M. CURIE EXPLAINING THE WONDERS OF RADIUM AT THE
SORBONNE. THIS EXPERIMENT WITH THE RADIUM LIGHT IS
DESCRIBED IN THE ARTICLE.
In order to demonstrate that radium throws off heat continually he took two glass vessels, one containing a thermometer and a tube of radium, the other containing a thermometer and no radium. Both vessels were closed with cotton, and it was presently seen that the thermometer in the vessel containing the radium registered constantly 5·4 degrees F. higher than the thermometer which was not so influenced.
The most striking experiment presented by M. Curie in his London lecture was one devised by him to prove the existence of radium emanations, a kind of gaseous product (quite different from the rays) which this extraordinary metal seems to throw off constantly as it throws off heat and light. These emanations may be regarded as an invisible vapour of radium, like water vapour, only infinitely more subtle, which settles upon all objects that it approaches and confers upon them, for a time at least, the mysterious properties of radium itself. Thus the yellow powder sulphide of zinc bursts into a brilliant glow under the stimulus of radium emanations, and to make it clear that this effect is due to the emanations and not to the rays M. Curie constructed an apparatus in which a glass tube, R, containing a solution of radium is connected with two glass bulbs, A and B, containing sulphide of zinc.
The experiment is begun by exhausting the air from the two bulbs A and B, by means of air-pump connections through the tube E. The air is not exhausted, however, from the tube R, over which the stop-cock F is closed, and within which the emanations have been allowed to accumulate. The room is now darkened, and it is seen that so long as the stop-cock F remains closed there is no glow in the bulbs A and B, but as soon as the stop-cock F is opened both bulbs shine brilliantly, so that the light is plainly visible at a distance of several hundred yards. Now, obviously, if this effect were due to the radium rays, it would be produced whether the stop-cock F were open or closed, since the radium rays pass freely through glass and need not follow the tube S in order to reach the bulbs A and B. It is therefore clear that the sudden light in the bulbs is due to the passage of something out of the tube R, and through the tube S, that something being kept back by the glass of the bulb R until the stop-cock F is opened. So we conclude that the emanations of radium cannot pass through glass, and are a manifestation quite distinct from the rays of radium, which can pass through but do not influence the sulphide of zinc.
This point having been established, M. Curie proceeded to the most sensational part of his demonstration, by closing the stop-cock F and then placing the lower bulb B, still radiant, in a vessel G containing liquid air, the result being that the light in the bulb B gradually grew stronger while the light in the bulb A diminished, until, presently, all the light seemed concentrated in B and gone from A, the conclusion being that the intense cold of liquid air had produced some change in the emanations, had possibly reduced them from a gas to a liquid, thus withdrawing them from A to B and checking the one glow while increasing the other.
In talking with Sir William Crookes, M. Curie was interested to learn that the English scientist had just devised a curious little instrument which he has named the spinthariscope, and which allows one to actually see the emanations from radium and to realize as never before the extraordinary atomic disintegration that is going on ceaselessly in this strange metal. The spinthariscope is a small microscope that allows one to look at a tiny fragment of radium, about one-twentieth of a milligramme, supported on a little wire over a screen spread with sulphide of zinc.
The experiment must be made in a darkened room after the eye has gradually acquired its greatest sensitiveness to light. To the eye thus sensitive and looking intently through the lenses the screen appears like a heaven of flashing meteors, among which stars shine forth suddenly and die away. Near the central radium speck the fire shower is most brilliant, while towards the rim of the circle it grows fainter. And this goes on continuously as the metal throws off its emanations; these myriad bursting blazing stars are the emanations—at least, we may assume it—and become visible as the scattered radium dust or radium vapour impinges speck by speck upon the screen, which, for each tiny fragment, flashes back a responsive phosphorescence. M. Curie spoke of this vision, that was really contained within the area of a two-cent piece, as one of the most beautiful and impressive he had ever witnessed; it was as if he had been allowed to assist at the birth of a universe or at the death of a molecule.
Dwelling upon the extreme attenuation of these radium emanations, M. Curie mentioned a recent experiment, in which he had used a platinum box pierced by two holes so extremely small that the box would retain a vacuum, yet not small enough to resist the passage of radium emanations.
In view of the extreme rarity and costliness of radium, it is evident that its emanations may be put to many important uses in and out of the laboratory, since they bestow upon indifferent objects—a plate, a piece of iron, an old shoe, anything—the very properties of radium itself. Thus a scientist or a doctor unable to procure the metal radium may easily experiment with a bit of wood or glass rendered radio-active—that is, charged by radium emanations, and capable of replacing the original metal as long as the charge keeps its potency. This period has been determined by the Curies after observations extending over weeks and months, and applied to all sorts of substances, copper, aluminium, lead, rubber, wax, celluloid, paraffin, no fewer than fifty in all, the resulting conclusions being formulated in a precise law as follows:—