RADIO-ACTIVITY 179 



active processes : the impossibility of changing 

 the amount of activity by any ordinary chemical 

 or physical operations. 



Since the phenomena of radio-activity have 

 been well known, and the various types of radia- 

 tion and emanation which proceed from radio- 

 active materials clearly distinguished, traces of the 

 property have been found to be disseminated very 

 widely. Mr C. T. R. Wilson, for example, 

 detected radio-activity in newly-fallen rain and 

 snow ; when evaporated they leave a residue 

 which discharges an electroscope. Again, Sir 

 J. J. Thomson found that when air is bubbled 

 through various samples of water from deep 

 wells, or when the water is boiled and the dis- 

 solved air driven off and collected, there is present 

 in the air a radio-active gas, which behaves as 

 though it were the emanation from some active 

 substance of which slight traces are contained in 

 the water. The air loses its active properties, 

 while the water regains a small part, and after 

 some days will again yield a supply of active gas. 

 The rate of recovery and decay seem to be 

 about the same as for the radium emanation, 

 and this suggests that the active material is 

 radium in minute quantity. 



Again, M'Lennan, Rutherford and Cooke, and 

 Strutt found that the rate of leak in a closed 

 vessel depends on the nature of the walls of the 

 vessel. But Strutt detected some variation in 

 the rate of leak with different samples of the 

 same material, and Cooke diminished the rate of 

 leak in a brass electroscope by carefully cleaning 

 the walls. Probably this result is to be explained 



