10 



SCIENCE 



[N. S. Vol. XLIX. No. 1253 



Soddy points out, if thorium (over 232) lost 

 six atoms of helium, and he and Honigschmid 

 have found quantitative evidence of its exist- 

 ence in thorium minerals. 



After reviewing aU the data, Professor F. W. 

 Clarke has brought forward an interesting and 

 reasonahle hypothesis explaining the difference 

 between the several kinds of lead. He points 

 out that whereas we have every reason to be- 

 lieve that uranium and thorium lead are the 

 results of disint^ration of heavier atoms, 

 ordinary lead may be imagined to be the prod- 

 uct of a far earlier synthesis or evolution from 

 smaller atoms. The hypothesis might be sup- 

 ported by the analogy of the synthesis and 

 decomposition of organic substances, which by 

 no means always foUo-w similar paths; it seems 

 to be consistent with most, if not all, of the 

 facts now known. 



On the other hand, may not the uniformity 

 of ordinary lead and its difference from either 

 of the radioactive leads be almost equally ca- 

 pable of interpretation in quite a different 

 fashion? Whenever, in the inconceivably dis- 

 tant past, the element lead was evolved, it is 

 hardly to be supposed that uranium-lead and 

 thoriuna-lead could have been entirely absent. 

 The conditions must have been chaotic and 

 favorable to mixture. Wlien the two or more 

 forms were mixed, n»ne of the processes of na- 

 ture wovdd separate them. Therefore they 

 must appear eons afterwards in an equably 

 mixed state on earth, constituting our ordinary 

 lead. There may have been more than two 

 forms of lead; but two forms, one possessing 

 an atomic weight 206 and the other, an atomic 

 weight over 208, would account for all the 

 facts. The identity in nature of all the com- 

 mon lead on earth might indicate merely that 

 one time all the matter now constituting the 

 earth was liquid or gaseous in violent agita- 

 tion, so that all the kinds of lead were thor- 

 oughly commingled before solidification. This 

 explanation, if it could be confirmed, would 

 furnish important evidence concerning the 

 early history of planets. So far afield may a 

 difference in weight amounting to two units in 

 the twenty-fourth decimal place, between two 

 kinds of atoms so small as to be far beyond the 

 possible range of our most piercing means of 



actual observation, carry the inquiring in- 

 vestigator ! 



The true answers to these questions are not 

 to be found by speculation, such as that just 

 detailed, however suggestive such speculation 

 may be. They are to be found by careful ob- 

 servation. For example, the doubt as to the 

 nature of ordinary lead can only be decided 

 by discovering whether or not it may be sepa- 

 rated into two constituents. Since weight (or 

 mass) is the quality distinguishing between 

 the several isotopes or kinds of lead, weight 

 (or mass) must be made the basis of separa- 

 tion. Hence the only hope of separating iso- 

 topes of lead lies in the method of fractional 

 diffusion, as has been already suggested by 

 many other experimenters on this subject. 

 Promising preliminary experiments prepara- 

 tory to such an undertaking have already been 

 begun at Harvard, and before long more light 

 may be obtained. 



The idea that other elementary substances 

 also may be mixtures of two or more isotopes 

 has been advanced by several chemists. Espe- 

 cially if ordinary lead should really be found 

 to be thus complicated, many, if not all, other 

 elements should be tested in the same way. 

 The outcome, while not in the least affecting 

 our table of atomic weights as far as practical 

 purposes are concerned, might lead to highly 

 interesting theoretical conclusions. 



How can such remote scientific knowledge, 

 even if it satisfies our ever-insistent intellec- 

 tual curiosity, be of any practical use? Who 

 can tell? It must be admitted that the rela- 

 tionship is apparently slight as regards any 

 immediate application, but one can never 

 know how soon any new knowledge concerning 

 the nature of things may bear unexpected 

 fruit. Faraday had no conception of the elec- 

 tric locomotive or the power-plants of Niagara 

 when he performed those crucial experiments 

 with magnets and wires that laid the basis for 

 the dynamo. ^Nearly fifty years elapsed be- 

 fore his experiments on electric induction in 

 moving wires bore fruit in a practical electric 

 lighting system ; and yet more years before the 

 trolley car, depending equally upon the prin- 

 ciples discovered by Faraday, became an every- 

 day occurrence. At the time of discovery, even 



