482 POPULAR SCIENCE MONTHLY. 



season of 1895 that Professor Eontgen of Wurzburg, Germany, ex- 

 hibited to the Physical Society of Berlin the first X-ray photographs. 

 These photographs showed that from a vacuum bulb in which an elec- 

 trical 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 Eontgen 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 Eontgen made his first announce- 

 ment. 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. JSTow it had long been known that there are some natural sub- 

 stances, 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 photo- 

 graphic plates. It accordingly very naturally occurred to some scien- 

 tists that the X-rays might perhaps be due to this fluorescent light 

 which came from a vacuum bulb, rather than to any immediate in- 

 fluence 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, vsdthin 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 



