stance into light of longer wave-length. How that change is brought about we do not know. Many substances only show phosphorescent effects if they are not quite chemically pure, and this renders it possible that the cause is some kind of chemical action. On the other hand, there are facts, such as the luminous effects produced by cleavage and friction, which seem to suggest a mechanical cause. Moreover, phosphorescence is, to a certain degree, a function of the temperature. Thus various materials-paper, for instance-can be made brilliantly luminous if they are at the temperature of liquid air, while certain crystals and various kinds of glass become phosphorescent without any other agency if they are heated. Again, if a substance which has been rendered phosphorescent by light be heated while it is still luminous, the effect is, first, great increase of brightness, and, next, far more rapid extinction. So sensitive is phosphorescence to the radiation of heat, that even some of the visible rays at the red end of the solar spectrum, and still more the invisible heat rays, suffice in certain cases to extinguish the light, after having first caused a brief increase of activity. These and other curious interactions between heat, light, and phosphorescence show that the phenomena are, in any case, extremely complicated. Possibly there is really a close link between phosphorescence and radio-activity, so that knowledge concerning the one may throw light on the other.

A principle which has produced great results in modern research is that it is worth while to seek elsewhere for what is known to exist anywhere. It was this principle which inspired Becquerel when he made experiments with fluorescent salts, in the hope of finding radiations which should, like the Röntgen rays, act on the photographic plate through substances opaque to light. He found far more than he had sought, but it was some time before the evidently complex nature of the spontaneously emitted uranium radiation he had detected was thoroughly understood; not, indeed, till after the discovery of that superlatively radio-active element so aptly named radium. It was then seen that part of the radiation can be bent out of its course by a strong magnetic field in precisely the same manner as cathode rays can be bent aside. This part forms the B-rays. Later on it was found possible in the case of radium, if the magnetic field was sufficiently intense, to deflect slightly a considerable portion of the remaining radiation in the opposite direction. This portion constitutes the a-rays. The y-rays are, like the Röntgen rays, unaffected by magnetism. Like the Röntgen rays also, they traverse a prism without refraction. Very little is known about them because of their exceeding penetrativeness; on which account it is possible that a great proportion of this radiation escapes detection altogether, for rays which traverse substances without any check can produce no perceptible effects at all.

The photographs obtained by making the radiations permanently record their own path furnish valuable data for the mathematician

and for the experimentalist. Thus it is clearly seen that, under the influence of magnetism, the B-rays describe circles of varying radius; whence it follows that they vary in velocity. It is also clearly seen that the B- and y-rays are perfectly distinct, for there is marked discontinuity between the least deflected B-rays and the totally undeflected y-rays. Furthermore, the photographs show that it is the y-rays and the least deflected B-rays which most easily penetrate obstacles placed in their path; but where B- or y-rays are checked by the substances they traverse, they give rise to secondary rays emanating from those substances-rays not due to reflection or diffusion, but analogous rather to phosphorescence, for they have not precisely the same properties as the rays which call them forth. The a-rays cannot pass through obstacles, and are totally absorbed even by air at a very short distance from their source.

The chief difference between positive and negative radiation, wheresoever found, is this. Negative radiation is formed of those inconceivably minute particles called electrons, which some physicists believe may consist entirely of electricity; while positive radiation is formed of particles which seem to be of the order of atoms, and which, hence, are, when compared with electrons, of enormous size and mass. The velocity of the radiations varies greatly. In the cathode rays it is one-fifth that of light; in the B-rays of radium the highest value is about one-third that of light. 'Slow' negative rays, such as some of those which can be drawn out of metal by the agency of the light of the electric arc, or other source rich in ultra-violet rays, have a velocity which is about a hundredth that of light. It is interesting to note that the feeble magnetism of the earth suffices to curve the slower radiations. The apparent convergence of the rays of an aurora borealis is an optical effect believed to be due to this cause. Positive radiation is more difficult to study, and little is known about it yet. The a-rays of radium have a velocity which is a twentieth that of light. In uranium radiation there seem to be no a-rays; but since wherever electricity of one sign is made manifest an equal quantity of electricity of the opposite sign is liberated somewhere, the probability is that in this and in other cases where we perceive negative radiation alone, the positive charge is left on atoms which remain in the substance itself.

