1908] on Recent Researches in Radio-activity. 81 



has a period of more than 800 days. Mesothoriiim apparently emits 

 /? rays, and has a still longer period of transformation, the exact 

 value of whicli has not yet been accurately determined. Since 

 thorium is used commercially on a large scale, there is every prospect 

 that we shall soon be able to obtain considerable quantities of very 

 active preparations of mesothorium and radiothorium. The separa- 

 tion of these bodies from thorium does not in any way alter its 

 commercial value. It is to be hoped that if these active preparations 

 are separated in c^uantity, the physicist and chemist may be able to 

 obtain a supply of very active material at a reasonable cost, and that 

 there will not be an attempt to compete Avith the ridiculously liigh 

 prices charged for radium. 



From the radio-active point of view, the radio-elements are only 

 distinguished from their families of products by their comparatively 

 long period of transformation. Now we have reason to believe that 

 radium itself is transformed according to the laws of other radio- 

 active products with a period of about 2000 years. If this be the 

 case, in order to keep up its supply in a mineral, radium must be 

 produced from another substance of relatively long period of trans- 

 formation. The search for this elusive parent of radium has been 

 one of almost dramatic interest, and illustrates tlie great importance 

 of the theory as a guide to the experimenter. The view that radium 

 was a substance in continuous transformation was put forward by 

 Rutherford and Soddy in 1903. The most probable parent of radium 

 appeared to be uranium, which has a period of transformation of the 

 order of 1000 million years. If this were the case, uranium, initially 

 freed from radium, should in the course of time grow radium, i.e. 

 radium should again appear in the uranium. This has been tested 

 independently by Soddy and Boltwood, and both have shown that in 

 carefully prepared uranium solutions there is no appreciable growth 

 of radium in the course of several years. The rate of production of 

 radium, if it occurs at all, is certainly less than rgVo ^^ the amount 

 to be expected from theory. This would appear at first sight to put 

 out of count the view that uranium is the parent of radium. This, 

 however, is by no means the case, for such a result could be very 

 easily explained if one or more substances of very slow period of 

 transformation appeared between uranium and radium. It is obvious 

 that the necessity of forming such an intermediate product would 

 greatly lengthen the time required before an apj^reciable amount of 

 radium appeared. 



There is, however, another indirect but very simple method of 

 attack to settle the parentage of radium. If radium is derived from 

 the transformation of uranium, however many unknown products 

 intervene, the ratio between the amount of .radium and uranium in 

 old minerals should be a definite constant. This is obviously the 

 case, provided sufficient time has elapsed for the amount of radium 

 to have reached its equilibrium value. The constancy of this relation 



