72 THE KINGDOM OF MAN 



substance which shines in the dark after exposure to sun- 

 light that is to say, is phosphorescent. Other sulphides 

 and the minerals fluor-spar, apatite, some gems, and, in 

 fact, a whole list of substances have, under different con- 

 ditions of treatment, this power of phosphorescence or 

 shining in the dark without combustion or chemical 

 change. All, however, require some special treatment, 

 such as exposure to sunlight or heat or pressure, to elicit 

 the phosphorescence, which is of short duration only. 

 Many of the compounds of a somewhat uncommon 

 metallic element, called uranium, used for giving a fine 

 green colour to glass, are phosphorescent substances, 

 and it was, fortunately, one of them which Henri Bec- 

 querel chose for experiment. Henri Becquerel is pro- 

 fessor 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 Bec- 

 querel'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 ex- 

 periment 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 

 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 was produced. Penetrating rays 



