MODERN THEORIES OF ELECTRICITY AND MATTER. 113 



the rapid discharge of an electroscope. The energy of the rays is 

 so great that the discharge is produced even across a thick metallic 

 screen, for the rays can traverse such a screen. 



Some of the rays comprise electrified particles moving ^Yitll very 

 great velocity. Some are charged positively, and their dimensions 

 are comparable with those of atoms; Avhile others are negative elec- 

 trons, whose electric charge may be shown by direct experiments. 

 Admitting that all these projectiles come from the atoms of radium 

 themselves, it is difficult to avoid the conclusion that the departure 

 of a positive particle must necessarily cause a modification of the 

 atom which expels it. 



Among the electrons emitted there are some whose velocity is enor- 

 mous, and is in fact no less than nine-tenths the velocity of light. 

 It has been found that the nuiss of these ])rojectiles (which are the 

 most rapid that we know of) is greater than that of slower-moving 

 electrons, and this result may be considered as a confirmation of the 

 theory according to which the mass of an electron is regarded as the 

 result of electro-magnetic phenomena. 



The energy of the rays of radium is also manifested by their capacity 

 for exciting the luminosity of various phosphorescent substances. 

 Radium salts are, indeed, themselves luminous, and the light is 

 readily visible in certain conditions. 



Here are now a new series of facts which are interpreted by the 

 theory of radio-actiA'e transformation. Kadium disengages continu- 

 ously a substance which behaves like a gaseous radio-active material, 

 and which has received the name of the einanation. Air which has 

 been in contact with a solution of radium salts is charged with the 

 emanation, and may be drawn away and studied. Air containing the 

 emanation ijs strongly conducting. A sealed glass tube in which the 

 emanation has been imprisoned acts on the outside like a radioactive 

 substance, and is able, for example, to discharge an electroscope. 

 When the emanation is drawn into a flask containing zinc sulphide, 

 the latter becomes luminous. The emanation is an unstable gas and 

 spontaneously disappears, even from a sealed glass tube, at a rate in 

 accord with a strict law, by which a given quantity of emanation 

 diminishes by half in al)out four days. The emanation possesses 

 the property of imparting radioactivity to all the bodies in contact 

 with it, and such bodies are said to possess induced radio-activity. 

 Here is a tube of o-lass which did contain emanation, but contains 

 it no longer, for it has been purged by a current of air; nevertheless 

 this tube continues to act as a radioactive body, and is able to dis- 

 charge an electroscope. But this induced radioactivity is even less 

 durable than the emanation ; it disappears spontaneously and dimin- 

 ishes by half in a half hour, 



SM 1906 8 



