Thus we see why it is that gases may become conductors under the influence of certain radiations, or of the combustion of flames. It has been known for a long time, however, that without any of these influences a gas cannot prevent the passage of electricity when the field is sufficiently strong. The phenomena of the disruptive discharge, including the spark, the arc, and the brush discharges, have long been known, and they take various and complicated forms in the air at different pressures; but until recently they have been very little understood. The theory of gaseous ions has thrown a new light upon this manner of discharge. As a result of recent researches, the disruptive discharges can be explained by assuming that the ions which have acquired a sufficient velocity under the action of an electric field are able to act as projectiles, which, coming in contact with the molecules of gas, ionize them by the shock which they produce. Negative ions are much more active ionizing agents than the positive ions, and can produce these effects in more feeble fields. It may be conceived, then, that the ions being multiplied by the shock of those already present, the conductibility of the gas becomes very great when the field is sufficiently strong, and the ionized gas is then luminous.
The cathode rays, which are produced when the discharge is made to pass in a tube containing a gas under low pressure, are the electrons sent off by the cathode with a great velocity. Since these electrons and the positive ions have different properties, the discharge tube takes on the well-known dis-symmetrical appearance, which the theory of the ions readily explains, but for which no other interpretation has sufficed.
The Roentgen rays, which are emitted from a Crookes tube, are believed to be in reality electromagnetic waves whose wave length is very short. Such waves as these are supposed to be emitted by an electron whenever it is subjected to an abrupt acceleration, such as is produced, for example, when the electrons of a metal are put in vibration by the impact of cathode rays.
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In accordance with what has been said, all gases which show themselves conducting contain the charged centers which we call gaseous ions. The presence of these charged centers may be made evident by means of a very curious experiment, which utilizes the property which ions have of promoting the condensation of supersaturated water vapor. When the volume of a certain mass of saturated water vapor is quickly increased the vapor condenses to the extent to which it is supersaturated, but if the supersaturation is not very great, and if the vessel contains no dust, there is no noticeable condensation at the moment of change, and the gas remains transparent; but when the gas contains ions the condensation takes place readily—that is to say, with a smaller expansion. It is easy to regulate the expansion so that there will be no condensation when the gas is not ionized but an abundant condensation if ionized. In the latter case the condensation manifests itself by the formation of an opaque cloud which fills the receptacle. Investigation of this phenomenon has shown that the globules of water, which constitute the cloud, form themselves upon the ions, each of which serves as a center for one of them. Ingenious experiments have made it possible to count the globules present in a cubic centimeter of cloud and thus to obtain the number of ions present in this volume. By measuring, in addition, the total charge of the ions of each sign in a cubic centimeter the individual charge of the ions is determined—that is to say, the charge of a single atom of electricity. This charge is equal to 3.4 times 10–10 electrostatic units. In order to show this phenomenon the gas may be ionized by the introduction of a glowing platinum wire, and it will be recognized that there is an energetic ionization of the gas surrounding the incandescent body.
We will now pass to the essential facts revealed by the study of radio-active substances, and examine them from the point of view of the hypothesis of the atomic transformation of matter. Among the radioactive elements, some appear to be permanently active (uranium, thorium, radium, actinium) while others lose their radioactivity little by little (polonium). The most powerful representative of the permanently radioactive substances is radium. According to the theory of transformation this substance changes very slowly, so that a given mass of radium would lose half its weight only in several thousand years. Consequently, the quantity of radium which disappears from a gramme of this substance in an hour is absolutely inaccessible to experiments. However, a gramme of radium disengages each hour about 100 calories of heat. To conceive how enormous this disengagement of heat is, we remark that during the life attributable to radium the complete transformation of a gramme of this substance would produce as much heat as the combustion of a ton of coal. The transformation of radium, then, if transformation there be, is not to be regarded as an ordinary chemical reaction, for the quantity of heat involved is of a far higher order. One is led to conceive, rather, that the atoms themselves are transformed, for the quantities of energy put in play in the formation of atoms are probably considerable.
Indeed, the phenomena of radioactivity has a palpably atomic character, which was brought to light in the beginning of researches on the subject. It was precisely the absolute conviction that we were dealing with an atomic phenomenon which led M. Curie and me to the discovery of radium. If the radioactivity can not be separated from the atom it is very difficult to conceive anything but the atom itself involved in the transformation.
The effects produced by radium are very powerful considering how small is the quantity of this substance at disposal for experiments. There is a spontaneous and continuous emission of rays, analogous to those which we know are produced by means of an induction coil in a Crookes tube, and these rays produce ionization of gas in the same manner. They are able, for example, to produce the rapid discharge of an electroscope. The energy of the rays is so great that the discharge is produced even across a thick metallic screen, for the rays can traverse such a screen.
Some of the rays comprise electrified particles moving with very great velocity. Some are charged positively, and their dimensions are comparable with those of atoms; while others are negative electrons, whose electric charge may be shown by direct experiments. Admitting that all these projectiles come from the atoms of radium themselves, it is difficult to avoid the conclusion that the departure of a positive particle must necessarily cause a modification of the atom which expels it.
