.
If this electrode is made concave the streamer that proceeds from it (and which is known as the ray°) may be made to converge to a focus, diverging again after passing the focus, so that the complete streamer has the general form of a double cone, one base of which rests upon the cathode, while the other rests upon the glass wall of the tube, opposite to the cathode. Where the cathode ray strikes the glass, the glass is excited to fluorescence, and it also be comes heated at this point. The whole phenom enon, in fact, is of such a nature as to strongly suggest that the cathode ray consists of a tor rent of material particles; and it is natural to assume that the molecules of the residual gas within the vacuum tube receive electrical charges as they come in contact with the cathode, being then violently repelled from it in a direction normal to its surface. Crookes found, in fact, that this °projectile hypothesis" agrees well with practically all of the phenom ena that he observed in these high vacuums. He observed, for example, that the cathode ray is apparently cut off by the interposition of any solid matter, even by a very thin film of mica, which would presumably be transparent to ether-waves; and he constructed many ingeni ous and beautiful forms of tube for showing that the cathode ray is capable of exerting pre cisely such mechanical effects as would be ex pected if the projectile hypothesis were correct. In one of the most interesting of these tubes the cathode ray is caused to strike against one side of a little paddle-wheel, the wheel being thereby caused to revolve just as a water-wheel revolves when its lower half is immersed in a running stream of water. The direction of rotation in this form of tube may be reversed readily by reversing the polarity of the two electrodes. Objections may be urged to this simple projectile explanation, however. For ex ample, there are reasons for doubting if an iso lated molecule of gas can receive an electrical charge in the way that the molecules of the residual gas have been assumed to be charged by the cathode. Despite this objection, it must still be regarded as probable that the projectile hypothesis of Crookes is true in some form or other. According to the electron theory the cathode ray consists, not of whole gas mole cules but of tiny particles that are split off from the molecules. These ultimate particles, or elec trons, certainly do carry electrical charges, and it is quite likely that the mechanical and other effects that are observed in vacuums where the pressure is not more than the millionth part of an atmosphere are due to the projectile-like motions of the free electrons. Crookes' experi ments were carried out before the idea of elec trons was developed; but it should be said, in justice to him, that in describing his work he frequently emphasized his belief that the matter in his tubes is in a state quite different, in some respects, from any state with which we had previously been familiar; and he called this, provisionally, the °fourth state° of matter, since he considered it to be as different from a gas as a gas is from a liquid or a solid. A vast amount of research has been carried out, in recent years, in connection with the phenomena observed in high vacuums, and the subject must now be considered in its relation to the electron theory, radioactivity, and X-rays, if it is to be properly understood. See ELECTRON THEORY; MOLECULAR THEORY; RADIOACTIVITY; X-RAY; and consult bibliographies given.