PRESENT PROBLEMS OF RADIOACTIVITY 173 



stance is proportional to the number of atoms which break up per 

 second ; and since each atom in breaking up gives rise to one atom 

 of the next product together with a or /? particles or both, the activity 

 of the parent is a measure of the rate of production of the succeeding 

 product. 



Of these radioactive products, the radium emanation has been very 

 closely studied on account of its existence in the gaseous state. It has 

 been shown to be produced by radium at a constant rate. The amount 

 of emanation stored up in a given mass of radium reaches a maximum 

 value when the rate of supply of fresh emanation balances the rate of 

 change of the emanation present. 



If q be the number of atoms of emanation produced per second by 

 the radium and N the maximum number present when radioactive 

 equilibrium is reached, then N = *, where X is the constant of 

 change of the emanation. This relation has been verified experi- 

 mentally. The emanation is found to diffuse through air like a gas of 

 heavy molecular weight. It is unattacked by chemical reagents, and 

 in that respect resembles the inert gases of the argon family. It con- 

 denses at a definite temperature 150C. Its constant of change is 

 unaffected between the limits of temperature of 450C and 180C. 

 Since the emanation changes into a non-volatile type of matter which 

 is deposited on the surface of vessels, it was to be expected that the 

 volume of the emanation should decrease according to the same law, 

 as it lost its activity. These deductions, based on the theory, have 

 been confirmed in a striking manner by the experiments of Ramsay 

 and Soddy. The radium emanation was chemically isolated and found 

 to be a gas which obeyed Boyle's law. The volume of the emanation 

 observed was of the same order as had been predicted before its sepa- 

 ration. The volume was found to decrease with the time according to 

 the same law as the emanation lost its activity. Ramsay and Collie 

 found that the emanation had a new and definite spectrum similar in 

 some respects to that of the argon group of gases. 



There can thus be no doubt that the emanation is a transition sub- 

 stance with remarkable properties. Chemically it behaves like an 

 inert gas, and has a definite spectrum, and is condensed by cold. But, 

 on the other hand, the gas is not permanent, but disappears, and is 

 changed into other types of matter. It emits during its changes about 

 one million times as much energy as is emitted during any known 

 chemical change. 



From the similarity of the behavior of the emanation of thorium 

 and actinium to that of radium, we may safely conclude that these 

 also are new gases which have only a limited life and change into other 

 substances. 



The non-volatile products of the radioactive bodies can be dis- 

 solved in strong acids and show definite chemical behavior in solution. 



