The fact that the rays when they pass through a gas ionize it
and make it a conductor of electricity furnishes the best means of
measuring their intensity, as the measurement of the amount of
conductivity they produce in a gas is both more accurate and more
convenient than measurements of photographic or phosphorescent
effects. Rontgen rays when they pass through matter produce-as
Perrin (Complex rendus, 124, p. 455), Sagnac (Jour. de Phys., 1899,
(3), 8, and J. Townsend (Proc. Camb. Phil. Soc., 1899, 10, p. 217, have
shown-secondary Rontgen rays as well as cathodic rays. A very
complete investigation of this subject has been made by Barkla
and Sadler (Barkla, Phil. Mag., June 1906, pp. 8I2~828; Barkla
and Sadler, Phil. Mag., October 1908, pp. 550-584; Sadler, Phil.
Mag., July 1909, p. 107; Sadler, Phil. Mag., March 1910, p. 337).
They have shown that the secondary Rontgen rays are of two kinds:
one kind is of the same type as the primary incident ray and may be
regarded as scattered primary rays, the other kind depends only
on the matter struck by the rays-their quality is independent of
that of the incident ray. When the atomic weight of the element
exposed to the primary rays was less than that of calcium, Barkla
and Sadler could only detect the first type of ray, i e. the secondary
radiation consisted entirely of scattered primary radiation;
elements with atomic weights greater than that of calcium gave
out, in addition to the scattered primary radiation, Rontgen rays
characteristic of the element and independent of the quality of the
primary rays. The higher the atomic weight of the metal the more
penetrating are the characteristic rays it gives out. This is shown
in the table, which gives for the different elements the reciprocal
of the distance, measured in centimetres, through which the rays
from the element can pass through aluminium before their energy
sinks to I/2-7 of the value it had when entering the aluminium; this
quantity is denoted in the table by A.
Element. Atomic weight. X.
Chromium 52 367
Iron . 55-9 239
Cobalt . 59-o 193-2
Nickel 58-7? (61-3) 159-5
Copper 63'6 128-9
Zinc 65-4 106-3
Arsenic . 75-0 60-7
Selenium 79-2 51-0
Strontium . 87-6 35-2
Molybdenum 96-o 12-7
Rhodium 103-o 8-44
Silver .. . . 107-9 6-75
Tin ... . II9°O 4-33
The radiation from chromium cannot pass through more than a lew centimetres of air without being absorbed, while that from tin is as penetrating as that given out by a fairly efficient Rontgen tube. Barkla and Sadler found that the radiation characteristic of the metal is not excited unless the primary radiation is more penetrating than the characteristic radiation. Thus the characteristic radiation from silver can excite the characteristic radiation from iron, but the characteristic radiation from iron cannot excite that from silver. We may compare this result with Stokes's rule for phosphorescence, that the phosphorescent light is of longer wave-length than the light which excites it.
The discovery that each element gives out a characteristic radiation (or, as still more recent work indicates, a line spectrum of characteristic radiation) is one of the utmost importance. It gives us, for example, the means of getting homogeneous Rbntgen radiation of a perfectly definite type: it is also of fundamental importance in connexion with any theory of the Rontgen rays. We have seen that there is no evidence of refraction of the Rontgen rays; it would be interesting to try if this were the case when the rays passing through the refracting substance are those characteristic of the substance.
