RADIO-ACTIVITY 173 



number of a particles in a narrow pencil of the 

 rays, and recalculated from his results several 

 radio-active constants. 



In the year 1900 Rutherford made another 

 striking discovery. The radiation from thorium 

 was known to be very capricious, being affected 

 especially by slight currents of air passing over the 

 surface of the active material. Rutherford traced 

 this effect to the emission of a substance which 

 behaved like a heavy gas having temporary 

 radio-active properties. This emanation, as it 

 was named, is to be distinguished clearly from 

 the radiations previously described, which travel 

 in straight lines with velocities approaching that 

 of light. The emanation diffuses slowly through 

 the atmosphere, as would the vapour of a volatile 

 liquid. It acts as an independent source of 

 straight line radiations, but suffers a decay of 

 activity with time. 



Similar emanations are given off by radium 

 and actinium, but not by polonium or uranium. 

 The emanations seem to be very inert chemically, 

 in this resembling gases of the argon group. 

 They pass unchanged through acids or hot tubes, 

 but are condensed at the temperature of liquid 

 air, evaporating again as the tube is warmed. 

 By taking advantage of this property, many pretty 

 lecture-room experiments may be performed. For 

 example, a quantity of radium emanation is con- 

 densed in a tube surrounded with liquid air. The 

 tube is connected with others, and, if the liquid 

 air be removed, the emanation can be traced as 

 it diffuses, by the fluorescence it excites on the 

 glass, or on small pieces of paper covered with 

 zinc sulphide, which are placed here and there 



