One of the first and most important results to which the study of X-rays led was the discovery that their passage through gases rendered the latter conductors of electricity. It was found later that this effect was in reality due to the ionisation which the fast moving electrons ejected from the atoms produce along their paths. The new radiations showed themselves capable of produc ing a considerable effect on photographic plates, a phenomenon also very probably due to a secondary action.
These two effects allow us to estimate the intensity of X-rays by measuring the ionisation current produced or the degree of blackening of photographic emulsions.
In addition, X-rays produce a certain number of chemical actions and the well-known phenomenon of luminescence in many substances. To-day barium platinocyanide and calcium tungstate are most commonly employed, either as fluorescent screens for visual observation, or as intensifiers to shorten photographic exposures.
All these remarkable properties have attracted keen attention; but what actually gives X-rays a place apart in the realm of Physics, is that by revealing simple general laws of very wide application in natural philosophy they yield us information con cerning the interior parts of atoms, much better than do the other radiations.
It is the laws of X-rays which have most effectively helped to clarify the ideas of modern physicists on the structure of atoms. The study of Röntgen rays is so closely bound up with these conceptions that it is necessary to outline the latter at this point.
As a result of the theories of Sir Ernest Rutherford and Niels Bohr the atom of an element of atomic number N (that is to say in the Nth place in the natural order of the elements) is regarded as composed of a nucleus possessing a total positive charge equal to N times the elementary charge surrounded by a swarm of N electrons distributed in a certain manner around it. (See also ATOMS.) We understand, especially from the ideas of Bohr, that these electrons may be divided into several classes, characterised by the work which must be expended to extract from the atom an electron of a certain class. In conformity with the quantum theory we may define for each of the,se electronic levels a fre quency vp related to the work Wp necessary to extract the electron from the atom by the relation— IV = • We denote by the letters K, L, M, etc., successive levels proceed ing outwards from the nucleus. This being premised it is con
venient to recall that the fundamental hypotheses of Bohr's theory are as follows : A spectral line is emitted by an atom when an electron passes from one level to another which is nearer the nucleus, e.g., from the M to the K level, and this spectral line (which is emitted with an energy hp) possesses a frequency v defined by— Conversely the transport of an electron from one level to another further from the nucleus, e.g., from K to M, would correspond to the absorption of energy of a definite frequency; in the case of luminous radiation, this is called a resonance absorption; but as we shall see later, this process does not, in general, appear to be realised with X-rays.
When an electron is removed from one level it seems nearly always to be taken right out of the atom, which corresponds, in the case in which the first level is, for example, the L level, to a transport of an electron from this level to infinity with absorption of a wave of frequency which would be It is only by the absorption of energy of a frequency at least equal to v, that an electron of the L ring may be extracted from the atom. In passing through this frequency the incident radiation will suddenly become susceptible to a new mechanism of absorp tion and we ought to find a sudden rise in the curve of its absorp tion coefficient.
Bohr's theory enables us to predict the lines of a spectrum, when the various levels are stated. Among all these lines we may consider separately those which have the same final level, because their manifestation is conditioned by the same initial phenomenon : the existence of a vacant position in this level. Experiment shows that these lines actually have their origin in a uniform mechanism and form a natural group, a "series" of lines which will bear the name of the final level. If an exciting radiation, falling on a small portion of matter which contains a large number of atoms, is capable of ejecting an electron from the K ring of a number of them, it is natural to suppose that in some of the atoms, for example, the space will be filled up by an electron coming from the L ring with emission of the line the vacancy thus created on the L ring will be filled at the expense of the M ring with the emission of the line In other atoms it will be an electron from the M ring that will jump directly to the K ring emitting the line and so on.