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as if it had never been interrupted, and goes on till the last element, uranium, is reached. This was a' veritable cryptogram challenging interpretation, and although far from deciphered the first step in the finding of the key has now been taken. The Periodic Law is Nature as it is, not as we would have it, or as we would have nade it, if the making of it had been ours. There are some curious minor exceptions even in its very arbitrary regularities. At first, also, gaps had to be left for missing elements to satisfy the scheme, and so the existence of elements not yet discovered, and even their very properties, were predicted, and in the majority of cases these predictions have been verified by the subsequent discovery of the missing members.
With regard to the very simplest constituents, into which the material universe has been resolved, there is thus a veritable tangle of complex relationships in contrast to that craving for simplicity, symmetry, and order which the mind is always attempting to satisfy in its interpretations of the external world.
In 1896 one of the elements, uranium, the last on the list, was discovered by Becquerel in Paris to possess a new property.
It was described as radioactive to signify that it was continually and spontaneously emitting a new kind of radiation, analogous in its chief characteristics to the X-rays of Röntgen, discovered the year previously. M. and Mme. Curie then showed that thorium, the element next to uranium in atomic weight, possessed a similar property, but, with the doubtful exception of two others, potassium and rubidium, none of the other elements then known show the least evidence of radioactivity. Going back to the natural minerals in which uranium occurs, such as pitchblende, M. and Mme. Curie discovered therein several intensely
radioactive new elements in almost infinitesimal quantity, the best known of which is radium. The radium is present in pitchblende in very minute quantity, not more than one part in five or ten millions of the mineral at most. Small as the quantity was, they succeeded in isolating the compounds of radium in the pure state, and ultimately accumulated enough, not only for a detailed investigation of its extraordinary radioactivity, but also of its chemical character, spectrum, and atomic weight. They found its atomic weight to be 226, which is next to that of uranium, 238, and thorium, 232. This and its chemical character put it into a position in the periodic table in the family of the alkaline-earth elements, comprising calcium, 40, strontium, 85, and barium, 137. In its whole character it has the closest resemblance to the latter element, and can only be separated from it by prolonged and tedious fractionation processes. Chemically it was normal in every respect, and its chemical character could have been predicted from the Periodic Law before its discovery. But in addition to its chemical character it had a whole new set of surprising radioactive properties in a very intense degree.
These discoveries naturally aroused the very greatest scientific interest. The very existence of radium, a substance capable of giving off spontaneously powerful new radiations which can be transformed into light and heat, and, indeed, not only capable of doing this, but, so far as we know, incapable of not doing it, ran counter to every principle of physical science. For whence comes the energy that is being given out in the process? So soon as pure radium compounds became available, the amount of this energy was measured and it was found to be sufficient to heat a quantity of water equal to the weight of the radium from the freezingpoint to the boiling-point every three-quarters of an hour. In the combustion of fuel, from which the world draws by far the greater part of the energy it needs, the heat evolved is sufficient to raise a weight of water some 80 to 100 times the weight of fuel from the freezing-point to the boiling-point. Hence radium, weight for weight, gives out as much heat as the best fuel every three days, and in the fifteen years that have elapsed since it was first isolated, a quantity of energy nearly two thousand times as much as is obtainable from fuel has been given out by the radium, and the supply as yet shows no sign of exhaustion.
Before, however, these questions could be asked in this definite quantitative form they had been answered, from a detailed investigation of the radioactivity of the element thorium. Professor, now Sir Ernest, Rutherford, at M'Gill University, Montreal, and now at Manchester University, was one of the leading and most active physicists in the investigation of the new property, and, when the writer joined him in Montreal in 1901, had made a large number of very startling and fundamental discoveries, and had developed the refined methods of investigation and measurement which, more than anything else, contributed to the rapid solution of the problem. The apparently steady and continuous outpouring of the radiations from thorium was found to be a most complex process, in which new substances were being continually produced. These new substances are endowed with a temporary or transient radioactivity, which in the course of time decays away and disappears. Simple methods of chemical analysis sufficed to remove from thorium altogether infinitesimal quantities of substances, to which, however, by far the greater part of the radioactivity was due. After removal the activity of these sub
THE CHANGEABLENESS OF THE ELEMENTS 93
stances steadily and continuously decayed. But the thorium from which they had been removed and which was thereby rendered nearly non-radioactive, gradually recovered its original activity again. Investigation proved that the thorium was in fact continually growing a fresh crop of these radioactive constituents. As fast as it was purified from them by a chemical process, more began to form. The quantities of material involved in these processes are so minute that they are far beyond the limit of detection by the balance or the spectroscope. Indeed, it is estimated that geological epochs of time would have to elapse in the case of thorium before a weighable quantity of the new materials was formed. Nevertheless the characteristic radioactivity they produce enables them to be followed and dealt with as easily, or perhaps more easily, than ordinary substances in weighable amount. Moreover, in certain cases the radioactive products are gasescalled the radioactive emanations and in these cases no chemical separation is needed, as they diffuse away by themselves from the radioactive substance into the surrounding air and are the cause of many striking phenomena.
The doctrine of the unchangeableness of the chemical elements is no fixed article of belief in the chemists' creed, but is simply the expression of the facts known before the discovery of radioactivity, that in all material changes known the chemical elements do not essentially change. When a chemical element or one of its compounds is purified, it remains pure unless it is again mixed with other substances. The discovery that the radioactive substances are continually producing from themselves entirely different chemical elements overthrew the doctrine of the unchangeableness of the elements so far as those that are radioactive are concerned. Since
these new changes cannot yet be artificially produced or imitated, the doctrine of the unchangeability of the elements remains in this limited sense still true.
Sir Ernest Rutherford and the writer were forced to the conclusion that the element thorium, and ultimately all the radio-elements, are in the process of slow spontaneous change. Their radioactivity is due in large measure to minute quantities of impurities, of totally different chemical character from themselves, that can be readily and completely removed by simple purification processes. But, once removed, the substances so purified do not remain pure. At a perfectly definite rate they regrow or produce the radioactive impurities, and these can be again separated as often as desired. Once separated, the radioactivity of the products dies away or decays, and the apparently steady continuous emission of rays from the parent substance is due to an equilibrium, in which new radioactive products are formed as fast as the radioactivity of those already produced disappears. Very rapidly a complete and satisfactory theory of the whole phenomena was developed, and fourteen years of further development of the science has not necessitated any modification. The atoms of the radio-elements are not permanently stable. After a term of existence which may be long or short, according to the nature of the atom in question, and which for the individual atoms of the same radio-element may have any actual value, but is for the average of all the atoms of any one kind a perfectly definite period, known as the period of average life, the atom explodes. Fragments are expelled from it at hitherto unknown velocities constituting the rays, of which more anon. What is left is the new atom of a new element, totally different from the parent. The radio-elements are in course of spontaneous transmutation into other elements,