March 5, 1908] 



NA TURE 



constituent of the radio-active substance separated by 

 Hofmann and called by him radio-lead. Radium G is 

 identical with the first radio-active substance separated from 

 pitchblende by Madame Curie, viz. polonium. We are thus 

 sure that these bodies are transformation products of 

 radium. It will be seen that I have added another product 

 of period 4-5 days between radium D and polonium. The 

 presence of such a product has been shown by Meyer and 

 Schweidler. 



In the case of thorium, a very long list of products is 

 now known. For several years thorium X was thought 

 to be the first product of thorium, but Hahn has recently 

 shown that at least two other products of slow transforma- 

 tion intervene, which he has called mesothorium and radio- 

 thorium. The radiothorium emits o rays, and has a 

 period of more than 800 days. Mesothorium apparently 

 emits & rays, and has a still longer period of transforma- 

 tion, the exact value of which 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 separation of these 

 bodies from thorium does not 



in any way alter its commercial tit 



v.-jlue. It is to be hoped that if 

 these active preparations are 

 separated in quantity, the physi- 

 cist 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 with the 

 ridiculously high 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 thorium 

 long period of transformation. lo'i'RS 

 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, actinium 

 radium must be produced from ' 

 another substance of relatively « 



long period of transformation. 

 The search for this elusive parent 

 of radium has been one of almost 

 dramatic interest, and illustrates 



the great importance of the 



theory as a guide to the experi- uranium. 

 mcnter. The view that radium ixn'URS 

 was a substance in continuous 

 transformation was put forward 

 by Rutherford and Soddy in 



1003. 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 in- 

 dependently 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 i, 1000 of the amount to be ex- 

 pected from theory. This would appear at first sight to 

 put out of count the view that uranium is the parent of 

 radium. This, how'ever, is by no means the case, for such 

 a result could be very easily explained if one or more sub- 

 stances 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 appreciable 

 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 con- 

 stancy of this relation has been completely substantiated 

 by the independent work of Boltwood, Strutt, and McCoy. 

 It has been shown that the quantity of radium correspond- 

 ing to I gram of radium is 3.8X10-' gram, and is the 

 same for minerals obtained from all parts of the world. 

 Since the radium is always distributed throughout the mass 

 of uranium, we cannot expect to find nuggets of radium 

 like nuggets of gold, unless by some chance the radium 

 has been dissolved out of radio-active minerals and re- 

 deposited within the last few thousands of years. To 

 those who had faith in the disintegration theory, this 

 unique constant relation between the amounts of two 

 elements was a satisfactory proof that radium stood in a 

 genetic relation with uranium. A search was then made 

 for the unknown intervening product which, if isolated, 

 must grow radium at a rapid rate. A year or so ago 

 Boltwood observed that a preparation of actinium separated 



oorf'd 



22 DAYS 



Succession of Substances produced by the transformation of radium, thorium, actinium, : 

 The period of transformation of each substance is added below. 



from a uranium mineral did grow radium at a constant 

 but rapid rate. It thus appeared as if actinium were the 

 long-looked-for parent of radium, and that actinium and 

 its long family of products intervened between uranium X 

 and radium. I was, however, able to show that actinium 

 itself was not responsible for the growth of radium, but 

 another unknown substance separated with it. These 

 results were confirmed by Boltwood, who finally succeeded 

 in isolating a new substance from uranium minerals, which 

 was slowly transformed into radium. This substance, 

 which he' termed "ionium," has apparently chemical 

 properties similar to those of thorium, and emits o rays 

 of penetrating power less than those of uranium. 



The main previsions of the theory have thus been ex- 

 perimentally verified. Radium is a changing substance 

 the amount of w-hich is kept up by the disintegration of 

 another element, ionium. In order to complete the chain 

 of evidence, we require to show that uranium grows 

 ionium, and it is probable that evidence in this direction 

 will soon be forthcoming. We thus see that we are able 

 to link uranium, ionium, radium, and its long line of 

 descendants, into one family, with uranium as its first 

 parent. As uranium has a period of transformation of 



NO. 2001, VOL. ']']'\ 



