These are the famous “cathode rays” of which we have recently heard so much.  Apparently they cannot be produced except in a very high vacuum, where the pressure of the air is about 1-100th millionth of an atmosphere, or that which it is some 90 or 100 miles above the earth.  Mr Crookes regards them as a stream of airy particles electrified by contact with the cathode or negative discharging point, and repelled from it in straight lines.  The rarity of the air in the tube enables these particles to keep their line without being jostled by the other particles of air in the tube.  A molecular bombardment from the cathode is, in his opinion, going on, and when the shots, that is to say, the molecules of air, strike the wall of the tube, or any other body within the tube, the shock gives rise to phosphorescence or fluorescence and to heat.  This, in brief, is the celebrated hypothesis of “radiant matter,” which has been supported in the United Kingdom by champions such as Lord Kelvin, Sir Gabriel Stokes, and Professor Fitzgerald, but questioned abroad by Goldstem, Jaumann, Wiedemann, Ebert, and others.

Lenard, a young Hungarian, pupil of the illustrious Heinrich Hertz, was the first to inflict a serious blow on the hypothesis, by showing that the cathode rays could exist outside the tube in air at ordinary pressure.  Hertz had found that a thin foil of aluminium was penetrated by the rays, and Lenard made a tube having a “window” of aluminium, through which the rays darted into the open air.  Their path could be traced by the bluish phosphorescence which they excited in the air, and he succeeded in getting them to penetrate a thin metal box and take a photograph inside it.  But if the rays are a stream of radiant matter which can only exist in a high vacuum, how can they survive in air at ordinary pressure?  Lenard’s experiments certainly favour the hypothesis of their being waves in the luminiferous ether.

Professor Rontgen, of Wirzburg, profiting by Lenard’s results, accidentally discovered that the rays coming from a Crookes tube, through the glass itself, could photograph the bones in the living hand, coins inside a purse, and other objects covered up or hid in the dark.  Some bodies, such as flesh, paper, wood, ebonite, or vulcanised fibre, thin sheets of metal, and so on, are more or less transparent, and others, such as bones, carbon, quartz, thick plates of metal, are more or less opaque to the rays.  The human hand, for example, consisting of flesh and bones, allows the rays to pass easily through the flesh, but not through the bones.  Consequently, when it is interposed between the rays and a photographic plate, the skeleton inside is photographed on the plate.  A lead pencil photographed in this way shows only the black lead, and a razor with a horn handle only the blade.