191 1.] BOLTW'OOD— RADIOACTIVITY. 337 



supposed to consist of electromagnetic pulses in the ether. A rather 

 ingenious corpuscular theory as to the nature of these rays has been 

 proposed by Bragg, but has not met with general acceptance owing 

 to the fact that it appears to be not altogether in accord with the 

 experimental evidence. 



The origin of gamma rays seems to be very intimately connected 

 with the emission of beta particles, for the two types of radiation 

 have been observed to appear simultaneously and bear a certain defi- 

 nite relation to one another. A\'hen the expelled beta particle has a 

 high velocity the gamma ray is of a very penetrating character, while 

 if the velocity of the beta particle is low the gamma ray emitted has 

 but little power of penetrating ordinary matter. 



The disintegration of the radio-elements takes place according 

 to a verv simple law and the number of atoms of any radio-element 

 which undergo transformation in the unit time is a definite propor- 

 tion of the total number of atoms initially present. The number 

 of atoms A' remaining unchanged after any time t is given by 

 A'^Xoc'^*, where No is the initial number of atoms and A is the 

 fraction undergoing transformation in the unit of time. This frac- 

 tion has a fixed and invariable value for each separate radio-element 

 and for this reason is known as the constant of cluing c for the ele- 

 ment in question. It is a relatively simple matter to determine the 

 period of time required under these conditions for exactly half of 

 any given quantity of a radio-element to be converted into other 

 substances and this time is known as tlic half zxiluc period. The 

 rate of change and the corresponding half value period is a definite 

 characteristic for each of the radio-elements but is very different for 

 the dift'erent radioactive substances. The half-value period of ura- 

 nium, for example, is over five billion years while the half-value 

 period of certain other radio-elements is only a few seconds. 



Although the disintegration of some of the radio-elements has 

 been examined over wide extremes of temperature and pressure, and 

 under various other special conditions which would greatly influence 

 the course of ordinary chemical reactions, it has not been found pos- 

 sible to definitely alter or effect the rate at which transformation 

 takes place to the slightest measurable degree. It is therefore evident 



PROC. AMER. PHTL. SOC., L. 200 V, PRINTED JULY 3I, I9II. 



