218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



difference between the several kinds of lead. He points out that 

 whereas we have every reason to believe that uranium and thorium 

 lead are the results of disintegration of heavier atoms, ordinary lead 

 may be imagined to be the product of a far earlier synthesis or evolu- 

 tion from smaller atoms. The hypothesis might be supported by the 

 analogy of the synthesis and decomposition of organic substances, 

 which by no means always follow similar paths ; it seems to be con- 

 sistent 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 

 capable of interpretation in quite a different fashion? Whenever, 

 in the inconceivably distant past, the element lead was evolved, it is 

 hardly to be supposed that uranium-lead and thorium-lead could 

 have been entirely absent. The conditions must have been chaotic and 

 favorable to mixture. When the two or more forms were mixed, 

 none of the processes of nature would separate them. Therefore they 

 must appear millions of years 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 common lead on 

 earth might indicate merely that at one time all the matter now con- 

 stituting the earth was liquid or gaseous in violent agitation, so that 

 all the kinds of lead were thoroughly commingled before solidifica- 

 tion. This explanation, if it could be confirmed, would furnish im- 

 portant evidence concerning the early history of planets. So far 

 afield may a difference of half a per cent in w T eight 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 specula- 

 tion, such as that just detailed, however suggestive such speculation 

 may be. They are to be found by careful observation. For example, 

 the doubt as to the nature of ordinary lead can only be decided by 

 discovering whether or not it may be separated into two constituents. 

 Since weight (or mass) is the only known quality distinguishing 

 between the several isotopes or kinds of lead, weight (or mass) must 

 be made the basis of separation. Hence the chief hope of separating 

 isotopes of lead lies in the method of fractional diffusion, as has been 

 already suggested by many other experimenters on this subject. 

 Promising preliminary experiments preparatory to such an under- 

 taking 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- 



