a PBESIDENTS ADDRESS. 



Many of the coiupuuiidsof a .sumewhal uncommon metallic element, called 

 ui'anium, used for giving a line green colour to glass, are phosphorescent 

 substances, and it was, fortunately, one of them Avhich Henri Becquerel 

 chose for experiment. Henri Becquerel is professor in the Jardin des 

 Plantes of Paris ; his laboratory is a delightful old-fashioned building, 

 which had for me a special interest and sanctity when, a few years ago, I 

 visited him there, for, a hundred years before, it was the dwelling-house of 

 the great Cuvier. Here Henri Becquerel's father and grandfather — men 

 renowned throughout the world for their discoveries in mineralogy, 

 electricity, and light — had worked, and here he had himself gone almost 

 daily from his earliest childhood. Many an experiment bringing new 

 knowledge on the relations of light and electricity had Henri Becquerel 

 carried out in that quiet old-world place before the day on which, about 

 twelve years ago, he made the experimental inquiry, Does uranium give off 

 penetrating rays like Rontgen's rays ? He wrapped a photographic plate in 

 black paper, and on it placed and left lying there for twenty-four hours some 

 uranium salt. He had placed a cross, cut out in thin metallic copper, 

 under the uranium powder, so as to give some shape to the photographic 

 print should one be produced. It toas produced. Penetrating rays were 

 given off by the uranium : the black paper was penetrated, and the form 

 of the copper cross was printed on a dark ground. The copper was also 

 penetrated to some extent by the rays from the uranium, so that its 

 image was not left actually white. Only one step more remained before 

 Becquerel made his great discovery. It was known, as I stated just now, 

 that sulphide of calcium and similar substances become phosphorescent 

 when exposed to sunlight, and lose this phosphorescence after a few 

 hours. Becquerel thought at first that perhaps the uranium acquired its 

 power similarly by exposure to light ; but very soon, by experimenting 

 with uranium long kept in the dark, he found that the emission of 

 penetrating rays, giving photographic effects, was produced spontaneously. 

 The emission of rays by this particular fragment of uranium has shown 

 no sign of diminution since this discovery. The emission of penetrating 

 rays by uranium was soon found to be independent of its phosphorescence. 

 Phosphorescent bodies, as such, do not emit penetrating rays. Uranium 

 compounds, whether phosphorescent or not, emit, and continue to emit, 

 these penetrating rays, capable of passing through black paper and 

 metallic copper. They do not derive this property from the action of 

 light or any other treatment. The emission of these rays discovered 

 by Becquei-el is a new property of matter. It is called ' radio-activity,' 

 and the rays are called Becquerel rays. 



From this discovery by Becquerel to the detection and separation of 

 the new element radium is an easy step in thought, though one of 

 enormous labour and difficulty in practice. Professor Pierre Curie (whose 

 name I cannot mention without expressing the grief with which we all 

 heard in last April of the sad accident by which his life was taken) and 

 his wife, Madame Sklodowski Curie, incited by Becquerel's discovery 



