198 THE KADIO- ACTIVITY OF MATTER. 



in the luhorutory several ^^ears ago, caused me to think that the phe- 

 nomenon would have been the same at an}- time it might ha^'e Ijeen 

 observed; it should, therefore, be permanent, that is to say, there 

 should not be any appreciable weakening after a very long time. This 

 is, in fact, what I have pro^'ed during the past six years. I will show 

 you thehrst proof I had of the spontaneity of the rays; these ra3^s have 

 traversed the black paper which covered the photographic plate, and 

 a thin strip of copper in the form of a cross (lig. 1). Here, again, is 

 the radiograph, made about the same time, of an aluminum medal, 

 fig. li; tlie unequal absorption of the different thicknesses has caused 

 the appearance of the efhgy thereon. After the \'ery iirst observation 

 J observed that the new radiations would discharge electrified bodies 

 at some distance in the air, a phenomenon which gives us a second 

 method for studying these rays; the photographic method is specially 

 qualitative, while the electrometer furnishes numerical elements of 

 comparison. 



In the course of these first ol)servations, I was led away from the 

 path toward which later experiments brought me back l)y several facts, 

 of which the following is the principal: Having protected a photo- 

 graphic plate by means of a sheet of aluminum '2 nuu. in thickness, 

 and having arranged on the aluminum several samples of phosphor- 

 escent powders, placed on separat(^ plates of glass, and co^'ered with 

 small tubes like clock shades (fig. 3), the photographic proof, obtained 

 after forty-eight hours, showed silhouettes of the plates of glass (fig. 4), 

 just as if the}^ had been produced by the total refraction and reflection 

 of rays identical with those of light, ])ut which nmst have traversed 

 the 2 nmi. of aluminum. This photograph is unique: 1 have never 

 been able to reproduce it or obtain any action with the same sample 

 of sulphide of calcium, nor with any other phosphorescent pi'cparation. 

 At about the same time M. Niewenglowski obtained an impression 

 with sulphide of calcium, and M. Troost with hexagonal l)lende. To 

 this day I do not know the cause of the activit}^ of these products or 

 its disappearance. These facts, and some others, gave me the idea that 

 the new rays nnght ))e a transversal movemcMit of the either analogous to 

 that of light; ))ut the absence of refraction and a large numbei- of other 

 experiments made me abandon this hypothesis. 



In this same year, iSiXi, 1 found that all the uranium salts emitted 

 rays of n. similar nature, that the radiant ])roperty is an atomic one 

 belonging to the ehMuent uranium, and electric measurements showed 

 me that metallic uranium was about thre(> and a half tim(\s more active 

 in ionizing air than is th(> double sulphate of uranium and i)otassium. 

 The same method enables us to study the role played ))v tho gases in 

 the discharge, and to observe that a sj)here of electrified uranium 

 retains its charge in vacuo, while in air it loses it. The rate of the 

 fall of potential is s(Misi])ly ])ro])ortional to the potential if the latter is 



