rate, the problem is soluble. Undoubtedly the near future will show marked advances in this field of enquiry.

The problem of the nature of light emission is being attacked also by experimental physicists, the most prominent of whom is Prof. Wood of Johns Hopkins University. For some years he has been investigating the fluorescent properties of vapours. The general reader will recall that certain substances emit light when stimulated by light from some external source. They are said to fluoresce, or, if the light continues to be emitted on removal of the outside source-to phosphoresce. It is possible, therefore, to cause bodies to emit light under the stimulus of an external "exciting" source. The general reason for this is evident. The exciting light waves, being electrical in nature, set into vibration the electrons within the atoms of the substance on which they fall, thus causing a light emission from the latter. Now it will at once be clear that for fluorescence to take place, there must be some sort of "tuning" between the frequency of the vibrations falling on an atom and the natural frequency of the vibrations which itself would emit. In other words we cannot expect to obtain fluorescence for all vibrations which may fall on a body, and experiment shows that this is the case. As both the exciting light and the fluorescent light may be analyzed with a spectroscope, we have a means of examining any relation which may exist between them. We are thus supplied with another method of attacking the problem of the nature of light emission.

In this field Prof. Wood has carried out already several brilliant researches. His work has been confined to a study of the fluorescent properties of vapours, substances which, on account of their comparative simplicity of structure, render the problem much less complex. He has found that even the slightest variation in the nature of the exciting vibrations, produces marked differences in the fluorescent light. By controlling the vibrations of the source within very narrow limits, he hopes to throw considerable light on our knowledge of the internal vibrations of the atom. The brilliant work he has accomplished already is a voucher that much may be expected from him in the future.

J. K. R.

THE QUANTUM THEORY.

One of the greatest triumphs of modern Physics is the firm establishment of the atomicity of matter; that matter is built up of definite sized units. Ostwald, the great German chemist, and his band of followers were for many years the chief opponents of this theory, but in face of the accumulated evidence collected by the physical chemists, Ostwald was led to accept the atomic theory. Now, we are able to count the molecular units in a given mass of matter, measure their size and follow their movements. We have learned also, that instead of being a simple thing, as originally supposed, the atom has electrical constituents. Then again, it has been discovered that the electrical part of the atom of matter has itself an atomic structure. The value of the charge of an elementary electrical unit has been definitely measured in many ways, and has been found invariable.

The latest atomic theory deals with energy and is called the quantum theory. Of course, we all recognize electricity as a manifestation of energy and now scientists place matter in the same category; so that the quantum theory is really the old atomic theory generalized to apply to all kinds of energy. It was first proposed by Planck about ten years ago in order to account for the way in which heat energy leaves a heated body. Many attempts had previously been made to develop an equation expressing the relation between the temperature of a body, and the rate at which heat left it, or in other words its rate of cooling. Newton, Du Long and Petit, and various others had worked out empirical formulae, which. however, held only for small ranges of temperature. Wien and Rayleigh, mathematical physicists, had each, on theoretical grounds, developed a formula, which was found incorrect when tested by experiment. The only assumption they had made was that heat energy is continuously emitted by a hot body. Planck attacked the same problem and assumed from the beginning, that the emission of energy from the heated body is not continuous, as they supposed, but is sent off at intervals in small units or so called "quanta." From this he deduced a relation which, contrary to all the other ones, agreed well with the experimental facts. Since then Planck's idea has been applied to all kinds of interchanges of energy. Nerst, a chemist, has

shown that the quantum theory will account for the way in which the specific heats of bodies change as they are cooled to very low or heated to very high temperatures, and Einstein and J. J. Thomson have formulated a quantum theory of light which is radically different from the older wave theory. The Einstein-Thomson theory is that light, instead of being given out continuously by an illuminated body and propogated in the form of waves through the ether, is sent off from the body by means of energy spots, each spot containing an integral number of energy units or quanta. Accordingly, a beam of light is not a continuous wave structure but consists of energy spots distributed at random through it, all of which are moving with the same velocity. It is rather remarkable that this theory resembles Newton's corpuscular theory which was abandoned in favour of the wave theory.

In view of recent work, it is necessary to adopt an atomic or quantum theory with regard to X-rays as opposed to a wave theory. An X-ray pulse produced by the stopping of a cathode particle hurls out an electron from a molecule. It is found that the amount of energy required to throw an electron from a molecule is the same as that required to stop the electron which produced the X-ray. The X-ray, then, must simply transfer energy from one electron to another. To do this, it is apparent that the X-ray cannot be a spherical wave pulse which spreads from the point where it is made. Rather, it must be a form of localised energy which travels directly to the place where it ejects an electron. Only in this way could the same amount of energy be transferred from the place where the X-ray is made to the place where it is used up in driving an electron from a molecule of matter. The X-rays then must have an atomic structure.

When we come to consider the late experiments of Laue, Friedrich, and Knipping, and those of Bragg and others, we find that X-rays are very similar to ordinary light rays in their properties. It is most probable then, that ordinary light is built up in the same way as X-rays, and that Einstein and Thomson's quantum theory of light is correct.

Whatever the ultimate fate of the quantum theory, it seems probable that it will do its part in pointing the way to

new investigations. We have tried to show how it has corelated experimental data, which no other theory could do. Any theory is strong in proportion to its power to do these things and the quantum theory seems to have won its place among the other theories of natural phenomena, when judged by such a standard.

V. E. P.

GREAT BRITAIN.

The Constitutional Issue.

These lines are written while the fate of the Government proposals for the temporary exclusion of parts of Ulster is still in the balance. As the accounts cabled to this side of the Atlantic are not remarkable for their restraint, the situation is probably a good deal less desperate than it is made out to be. When the insurance companies begin to refuse or to increase considerably rates for war risks in Ulster, we may fear a serious struggle.

The terms proposed by Mr. Asquith seem to remove some, at least, of the Ulster grievances. There is not and could not be any further concessions to Protestants in the South. The Liberal contention has been that their religious interests are adequately protected already. Nor could all Ulster be left out of the Bill, for that would contract out of its scope the Nationalist half of the province. Equally any provision which kept any part of Ireland permanently apart from the national life could not be accepted. (It may be remembered that even Trinity College in Dublin, a centre of Protestantism, refused to acquiesce in a Unionist amendment which left it subject to Imperial Parliament.) Separate counties are therefore to contract themselves out of the operation of the Bill; and the exclusion is to last for six years. Within six years two general elections will be held, and if a Unionist Government is returned at either, the time limit may be indefinitely extended. If the Unionists still remain out of office, can Ulster expect better terms? The Unionists have concentrated their criticism upon the time limit, which they wish to extend indefinitely. If that were the true point in dispute, a compromise should not be beyond the reach of reasonable men. But, unfortunately, matters are past the stage of reasoning, for two powerful influences within the Unionist party may well force a refusal of any compromise. There is the resistance of Ulster itself, and there is that section of the party which will use any weapon to defeat the Government. The present struggle is intended to defeat more than Home Rule; it is to