CONTEMPORARY ADVANCES IN PHYSICS 81 



our descendants have solved the other problems of physics, they may 

 be able to entertain themselves by keeping records of the behavior of 

 long-lived radioactive substances, and so determining half-periods with 

 an accuracy improving from millennium to millennium. 



Another and the most picturesque of all the ways of determining a 

 disintegration-constant consists in counting the atoms which in a 

 measured quantity of the substance disintegrate in each second. It 

 sounds almost unbelievable that this should be feasible; but it is 

 really practicable to count the alpha-particles which proceed from a 

 radioactive substance, for they make individual visible scintillations 

 upon a fluorescent screen placed across their paths. If this device is 

 inconvenient, one can measure the total charge which the alpha- 

 particles carry into a chamber arranged to receive them, and divide it 

 by the specific charge borne by each, which is very accurately known. 

 The particles and consequently the transmuted atoms having been 

 counted, it is necessary to weigh the substance which is emitting them; 

 and this requirement is less easy to fulfil, being fulfillable in fact only 

 for three substances — radium, and the long-lived ancestors thorium 

 and uranium. Dividing the mass of the weighed sample by the mass of 

 an atom, and dividing the quotient into the number of alpha-particles 

 emitted per second, we obtain the value of Xi. This of course does not 

 prove that the transmutation is actually proceeding according to the 

 exponential law; that is proved only for certain substances of which 

 the half-periods amount to a few months, days or hours. Nevertheless 

 we assume it, and multiply the reciprocal of Xi so measured by loge 2, 

 and call the product the half-period. The values thus obtained are 

 close to 1700 years for radium, agreeing well with the results of the 

 method just above described; 4.7 billions of years for uranium I, 

 agreeing with the result derived from the relative proportions of 

 uranium and radium in the rocks; and 22 billions of years for thorium. 



There are yet other ways of estimating half-periods, some of them 

 very ingenious. Extremely short-lived substances require special 

 methods. Thoron, a gas with the half-period of fifty-four seconds, is 

 blown with a measured velocity through a tube along which various 

 electrodes are placed for measuring its activity as it flows past them. 

 Actinium A, of which the half-period is only .002 second, is projected 

 upon the rim of a rapidly revolving wheel, and whirled past various 

 instruments which measure its activity at successive points of its 

 transit through space and time. The projection is due to a very 

 simple but none the less striking natural phenomenon; when an alpha- 

 particle is fired out of an atom of its parent-substance actinon, the 

 residual particle — the atom of actinium A — rebounds or recoils like the 

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