The effect which is by far the most sensitive test of the existence of these invisible radiations, and which is, moreover, the only effect capable of quantitative measurement, is that of rendering air conductive to electricity. In the phraseology of that theory which is at present held to be the best means of co-ordinating the facts, the radiations ionise the air. According to this theory, the impact of the radiations causes a certain atomic dislocation in some of the particles of the air, so that these particles are separated into those positive and negative parts which, in all matter, neutralise one another when united -parts similar to those of which the charged radiations themselves are

composed. It is the movement of these parts under the influence of electric forces which constitutes the current. Independently of any theory, we know as experimentally proved facts that the change in the air which makes it conductive is accompanied by the formation of centres upon which water-vapour can condense, for air which was dust free and perfectly clear may become cloudy after ionisation; that these centres are positively and negatively charged, for they can be drawn away by an electric field; that their velocity is not high, for they can be blown out of their course by even a feeble current of air; and that the removal of these 'ions' destroys the conductibility of the air. Hence it is a legitimate inference, and independent of any hypothesis as to their nature, that the conductibility is due to the ions. It is the more necessary to distinguish between proved facts, which are an abiding possession, and the more or less ephemeral theories based upon those facts, because physicists now look upon theories of any kind as little else but convenient tools. The merit of a theory,' it has been recently said, 'consists not in being true, for no theories are true, but in being fertile '—that is to say, in being not only a satisfactory and self-consistent representation of the totality of the facts, so far as we know them, but also in suggesting by the images used in which direction to seek for further knowledge. When, as is the case with the theory of ions, calculations made on the suppositions involved in the pictorial representation lead to farreaching conclusions, which have been verified when put to the test of experiment and observation, then the theory is certainly fertile; and a theory can only be fertile, one would imagine, in virtue of bearing, in however remote a degree, some resemblance to the truth.

By the test of ionisation it would appear from the researches of several physicists that radio-activity is, in a feeble degree, a property of very many substances, and, indeed, perhaps of all.

An exceedingly interesting series of observations made by the German physicists Elster and Geitel has proved the universality of radio-activity from another point of view. About ten years ago, while studying atmospheric electricity, they found that even in the driest air, and in spite of all precautions, it was not possible to keep an instrument charged for any length of time without some loss. As it was necessary for their observations that they should be able to have entire confidence in their tools, they tested their instruments by leaving them charged for some time in vacuo. There being then no loss of charge, there was evidently no leakage through insufficient insulation of the supports in the instruments themselves, and the loss could only be due to a certain slight conductibility of atmospheric air, for which they could not account. It was known that air can be ionised by ultra-violet light, and they were inclined at first to attribute the conductibility to ionisation of the atmosphere by ultra-violet sunlight. But when, in order to test this supposition, they conducted

