season of 1895 that Professor Röntgen of Wurzburg, Germany, exhibited to the Physical Society of Berlin the first X-ray photographs. These photographs showed that from a vacuum bulb in which an electrical discharge was passing some sort of radiation was emitted, which was like light in that it produced an effect upon the photographic plate, but was unlike light, first, in that it was wholly invisible, and, second, in that it was able to pass easily through many substances which are perfectly opaque to ordinary light, such, for example, as cardboard, wood, leather and, notably, the flesh of the human hand. This discovery would probably have attracted little attention outside of scientific circles had it not been for this last-mentioned remarkable property, but the idea of obtaining photographs of the skeleton of a living being was so startling, so uncanny, at that time, to the average mind, that the discovery took to itself wings and within two weeks had set the whole world agog. Scores of scientists in all countries dropped at once their pending researches and began to experiment upon these strange new rays which Röntgen had named X-rays because they were such a completely unknown quantity. A surprisingly small amount of new knowledge concerning the nature of X-rays themselves resulted from all this research. The X-rays are almost as much of an unknown quantity to-day as they were when Röntgen made his first announcement. As is so often the case, it was in unexpected directions that this wave of experimentation upon X-rays bore fruit. The discovery of radio-activity was not the least important result of this activity. It came about in this way.
The Discovery of Radio-activity.
It was noticed that an exhausted bulb which is emitting X-rays under the influence of electrical discharges is always aglow with a peculiar greenish-yellow light which is commonly known as fluorescent light. Now it had long been known that there are some natural substances, notably the mineral uranium and its compounds, which possess a similar property of emitting this yellowish-green light not only when they are in a vacuum tube through which electrical discharges are passing, but also when they are exposed to the invisible radiation from the sun, that is, to the so-called actinic or ultra-violet rays which are chiefly responsible for the effects which sunlight produces upon photographic plates. It accordingly very naturally occurred to some scientists that the X-rays might perhaps be due to this fluorescent light which came from a vacuum bulb, rather than to any immediate influence of the electrical discharge, and, if so, that they ought to be emitted not simply by a vacuum tube, but also by uranium when exposed to sunlight. It was in 1896, within a year of the discovery of X-rays, that Henri Becquerel, the fourth illustrious possessor of that illustrious name, devised some experiments to test this inference. His method
