THK KADIO-ACTIVITY OF MATTKK. 205 



ablr to tliosr produciMl by a vapor of a special nature. This phciioine- 

 noii was (liscovored in lSl»t) ]iy Mr. Kutlierfoi'd and l»y M. and Mnic. 

 Curie simultaneously. 



Mr. Kutlierford, while exaniinino- the radiations t'l'oni thorium, 

 observed that, besides the ordinary rays, there was another eileet pi'o- 

 duced by an "emanatiorr"' consisting of a sort of vapor ionizing the 

 air. This \apor is deposited on bodies, principally those electrified 

 negatively, and makes them momentarily radio-active. Mr. Ruther- 

 ford made some very interesting measurements of this phenomenon. 



At the same time, M. and ]\im(\ Curie discovered that, under the 

 inf1uenc(» of radiun;. bodies became temporarily radio-active. This is 

 not the secondary effect already described, ])ut a persistent phenome- 

 non which disappears comparatively slowly from the moment when 

 the action of the I'adium ceases. M. Curie has called this "' induced 

 radio-activity." and has made a \ery complete examination of it. He 

 hiis observed that the phenomenon is produced with great intensity in 

 a close space, that induced activity is the same on all l)odies and prac- 

 tically independent of the pressure inside the inclosed space, but that 

 the phenomenon is not produced if we maintain a complete vacuum l)y 

 removing the gases produced; solutions of salts of radium produce the 

 same eti'ect with greater intensity than the solid salts. Li(juids, water 

 of crystallization extracted from active salts, or the water separated 

 from an active solution by a semipermeable membrane of celluloid, 

 remain strongly radio-active; it is the same with the gases. These 

 excited bodies produce the same effects as radium; they emit a i)ene- 

 trating ray which traverses the glass vessels which contain them and 

 makes these lattei' luminous. Induced activity is gradually propagated 

 in gases in a sealed tube, even through capillary tul»es and imperceptible 

 cracks; bodies are excited the more as the volume of gas is greater in 

 proportion to their surface. Phosphorescent l)odies become luminous 

 wdien excited. In a recent work MM. Elster and (ieitel have observed 

 that atmospheric air has properties analogous to those of excited gases, 

 and M. Ceitel has been able to collect on win's, negativelt' electrified, 

 traces of radio-active products. The cause of this radio-activity is a 

 prol)lem of the greatest interest. 



Finally, there is a remarkable method of induction, which is of such 

 a nature as to demand the gi-eatest res(M\(' in the conclusions which 

 might be fornmlated relative to the presence of new elements in radio- 

 active bodies. Evei'v inacti\'e substance which has been added to a 

 solution of a uranium or I'adium salt, and which has subseciuently 

 been removed by [)recipitation, has b<>come radio-active, and loses this 

 radio-activity very slowly. This fact was first observed by M. Curie 

 and M. Giesel, who rendered bisnmth radio-active in this manner. 

 In the case of uranium, a trace of l)arium, precii)itated in the form of 

 sulphate, became jiotably more active than the uranium; barium thus 

 excited emits onlv deviable ravs like uranium. 



