THE a-, - AND y-RAYS 97 



itself at the Radium Institute of Vienna a few months 

 ago, which gives the rate of change as ^-o^ih part 

 per year. This is more than fifty times slower than 

 the rate of change of radium itself, which has long 

 been established to be about ^th part per year. On 

 the other hand the original uranium is estimated 

 with fair probability to be changing 50,000 times 

 more slowly than ionium, or not much more than 

 io.ooo,ooo,ooo tn P art changing per annum. In the course 

 of 1,000,000,000 years a period beyond what even 

 the geologists claim as the total age of the earth 

 hardly more than 10 per cent, of a given quantity 

 of uranium would change through ionium, radium 

 and so on into other elements. Yet, as has been 

 mentioned, so delicate are our methods, that had 

 radium been the first direct product of the change, 

 an hour's observation on a kilogram of purified 

 uranium would have sufficed to have established the 

 growth beyond all doubt. As it is, the problem took 

 thirteen years. Uranium and thorium are the only 

 two primary radio-elements in the process of change. 

 All the other radio-elements known, and they number 

 thirty-three, are produced from one or other of them 

 in the course of their long sequence of changes. 



But what of the rays themselves, the expulsion of 

 which first drew attention to the phenomenon, and 

 which have furnished the necessary experimental 

 means for the study of the whole problem? Like 

 the X-rays, they do not recognise the optical 

 properties, transparency and opacity, nor, to a great 

 extent, the chemical nature of the matter in their 

 path. They plough through everything, affected 

 primarily only by the density of the absorbing 

 medium, or by the actual mass of the material in 

 their way. Physicists recognise three distinct types 

 of rays the a-, the /3- and the y-rays, the first 

 stopped completely by a sheet of notepaper, but by 