experiments in the air of caves and cellars, they found that the conductibility, instead of being less than in air exposed to sunlight, was, on the contrary, very much greater. While they were still searching for the cause of the ionisation, which was evidently not due to sunlight-and, indeed, the rays which cause ionisation are largely absorbed in the upper regions of the atmosphere-progress was being made in the study of radio-activity. Almost simultaneously, in 1899, Rutherford discovered with compounds of thorium, and Curie with compounds of radium, that, in addition to the radiations, these elements emit something else. This something else, to which Rutherford gave the name emanation, cannot be weighed, gives no clearly distinctive lines when examined spectroscopically, has none of the mechanical properties of a gas, does not act chemically in any way we can detect, and, indeed, yields, so to speak, no evidence whatever for its existence, save that where it passes or where it settles, there it gives rise to radio-activity. Any substance whatever which is left for some time in the vicinity of the radio-active salt becomes itself temporarily radio-active. The emanation diffuses throughout an enclosed space as a gas would diffuse, only, apparently, it passes through very narrow openings with more ease; it is checked by everything that checks a gas; it can, like a gas, be pumped or blown out of a vessel; it disappears at the temperature of liquid air, and reappears when the temperature is raised; its absence or presence being in every case manifested by the absence or presence of the induced radio-activity. This induced radio-activity can be measured in the usual waynamely, by the extent to which it renders air conductive; and it has been found that when radium emanation is left in a closed vessel without the radium salt which has given rise to it, this definite amount, whatever it may be, diminishes by half in four days. If, however, the vessel be open to the air, then the emanation diminishes by half in twenty-eight minutes. With actinium, which is very active thorium, the emanation diminishes by half in a closed vessel in three seconds. Constants of time such as these may serve to determine the nature of a radio-active substance, when it is found in quantities too small for any chemical test to be of the slightest avail.

The connection between the emanation and the radiations is as yet a matter for more or less plausible conjecture. The emanation disappears-that is to say, it becomes lost to our means of detectionand in disappearing it gives rise to radiations. Becquerel considers it best to look upon the emanation as the primary phenomenon, and to suppose that the radiations are always due to the break-up of emanation, whether that emanation be entangled, so to speak, in the pores of the substance itself, or whether it has diffused away from the substance and settled elsewhere. There is, however, no evidence for this explanation or for any other.

What we do know for certain is that the emanation is attracted by

negatively charged metal, and that it can thus be collected and concentrated. After this discovery, which was made as soon as the emanation itself was detected, Elster and Geitel conducted experiments to determine whether the ionisation of the atmosphere might be due to radio-activity. They fixed a cylinder, formed of thirty metres of wire, in the open air, and kept it negatively charged to a high potential. They found that if they rubbed the wire every few hours with a tiny bit of leather steeped in ammonia or in hydrochloric acid, the leather became radio-active, and that when they burnt the leather the ash was radio-active. By thus concentrating on a small surface the emanation collected on the whole cylinder during many hours, they were able to obtain, not only the ionisation effect, but also the photographic effect, for which much stronger radio-activity is required. It soon became evident that the atmosphere everywhere and always contains radio-active emanation, more or less, and the next question was: Whence does that emanation arise? Carefully conducted experiments proved that it is not due to any constituent of the air itself; it arises from the earth. Air taken from the soil may contain so much emanation that, if properly concentrated, it will even yield the phosphorescent effect. Water which has passed through the earth contains emanation in solution. This is especially the case with mineral waters, and it has been suggested that the curative properties may, in certain cases, be partly due to the radioactivity; if so, that would explain the puzzling fact that some waters lose their virtue when removed from their source, since, however carefully the vessel was closed, the emanation would nevertheless disappear. Whence this universally diffused emanation arises is not yet known; researches to determine the substances which produce it are being carried on now.

The amount of matter in question is so infinitesimal that experimenters have not yet been able to detect any loss of weight in their radio-active salts to account for the unceasingly emitted emanation. This is, however, not so strange as it may sound at first, for it is paralleled by facts with which we are perfectly familiar. Scent, which is on good grounds believed to be a material emanation, is not necessarily accompanied by loss of weight, not even when it is as strongly marked as in the case of musk. The fact is that where our senses do give us direct evidence they may be far more sensitive than any indirect means we can devise. Thus we know of the existence of a multitude of emanations by no other test than our sense of smell. Where, on the other hand, our senses fail us, there we may remain in total ignorance until we learn in some indirect way. The most striking example of this self-evident, though too often forgotten, fact is furnished by electricity. We are in the position as regards electricity of a deaf man, who only knows that there is sound when he sees motion or feels vibration; for it is only indirectly that we can perceive it, seeing that we lack an electric sense. Yet, step by step, by indirect