RADIO-ACTIVITY 175 



are considering untouched. In liquid hydrogen 

 most chemical activities are entirely suspended, 

 and these results, to whatever cause they may be 

 due, are very remarkable. It seems certain that, 

 even when we approach the absolute zero, all 

 the activities of radium are quite independent of 

 temperature. Such extraordinary results as these 

 point to a deep-seated difference in kind between 

 the radio-active processes and all chemical and 

 physical operations hitherto investigated. We 

 shall presently examine this point more closely. 



Unlike the "straight line" radiations of the 

 types a, /3, and 7, the emanations discovered by Sir 

 Ernest Rutherford are emitted much more freely 

 from some compounds of the radio-active element 

 than from others, while the rate of emission is 

 largely dependent on physical conditions, such as 

 the temperature of the system. By a striking 

 series of experiments, however, Rutherford traced 

 these differences to variations in the ease with 

 which, after formation, the emanation escapes 

 from the generating substance. 



Let us consider these results in more detail. 

 It is found, for example, that while the emanation 

 is given off very slowly from dry and solid radium 

 chloride, it is emitted freely from the same salt in 

 solution. This allows the problem to be submitted 

 to the test of quantitative experiment. The rate 

 of decay of the radium emanation is known ; its 

 activity falls to half value in '^.^ days. Thus, the 

 activity of the emanation stored in a solid radium 

 salt reaches a limit, when its rate of decay becomes 

 equal to the constant rate at which the emanation 

 is produced by the radium. On the hypotheses 

 that the emanation is formed at the same rate in 