Secondary Calhodic Rays.-The incidence of Rontgen rays on matter causes the matter to emit cathodic rays. The velocity of these rays is independent of the intensity of the primary Rontgen rays, but depends upon the “hardness” of the rays; it seems also to be independent of the nature of the matter exposed to the primary rays. The velocity of the cathodic rays increases as the hardness of the primary Rontgen rays increases. Innes (Proc. Roy. Soc. 79, p. 442) measured the velocity of the cathodic radiation excited by the rays from Rontgen tubes, and found velocities varying from 6-2 X 1o9cm./sec. to 8- 3X 109 cm./sec. according to the hardness of the rays given out by the tube. The cathodic rays given out under the action of the homogeneous secondary Rontgen radiation characteristic of the different elements have been studied by Sadler (Phil. Mag., March IQIO) and Beatty (Phil. Mag., August 1910). The following table giving the properties of the cathode rays excited by the radiation from various elements is taken from Beatty's paper; ti is the thickness of air at atmospheric pressure and temperature required to absorb one-half of the energy of the cathode particles, t2 is the corresponding quantity for hydrogen. Radiator. li iz
Iron . -00804 -0410
Copper. -0135 -0733
Zinc -01 64 -0909
Arsenic .... .. -0255
Tin ....... '1672 1 -37
The properties of the cathode rays excited by the radiation from tin correspond very closely with those produced in a discharge tube when the potential difference between the anode and cathode is about 30,000 volts. When Rontgen rays pass through a thin plate the cathodic radiation on the side the rays emerge is more intense than on the side they enter. Kaye (Phil. Trans. 209, p. 123) has shown that when cathode rays fall upon a metal two kinds of Rontgen rays are excited, one being the characteristic radiation of the metal and the other a kind independent of the nature of the metal and dependent only u on the velocity of the cathode rays. The faster the cathode rays the harder the Rontgen rays they produce. It would be interesting to see if there is any connexion between the velocity of the cathode rays required to excite Rontgen rays as hard as those given out say by tin and the velocity of the cathode rays which the radiation from tin produces when it falls upon any metal. Sadler has shown that metals can give off cathodic radiation even when the incident Rontgen rays are too soft to excite the characteristic Rontgen radiation of the metal, but that there is a large increase in the cathodic radiation as soon as the characteristic Rontgen radiation is excited. It is possible that the shock produced by the emission of these cathode particles starts the vibrations which give rise to the characteristic rays; the cathode particles emitted when the incident rays are too soft to excite the characteristic radiation coming from a different source from those tapped by the hard rays.
Absorption of Rontgen Rays.-The wide variations in the penetrating power of Rontgen rays from different sources is shown by the above table of the penetrating power of the characteristic rays of the different elements. Many experiments have been made on the penetration of the same rays for different substances. It is a rule to which there is no well established exception that the greater the density of the substance the greater is its power of absorbing the rays. The connexion, however, between the absorption and the density of the substance is not in general a. simple one, though there is evidence that for exceedingly hard rays the absorption is proportional to the density.
The power of any material to absorb rays is usually measured by a coefficient X, the definition of which is that a plate 1/A centimetres thick reduces the energy of the rays when they pass through it normally to I/e of their original value, where e is the base of the Napierian logarithms and equal to 2~7128 It has been shown that however the physical state of a substance may alter, -if, for example, it changes from the liquid to the gaseous, — A/D, where D is the density of the substance, remains constant. It has also been shown that if we have a mass M made up of masses Mi, M2, Mg, of substances having coefficients of absorption), lo, A3, and densities D1, D2, Da, . then if A/D for the mixture is given by the equation
MX/D=M1}, /D1-l-Mgliz/D2-l-Mah, /Di-ls
this equation is true whether the substances are chemically combined or chemically mixed. From this equation, when we know 7/D for a binary compound and for one of its constituents, we can find the value of A/D for the other constituent. By the use of this principle we can find the value of A/D for the elements which cannot be obtained in a free state. Benoist (Jour. de Phys. (7), 28, p. 289) has shown that if the values of A/D are plotted against the atomic weight we get a smooth curve; if we draw this curve it is evident that we have the means of determining the atomic weight of an element by measuring its transparency to Rontgen rays when in combinatiortwith elements whose transparency is known. Benoist has applied this method to determine the atomic weight of indium. The value of A/D for any one substance depends upon the type of ray used, and the ratio of the values of A/D for two substances may vary very greatly with the type of ray; this is especially the case when one of the substances is hydrogen. Thus Crowther (Proc. Roy. Soc., March 1909) has shown that the ratio of A for air to A for hydrogen varied from 100 for rays given out by a Rontgen tube at a comparatively high pressure when the rays were very soft to 5-56 when the pressure in the bulb was very low and the rays very hard. Beatty (Phil. Mag., August 1910) found that this ratio was as large as 175 for the characteristic rays given out by iron, copper, zinc and arsenic, but fell to 25'O for the rays from tin. Polarization of Rzinlgen Rays.-A great deal of attention has been paid to a phenomenon called the polarization of the
