Nature of

The mechanism of the production of this continuous back ground still remains obscure. Attempts were made in the first place to find in it the radiation which the electromagnetic theory predicts in the case when an electron undergoes a change in its velocity but we have a very imperfect acquaintance with the laws of decrease of velocity of the fast electrons when they penetrate into matter. It is certain that the old classical theories are not sufficient and that it is necessary to introduce the idea of quanta. The recent experiments of Davisson and Germer, G. P. Thomson, etc., confirming the conceptions of the wave mechanics, have thrown new light on the behaviour of the electrons when they travel through an obstacle. In this way will be found perhaps a satisfactory explanation of the experimental data concerning the continuous spectral background of the Röntgen rays.

Absorption.

The best known property of X-rays, which has attracted wide attention to this type of radiation, is their ability to penetrate substances quite opaque to ordinary light, but it must not be thought that all X-rays are equally penetrating. Their absorption depends in a simple and general way on the wave length of the rays and on the atomic number of the absorbing body; the higher the frequency of the rays and the lower the atomic number of the absorbing body, the greater will be the transparency. Certain rays are able to penetrate several centi metres of steel while others are stopped by a few millimetres of air.

The study of the gradual diminution of intensity undergone by a beam of X-rays in passing through matter has shown that it depends on two different causes. On one hand the presence of atoms of matter in the path of the beam causes a scattering of a fraction of the latter in all directions in space; on the other hand, the same atoms are capable of absorbing quanta of the incident radiation while undergoing a more or less intense ionisation.

The first phenomenon has been called "scattering" and may be effected moreover in two very different ways, with which we shall have to deal later. The second phenomenon is that of absorption properly so called; let us recall first how we may represent it on the Rutherford-Bohr model. Several groups of electrons exist in the atom and to remove an electron of any group from the atom, a definite amount of energy must be supplied. When the atom is placed in a radiation of frequency v, or when it is bombarded by quanta hv, the electrons whose energy of ejection is less than by may in certain favourable cases be ejected from the atom by the absorption of a quantum. Experiments have up to now always

indicated that quanta of radiant energy are absorbed as a whole; if then a quantity of energy W,, is necessary to remove an electron of the nth class, the latter will leave the atom after an absorption of a quantum and with kinetic energy hv This is Einstein's law of photoelectricity. After the departure of the electron, the atom remains in an abnormal ionised state; we know that it is then in a condition to emit one of the lines of the series characterised by the spectral term The phenomenon of absorption can thus be viewed in two dif ferent aspects according as attention is directed to the diminished intensity of the resulting X-ray beam or to the modifications pro duced by it in the state of the matter irradiated. The first point of view, to which we will keep in the following paragraphs, corresponds to the study of the law of absorption; the second leads to an examination of the photoelectric effect itself, that is to say the expulsion of electrons by the absorption of quanta of radiation, and is the subject of a later paragraph.

Let us consider solely the diminution in intensity of a beam of X-rays of wave length X passing through an element of atomic number N; the intensity of the beam after having passed through a thickness x of the absorbing material, is related to the initial intensity by the relation The absorption is said to follow the exponential law. The quan tity T, i.e., the coefficient of total absorption, is the sum of two other quantities a and ,u, the first of which expresses the effect of scattering and the second that of the true absorption. The scat tering, and therefore the coefficient a, varies but little with the atomic number, while on the other hand the absorption, and the coefficientµ increase rapidly with N. This difference in the vari ations of the two phenomena permits their contributions to the total diminution in intensity to be separated; in particular, for dense materials and X-rays of the more normal wave-lengths, scattering is practically negligible in comparison with absorption, thereby simplifying the study of the latter. It seems natural to introduce besides the coefficient .t of the exponential law, the coefficient kt/p of absorption per unit mass, where p denotes the density of the absorbing material, and the coefficient of atomic absorption related to the preceding by the atomic weight of the absorbing body and Avogadro's number.