xviii THE CLASSIFICATION OF THE ELEMENTS 467 



minerals have given mesothorium and radiothorium. Radium 

 has been prepared in sufficient quantity for an ordinary 

 determination of atomic weight, and the atomic weight of 

 niton, a gaseous emanation produced during the decay of 

 radium, has been determined by special modifications 

 of ordinary methods (Gray and Ramsay, Proc. Roy. Soc., 

 1911, A, 84, 536-550). The chemical properties of other 

 radio-elements can be inferred from the readiness or other- 

 wise with which they are carried down with, or can be 

 separated from, typical elements of smaller atomic weight. 

 On these grounds polonium has been placed in the same 

 family as tellurium, in a vacant space immediately after 

 bismuth. In the same way, actinium is assigned to the 

 same family as lanthanum, the first of the rare-earth 

 elements, and probably occupies the vacant place between 

 radium and thorium. The vacant place between thorium 

 and uranium is perhaps occupied by one of the disintegra- 

 tion-products of uranium (UrX 2 ) with an average life of 

 only i '65 minutes ; but no radioactive element, however 

 transient, has been found to occupy the vacant places 

 below caesium and iodine in the families of the alkali- 

 metals and the halogens respectively. 



The total number of radio-elements that have been sched- 

 uled is about 40. These are all crowded into the narrow 

 range from thallium (204) to uranium (238), covering only 

 12 places in the periodic classification of the elements. It 

 is already occupied by three ordinary elements (thallium, 

 lead and bismuth) in addition to the four radio-elements 

 of known atomic weight (niton, radium, thorium and 

 uranium). As none of the radio-elements are allied to the 

 alkali-metals or the halogens there are really only 10 places, 

 of which 7 are clearly occupied, leaving only 3 vacancies 

 for the 36 remaining radio-elements. This crowding is 

 explained by the existence of ISOTOPIC ELEMENTS, having 

 similar (or perhaps identical) chemical properties (and 



H H 2 



