may be secondary radiations reflected by a kind of resonance phenomenon.

Certain experiments by Professors Elster and Geitel, however, are not favourable to this point of view. If an active body be surrounded by a radioactive envelope, a screen should prevent this body from receiving any impression from outside, and yet there is no diminution apparent in the activity presented by a certain quantity of radium when it is lowered to a depth of 800 metres under ground, in a region containing a notable quantity of pitchblende. These negative results are, on the other hand, so many successes for the partisans of the explanation of radioactivity by atomic energy.

CHAPTER X

THE ETHER AND MATTER

§ 1. THE RELATIONS BETWEEN THE ETHER
AND MATTER

FOR Some time past it has been the more or less avowed ambition of physicists to construct with the particles of ether all possible forms of corporeal existence; but our knowledge of the inmost nature of things has hitherto seemed too limited for us to attempt such an enterprise with any chance of success. The electronic hypothesis, however, which has furnished a satisfactory image of the most curious phenomena produced in the bosom of matter, has also led to a more complete electromagnetic theory of the ether than that of Maxwell, and this twofold result has given birth to the hope of arriving by means of this hypothesis at a complete co-ordination of the physical world.

The phenomena whose study may bring us to the very threshold of the problem, are those in which the connections between matter and the ether

appear clearly and in a relatively simple manner. Thus in the phenomena of emission, ponderable matter is seen to give birth to waves which are transmitted by the ether, and by the phenomena of absorption it is proved that these waves disappear and excite modifications in the interior of the material bodies which receive them. We here catch in operation actual reciprocal actions and reactions between the ether and matter. If we could thoroughly comprehend these actions, we should no doubt be in a position to fill up the gap which separates the two regions separately conquered by physical science.

In recent years numerous researches have supplied valuable materials which ought to be utilized by those endeavouring to construct a theory of radiation. We are, perhaps, still ill informed as to the phenomena of luminescence in which undulations are produced in a complex manner, as in the case of a stick of moist phosphorus which is luminescent in the dark, or in that of a fluorescent screen. But we are very well acquainted with emission or absorption by incandescence, where the only transformation is that of calorific into radiating energy, or vice versa. It is in this case alone that can be correctly applied the celebrated demonstration by which Kirchhoff established, by considerations borrowed from thermodynamics, the proportional relations between the power of emission and that of absorption.

In treating of the measurement of temperature, I have already pointed out the experiments of Professors Lummer and Pringsheim and the theoretical researches of Stephan and Professor Wien. We may consider that at the present day the laws of the radiation of dark bodies are tolerably well known, and, in particular, the manner in which each elementary radiation increases with the temperature. A few doubts, however, subsist with respect to the law of the distribution of energy in the spectrum. In the case of real and solid bodies the results are naturally less simple than in that of dark bodies. One side of the question has been. specially studied on account of its great practical interest, that is to say, the fact that the relation of the luminous energy to the total amount radiated by a body varies with the nature of this last; and the knowledge of the conditions under which this relation becomes most considerable led to the discovery of incandescent lighting by gas in the Auer-Welsbach mantle, and to the substitution for the carbon thread in the electric light bulb of a filament of osmium or a small rod of magnesium, as in the Nernst lamp. Careful measurements effected by M. Fery have furnished, in particular, important information on the radiation of the white oxides; but the phenomena noticed have not yet found a satisfactory interpretation. Moreover, the radiation of calorific origin is here accompanied by a more or less

important luminescence, and the problem becomes very complex.

In the same way that, for the purpose of knowing the constitution of matter, it first occurred to us to investigate gases, which appear to be molecular edifices built on a more simple and uniform plan than solids, we ought naturally to think that an examination of the conditions in which emission and absorption are produced by gaseous bodies might be eminently profitable, and might perhaps reveal the mechanism by which the relations between the molecule of the ether and the molecule of matter might be established.

Unfortunately, if a gas is not absolutely incapable of emitting some sort of rays by simple heat, the radiation thus produced, no doubt by reason of the slightness of the mass in play, always remains of moderate intensity. In nearly all the experiments, new energies of chemical or electrical origin come into force. On incandescence, luminescence is superposed; and the advantage which might have been expected from the simplicity of the medium vanishes through the complication of the circumstances in which the phenomenon is produced.

Professor Pringsheim has succeeded, in certain cases, in finding the dividing line between the phenomena of luminescence and that of incandescence. Thus the former takes a predominating importance when the gas is rendered luminous by