Since Hertz’ experiments, it has been shown that a conducting body electrified negatively loses its charge if it be subjected to the action of the ultraviolet rays obtained from electric sparks, and it is recognized in more recent works:
(1) That this leak can only take place under the influence of the ultraviolet;
(2) That it is the same for all metals;
(3) That the discharge only takes pace when the charge of the metal is negative and not positive.
Elster, Geitel, and Branly, it is true, mentioned some time ago two or three metals which discharged in ordinary sunlight, and the last-named cited several bodies which show the positive leak; but these phenomena were considered as exceptional and as in no wise possessing a general character.
As the subject did not appear to me exhausted, I deemed it well to take it up anew. Although there is a certain difference between the phenomena of the discharge of a body already electrified and that of the production of effluves emanating from an unelectrified body and capable of acting on an electrified one as shown in the previous chapter, yet the two phenomena have the same cause --- namely, the dissociation of matter by light. No experimenter had suspected this cause before my researches.
The experiments I am going to set forth prove --- (1) that the so-called negative leak is also, though generally in a lesser degree, positive; (2) that the discharge takes place under the influence of the various regions of the spectrum, although the maximum occurs in the ultraviolet; (3) that the discharge is extremely different in the various bodies, the metals especially. These are, as will be seen, three propositions exactly contrary to those generally received and recapitulated above.
...How can the electric atoms proceeding from the dematerialization of matter preserve their individuality and transform themselves in vibrations of the ether?
All modern research leads is to consider these particles as constituted by whirls, analogous to gyroscopes, formed in the bosom of ether and connected with it by their lines of force. The question, therefore, reduces itself to this: how can a vortex formed in a fluid disappear into this fluid by causing vibrations in it?
Stated in this form, the solution of the problem presents no serious difficulties. It can be easily seen, in fact, how a vortex generated at the expense of a liquid can, when its equilibrium is disturbed, vanish by radiating away the energy it contains under the forms of vibration of the medium in which it is plunged. In this way, for example, a waterspout formed by a whirl of liquid loses its individuality and disappears in the ocean.
It is, no doubt, the same with the vibrations of the ether. They represent the last stage of the dematerialization of matter, the one preceding its final disappearance. After these ephemeral vibrations the ether returns to its repose, and matter has definitely disappeared. It has returned to the primitive ether from which hundreds of millions of ages and forces unknown to us can alone cause it to emerge, as it has emerged in the far-off ages when the first traces of our universe were outlined on the chaos. The beginning of things was, doubtless, nothing else than a re-beginning. Nothing leads to the belief that they had a real beginning, or that they can have an end.
We have just seen that all bodies, simple or compound, conductors or insulators, subjected to the action of light undergo dissociation. But among none of the bodies examined up to now do gases appear. Are we to suppose that they escape the common law?
This exception seemed improbable. Yet up to Lenard’s last researches the dissociation of gases by the action of light had not been observed. No doubt it was supposed that the discharge of electrified bodies, when struck by light, might be due to the action of the luminous rays in the air, but this hypothesis fell to the ground in face of these two facts --- first, that the discharge varies according to the metals, which would not be the fact if it were the air and not the metal which was acting; and second, that the discharge takes place still more rapidly in a vacuum than in the air.
The reason of this apparent indifference of gases, air especially, to the light which strikes them is very simple. Some metals are dissociable only in a very advanced region of the ultraviolet. If gases should happen to be dissociable only in still more advanced regions, the observation of their dissociation must be difficult, seeing that the air with slight density is as opaque as lead for the radiations of the extreme ultraviolet.
Now, as Lenard has shown (Annalen der Physik, Bd. 1, 1900), it is solely inthis extreme region of the ultraviolet that what was then called the ionization of gases, which is no other than their dissociation, is possible. He saw that it sufficed to bring the bodies under experiment to within a dew centimeters from the source of light --- from the electric spark --- for the discharge to be the same for all bodies, which shows that it is then the air which becomes the conductor and acts. It is light, and no other cause, which intervenes, for the interposition of thin glass stops all effect.
By a special arrangement, which there would be no advantage in describing here, Lenard has measured the wavelength of the radiations which produce the ionization of the air. They begin towards 0.180 microns, just at the limits of the electric spectrum as formerly known (0.185 microns), and extend as far as 0.140 microns. The discovery of these short radiations is, as is known, due to Schuman. By creating a vacuum in a spectrograph, he proved that the ultraviolet spectrum, which, from the incorrect measurements of Cornu and Mascart, were believed to be limited to 0.185 microns, in reality extended much farther. He ahs succeeded in photographing rays reaching as far as 0.100microns. It is probably the absorption exercised by the gelatin of the sensitive plates, and no doubt also by the material of the prism, which prevents further progress.
As we advance into the ultraviolet spectrum, all bodies, the air especially, become more and more opaque to the radiations. It would therefore be very surprising if the x-rays, which pass through all bodies, were constituted by the extreme ultraviolet, as some physicists have maintained.
Most bodies, including air of a thickness of 2 cm, and water 1 mm thick, are, in fact, absolutely opaque for these radiations of very short wavelength. There are hardly any transparent to them except quartz, fluorspar, gypsum, and rock salt, and even these only on condition of their surface not being roughened. Pure hydrogen is equally transparent.
The extremely refrangible radiations of light therefore dissociate, not only all solid bodies, but also the particles of the air they pass through, while radiations less refrangible possess no action on gases, and only dissociate the surface of the solid bodies they strike. These are two very different effects which may be superposed on each other, but which will not be confused if it be borne in mind that when it is the air that is decomposed, the nature of the metal struck and the state of its surface are points of no importance; while the leak varies considerably with the metal when it is the latter that becomes dissociated. Besides, the influence of the extreme ultraviolet can be almost entirely avoided by removing the source of light to a little distance, since a layer of air of 2 cm suffices to stop this region of the spectrum. If, therefore, the sparks from the electrodes are at several centimeters from the quartz window of the spark-box, no effect due to the decomposition of the air can be produced.
In comparing some of the experiments set forth so far, it will be noticed that those bodies which absorb most light are precisely those which are the most dissociable. For example, air, which absorbs the radiations below 0.185 microns, is decomposed by these radiations. Lamp-black, which completely absorbs light, is energetically dissociated by it, and disengages effluves in abundance. This explanation does not appear at first sight at all to tally with the fact that metals which have recently received a mirror polish are likewise the seat of an extremely abundant disengagement of effluves. The objection vanishes, however, when it is considered that polished metals which reflect visible light very well reflect very badly the invisible light of the ultraviolet extremity of the spectrum, and absorb the greater part of it. Now, it is precisely these absorbable and invisible radiations which produce most effect.
To give a clear idea of the properties of the various pars of the ultraviolet spectrum, I will put them in tabular form. It shows that the aptness of light to dissociate bodies increases with every step into the ultraviolet.
To sum up, the more we advance into the ultraviolet, the shorter the wavelength of the radiations become, the less penetration these radiations have; but their dissociating action on matter shows itself more and more energetically. At the extremity of the spectrum all bodies are dissociated, including gases, on which the other parts of the spectrum have no action. The dissociating action of the various luminous radiation is therefore in inverse ratio to their penetration (1).
[(1) See Wm Ramsay and Dr Spencer, Philosophical Magazine, October 1906.]
The law thus formulated was quite unforeseen previous to my researches. All earlier observations seemed to show that the rays at the ultraviolet end of the spectrum possessed so slight an energy as to be almost inappreciable by the most delicate thermometers. It is, however, these radiations which most quickly dissociate the most rigid bodies, such as steel, for example.
~~Gustave Le Bon