SOME CONrF.MPOR.IKV .IPr.iXCfS IN PHYSICS ll' AK^ 



the table, all have Intn analyzctl except nine; hut of the next twi-nty- 

 soven elements, only one (mercury) has been analyze<l. These 

 heavier elements and their comjiounds seem generally to he non- 

 volatile and so impregnable by the original method; while they are 

 difficult, if not impossible, to examine by Aston's new scheme, as 

 the traces of the ions ui^on the plate In-comc fainter with increasing 

 mass, and are already extremely faint for the elements in the fifth 

 row of the table. 



Among the eight known elements l)eyond the eighty-second, every 

 one has atoms of several different kinds, alike in physical and chemical 

 properties but different, it is presumed, in mass; but they differ also 

 in another quality, a much more striking quality — they difTer in their 

 degree of instability. Out of a great numlwr of atoms of a radio- 

 active substance, existing at a moment /. one-half will have disinte- 

 grated at a subsequent instant t+T; the interval 7", which is calk-d the 

 half-period of the substance, is the measure of its instability. Like 

 the atomic mass, this half-[x>riod may vary from one kind of atom to 

 another, though lioth kinds have almost identical chemical and physical 

 properties and lx;long to the same element. The three isotopes 

 of the eighty-sixth element, "emanation." have three entirely distinct 

 half-peri(xls: .54 seconds. 3.8.i days and 11.2 days. Moreover, not 

 only the rate but the manner of disintegration may l)e different for 

 different isotof)es of a single element. The six kinds of atoms which 

 share the ninetieth place in the [xricxlic table display this (li\ersity 

 of properties: 



Uranium A'l has a half-period of 23.8 days and its atoms emit 



electrons and electromagnetic waves when breaking up; 

 Uranium Y has a half-jK>riod of 24. G hours and emits electrons; 

 Ionium has a half-perifnl of 9X 10* years and emits helium nuclei; 

 Thorium has a half-period of 2.2X10'" years and emits helium 



nuclei; 

 Radiothorium has a half-jK-riod of l.itO years and eniit> helium 



nuclei; 

 Radioaclinium has a half-jx'riod of 1!> days and emits helium 



nuclei. 



Nor must it he supposed that if each of two isotof)es is stated to 

 emit helium nuclei, they are in that respect identical; for the energies 

 of the emittetl nuclei generally var\- from one isotope to another, so 

 that ever>' one of the six kinds of atoms listed alxjve differs from 

 every other not only in the rate but also in the manner of its dis- 

 integration — and likewise in its ancestry and its posterit\', in the 



