THE MYSTERY OF MATTER. 187 



bodies in contact with the radium emanation become coated 

 with a thin film of this deposit, and this is the cause of the 

 temporary radio-activity which radium excites in bodies close to 

 it. The deposit is intensely radio-active, and possesses dis- 

 tinguishing physical and chemical properties. 



Ramsay and Soddy have shown that in the coiirse of the 

 transformation of the emanation helium is produced. They 

 introduced some pure emanation into a vacuum-tube, and after 

 five days the complete spectrum of helium was observed. As 

 I have said, it is generally believed that the a particles are free 

 atoms of helium. 



Helixmi was first discovered in the spectriun of the Sun by 

 Janssen and Lockyer. A bright yellow line was noticed different 

 from that of any known element. Helium was afterwards 

 found to be one of the mono-atomic inert gases which are 

 present in small quantities in the atmosphere, and has also been 

 shown to exist in pitch-blende and other minerals from which 

 radium can be extracted. We may therefore look upon it as at 

 any rate a bye-product of the ti'ansformation of radium. 



The active deposit is much more radio-active even than the 

 radium emanation. The small quantity which is present in 

 a piece of radium is responsible for about half of the activity 

 manifested. It consists essentially of a mixture of three distinct 

 substances, which have been called radium A, radium B, and 

 radium C. These three products are formed by three successive 

 disintegrations of the atom. The emanation is first transformed 

 into radium A with the emission of an a particle, and radimn A 

 is exceedingly unstable, having an average life of only three 

 minutes. Emitting an a particle it is quickly transformed into 

 radium B. Radium B emits no rays : it has a period of about 

 26 minutes, and is transformed into radium C. Radium C has 

 a period of 19 minutes and is transformed into a comparatively 

 stable substance. The atom of radium C breaks vip with explosive 

 energy, emitting a, /3, and 7 rays. The a rays travel much faster 

 than those from any other radium product, and the ji corpuscles 

 are emitted with a speed approaching the velocity of light. 

 A piece of ordinary radium contains stored within it small 

 quantities of the emanation, of radium A, radium B, and radium 

 C, and the activity which it manifests proceeds not only from the 

 radium itself, but also from its transformation-products. These 

 three transformations have been called collectively the active 

 deposit of rapid change. The transformations of radium do not 

 come to an end with radiiim C, but continue through three moi-e 