Among the electrons emitted there are some whose velocity is enormous, and is in fact no less than nine-tenths the velocity of light. It has been found that the mass of these projectiles (which are the most rapid that we know of) is greater than that of slower-moving electrons, and this result may be considered as a confirmation of the theory according to which the mass of an electron is regarded as the result of electromagnetic phenomena.
The energy of the rays of radium is also manifested by their capacity for exciting the luminosity of various phosphorescent substances. Radium salts are, indeed, themselves luminous, and the light is readily visible in certain conditions.
Here are now a new series of facts which are interpreted by the theory of radio-active transformation. Radium disengages continuously a substance which behaves like a gaseous radio-active material and which has received the name of the emanation. Air which has been in contact with a solution of radium salts is charged with the emanation, and may be drawn away and studied. Air containing the emanation is strongly conducting. A sealed glass tube in which the emanation has been imprisoned acts on the outside like a radioactive substance, and is able, for example, to discharge an electroscope. When the emanation is drawn into a flask containing zinc sulphide, the latter becomes luminous. The emanation is an unstable gas and spontaneously disappears, even from a sealed glass tube, at a rate in accord with a strict law, by which a given quantity of emanation diminishes by half in about four days. The emanation possesses the property of imparting radioactivity to all the bodies in contact with it, and such bodies are said to possess induced radioactivity.
In the theory of atomic transformation the emanation of radium is the first product of disintegration and is transformed in its turn. The induced radioactivity to which it gives rise is considered as due to a solid radioactive material, which results from the transformation of the radium emanation. Three different radioactive materials are distinguished in the induced radioactivity, which constitute three successive terms of the transformation. Each transformation is also accompanied by the emission of rays, and the expelled particles are also counted among the resulting products.
Induced radioactivity does not disappear completely; but there remains after the lapse of a day a very feeble residue which persists in part for years, and which is believed to be adding new terms to the series of successive transformations.
A new fact of great interest has come to the support of the theory of the transmutation of radioactive substances, and has, indeed, made it almost indispensable. It has been proved that radium, a perfectly definite chemical element, produces continually another perfectly definite chemical element, helium (Ramsay and Soddy). It is admitted that helium is one of the products of the disintegration of the atom of radium, and it is noteworthy that helium occurs in all the radium-bearing minerals.
The theory of the radioactive transformation has been extended to all the radioactive bodies, and investigations have been made to determine if the radioactive substances heretofore considered as elements are not to be derived from one another. The origin of radium itself has been sought in uranium. It is well known that radium is found in the uranium-bearing minerals, and it appears from recent researches that the proportion between the quantities of radium and uranium is the same in all these minerals. Uranium may, then, be thought of as a mother substance, which disintegrates with extreme slowness, giving place to the production of radium and the products which succeed it. It appears also to be probable that the last term of the radioactive series is polonium. It may be recalled that uranium was the substance in which the property of radioactivity was discovered by M. Becquerel, and polonium is the first new substance which was discovered by the aid of radioactivity.
A series of analogous considerations has been established for another radioactive substance—thorium. In this case thorium as a primary substance generates radio-thorium, a substance recently discovered, which gives rise to the gaseous radioactive emanation of thorium and various products of radioactivity induced by this emanation. Actinium also gives place to a series of transformations similar to those of thorium, and it, like radium, produces helium.
All the radioactive substances which have been studied sufficiently from the point of view of their disintegration follow a law of decreasing progression, characterized by a constant coefficient. This coefficient may be defined as the time required for the diminution of the activity by half. These constants appear to be independent of the conditions of experiment, are characteristic of the substance to which they appertain, and seem to be capable of fixing an absolute scale of time. Thus the emanation of radium diminishes by half in about four days, while that of thorium diminishes by half in about one minute, and that of actinium in about four seconds.
I have already stated that the radioactivity is a general property of matter. If the theory of radioactive transformation continues to inspire a growing degree of confidence, it will result in an important consequence for geology, and will lead to a careful study of the proportions of the elements occurring in rocks, with a view to deduce their relative ages.
It is plain that the hypothesis of radioactive transformation is well adapted to the present state of the science of radioactivity. It was among those proposed by M. Curie and myself at the beginning of our researches on radioactivity; but it has received its precise development by Rutherford and Soddy, to whom it is for this reason generally attributed. It seems to me, however, better not to leave the domain of demonstrated facts, and not to lose sight of other explanations of radioactivity which have been proposed. The actual state of the science does not seem to me far enough advanced to warrant a positive conclusion.
In closing, the general importance of the phenomena of radioactivity may be recalled. For physics the radioactive substances constitute a new implement of research in consequence of the rays they emit, and they have actively contributed to the development of the theory of the conduction of gas and of the nature of the electron. By their numerous chemical and physiological effects, and their possible influence on meteorology, these substances extend their sphere of action in the domain of all the science of nature; and it is probable that their importance for the development of science will go on increasing. Finally, it has been shown that there is nothing absurd in supposing that the energy we receive from the sun may be in part, or even in total, due to the presence of radioactive bodies which it may contain.
