200 THE RADIO-ACTIVITY OF MATTER, 



The sample of rudiuni that M. and Mnie. Curie have h'lit me for the 

 purpose of this leetuve enabh^s me to show y<^'ii ^ f^^v "f these phe- 

 nomena — ionization of the air. himinosity, and pliosphoreseenee. 



I have observed by means of th(^ photoomph I now show (tio-. 5), 

 that the radio-activity of polonium will not traverse a thin sheet of 

 black paper forming- a small cylinder closed l)y aluminium or mica, 

 and at the bottom of which was placed the powdered material. The 

 rays from radium easily pass through this envelope. We shall see 

 that still more profound differences exist l)etween these two kinds of 

 YSiys. 



The radio-activity of radium restores to certain crystals and to glass 

 the property of becoming phosporescent l)y heat, which th(\v have lost 

 owing to a previous elevation of temperature. 



The phenomena of al)sorption, examined either by means of photog- 

 raphy, ]>y phosphorescence, or by ionization of the air, showed the 

 heterogeneitv of the class of radiations emitted. Sul)se([ue::t ()t)serv- 

 ations have enlarged the field of this research. 



Toward the end of the year 1899 M. Giesel, and then MM. Meyer 

 and Schweidler, observed that the rays of radio-active preparations 

 were deviated 1)V a magnetic held in the same manner as are the 

 cathodic rays. For ni}^ part, at about the same time, vrithout having 

 heard of these experiments, I observed the same phenomenon with 

 radium. The experiment can be made in the following manner: A 

 small paper box containing a few grains of the radio-active body is 

 placed horizontally on a photographic plate covered with black paper, 

 between the poles of a magnet; the rays are thrown entirely to one 

 side of the plate. I here show my two lirst photographs, one of which 

 shows a concentration on one pole of the magnet. 



Very shortly afterwards 1 observed thatth(^ rays from Y^oloniuni are 

 not deviated, and consequently that two kinds of rays exist, one (levi- 

 able and the other nondeviable. M. and Mme. Curie have made an 

 electric examiinition of this subject, which has proved the simultaneous 

 existence of these two kinds of rays in the radio-activity of radium, 

 their unequal permeability varying with the distanci^ from the absor))- 

 ing screens. The accompanying photograph (tig. (i) show.s these two 

 kinds of rays from i-adiun\ I have recently obser\ ed that uranium 

 emits only deviable raj's — that is, saving the existence of much less 

 active, nondeviable rays. In fact, there does exist a third kind of 

 rays which are not deviable, but are extremely penetrating; tlu\v have 

 been shown more particularly by M. Villard. 



Thus the activity of radio-active bodies c()m})ris(\s thr(H> kinds of 

 rays — rays which are deviable in a magnetic field, which appear to be 

 identical with cathodic rays, and two sorts of nondeviable rays, one 

 kind being ver}' easily' absorbed, the other resembling X-rays and 



