certain substances, notably in a certain class of compounds known as platinocyanides. If, for instance, a screen is coated with a layer of barium platinocyanide and interposed in the path of the rays, the screen becomes brilliantly fluorescent and may be used not only for detecting the rays but also for studying their relative transparency to different substances. For if different obstacles are interposed between the source of the radiation and the screen, those portions of the latter which are protected from the direct action of the rays will appear dark, whereas the rest of the screen will be fluorescent, and according as the obstacle interposed is more or less opaque to this particular kind of radiation the shadow cast on the screen will be darker or lighter. In this way, for example, the shadow of the bones of the hand may be projected upon such a screen. For the rays can pass tolerably freely through the tissues of the hand but cannot penetrate the bones, which therefore cast a shadow of themselves upon the fluorescent screen behind them. If the fluorescent screen is replaced by a photo

graphic plate, then a shadow photograph or radiograph of the bones will be obtained on developing the plate. If the degree of transparency of the rays to different materials is studied, it is found that, as a general rule, substances of high density are for the same rays more opaque than those whose density is less; but the transparency also depends upon the particular rays used. If the vacuum. in the tube from which the rays originate is very high it is found that the Röntgen rays emitted are far more penetrating than when the vacuum is less perfect, so that the results obtained with different tubes cannot be compared.

It appears, therefore, that the Röntgen rays possess many of the properties of light of so short a wave-length as not to be visible to the eye, or as it is usually called ultra-violet light; but there is one remarkable difference. It is well known that when ordinary light passes from one medium to another the direction of propagation of the waves of light is changed and the light is said to be refracted. Thus if light passes through a prism of glass

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the light which emerges is found no longer to be travelling in the same direction as before it entered the prism. If a similar experiment is made with the Röntgen rays using an aluminium prism which is fairly transparent to the rays, the radiation passes through without being deviated from its original path, so that the rays have not suffered refraction in passing through the prism. This fact and the other properties of the rays have been explained by a theory due to Stokes, according to which the rays consist not of a regular undulatory disturbance propagated through the ether, as is the case with light, but of an irregular series of pulses set up whenever one of the cathode ray corpuscles is stopped. This view is consistent with all the known phenomena connected with the rays, but the equally probable view is held by some that the Röntgen rays consist merely of light waves of exceedingly short wave-length, so that the precise nature of the rays is still uncertain. We shall see later that rays analogous to the Röntgen rays are emitted by many radioactive bodies.

CANAL RAYS

There is yet another type of rays which can be observed during the discharge of electricity through a rarefied gas and which have their analogue in the rays given out by radioactive substances.

It was shown by Goldstein that if the cathode of a discharge tube in which cathode rays are being produced is perforated by a number of holes, streams of light appear to pass out in straight lines behind the cathode through the holes. These streams were called by Goldstein "Kanalstrahlen," or canal rays. The properties of these rays have been carefully studied, and it was shown by Wien that they consist of positively charged particles moving with high velocities for which the

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value of was of the same order as for the

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hydrogen ion in electrolysis. The rays therefore consist of projected particles charged with electricity of an opposite sign to that carried by the cathode rays, whose mass is of the same order as ordinary atoms of matter, assuming that each particle carries a charge equal to that carried by the hydrogen ion in electrolysis.

CHAPTER II

ON THE METHODS OF MEASURING CURRENTS

THROUGH GASES

THE QUADRANT ELECTROMETER

ELECTRIC currents through gases are usually exceedingly small, and except in cases of very intense ionization it is inconvenient, if not impossible, to measure them by means of a galvanometer by any of the ordinary methods adopted for the measurement of currents flowing in metallic conductors. For this reason special methods have to be employed, and one of the most convenient instruments used for the purpose is the quadrant electrometer. The instrument, a simple form of which is shown in Fig. 3, was invented by Lord Kelvin, and consists essentially of a light piece of metal cut into the shape shown in Fig. 4 and called the needle, suspended in four hollow quadrant