CONSTITUTION OF MATTER — RUTHERFOED. 185 



period probably long compared with the age of many of the min- 

 erals in which uranium is found. 



The complete sequence of changes in the uranium-radium series 

 is shown in the diagram (fig. 4) . The nature of the radiation and the 

 half period of transformation is added for each element. In addition 

 to uranium, there are two other radioactive elements, thorium and 

 actinium, which are transformed with the appearance of a number 

 of new substances. The time at my disposal, however, is too short 

 to discuss these changes in detail. Thorium is known to be a pri- 

 mary element whose radioactive life is even longer than uranium, 

 but actinium is believed to be a branch descendant from some point 

 of the uranium series, and is thus to be regarded as a product of 

 that element. In all, 34 of these radioactive substances have been 



Q-^Qr 



UtI UrX,. UrX,. Ur2.. lo. Ra. Eman" 



iA-J"^ \J>2J^ \J,5J \3X/^ v^y^ \3AJ \82/' 



RaA Rab. RaC. RoD. RjE. , RjF. , Lead. 



(polonium) 



3-Oininb. 268 mins. 195 mins. l&^i^ean. 5^45. I36daijs. 



Fig. 4.— Successive substances produced by the transformation of the uranium atom. 



discovered, and the position of each in the three main radioactive 

 series has been determined. 



Each of these new substances is to be regarded as a distinct chemi- 

 cal element in the ordinary sense, but differs from ordinary stable 

 elements in the spontaneous emission of special radiations which 

 accompanies the disintegration of the atoms. The radioactive sub- 

 stances are thus transition elements which have a limited life and 

 which carry within themselves the seeds of their own destruction. 

 Not only are these transition elements distinguished by their types 

 of radiation but also by their distinct physical and chemical proper- 

 ties. The extraordinary differences in properties which sometimes 

 exist between a product and its parent substance is well illustrated 

 by the comparison of radium and its product, the emanation. Ra- 

 dium is a solid element of atomic weight, 22G, which has chemical 

 properties allied to barium, but is capable of separation from it. The 

 emanation is a heavy monatomic gas of atomic w^eight, 222, which, by 

 its absence of chemical properties, is allied to the well-known group 

 of rare gases — helium, argon, neon, xenon, and krypton. In some 

 cases the elements show almost identical physical and chemical prop- 



