April 3, 1919] 



NATURE 



95 



but apparently when once they are mixed no chemical 

 method can separate them, since the properties of 

 the different kinds are so nearly alike. The latest 

 attempt at the Gibbs Memorial Laboratory involved 

 une thousand fractional crystallisations of the Aus- 

 tralian lead nitrate, which is believed to contain both 

 ordinary and uranium-radium lead. The extreme 

 fraction of the crystals (representing the least soluble 

 portion, if any difference in solubility might exist) 

 gave within the limit of error the same atomic weight 

 as the extreme fraction of the mother liquor (repre- 

 senting the most soluble portion), thus confirming 

 the work of others in this direction. 



When wires constructed of two different metals are 

 joined, and the junction is heated, an electrical potential 

 or electromotive force is produced at the junction. 

 This property seemed, then, to be a highly interesting 

 one to test in order to find out how great may be 

 the similarity of the two kinds of lead. In fact, 

 wires made of radio-active lead and ordinary lead 

 tested in the Gibbs laboratory gave no measurable 

 thermo-electric effect, the wires acting as if they were 

 made of the same identical substance, although the 

 atomic weights and densities were different. No other 

 known case of this sort is known, so far as I am 

 aware. The melting points of the two kinds of lead 

 were likewise found, with the assistance of N. F. 

 Hall, to be identical within the probable accuracy of 

 the experiment. 



Let us bring all these results together into one 

 table, so that we may better grasp their combined 

 significance. 



Summed up in a few words, the situation appears 

 to be this : At least two kinds of lead exist— one. the 

 ordinary metal disseminated throughout the world, in 

 non-uraniferous ores ; another, a form of lead_ ap- 

 parently produced by the decomposition of uranium, 

 radium" being one of the intermediate products. \i 

 we leave out of consideration the probably inessential 

 difference in radio-activitv, the two kinds are very 

 closelv, if not exactly, alike in ever>- respect except- 

 ing atomic weight, density, and immediately related 

 properties involving weight, such as solubility. 

 Thorium-lead appears to be a third variety, with 

 -;imilar relations. Shall we call these substances 

 different elements, or the same? The best answer is 

 that proposed bv Soddy, who invented a new name 

 and called them' " isotopes " of the same element. 



Comparison of Properties of Different Kinds of 

 Lead.* 



Common Mixture Uranio- Percentage 

 lead (Australian) lea! difference 



ABC A-B AC 



Atomic weight 207-19 206-34 20608 0-42 0-54 



Density .: ms? " •28^ "'273 0-42 0-56 



Atomic volume ... 18-277 18-278 

 Melting point (abso- 

 lute)' 600-53 600-59 



Solubility (metal as 



nitrate) 37-28i 37-130 — 0-4I — 



Refractive index of 



nitrate 17815 I78'4 - o oi - 



Thermo-electric effect — — — o 00 



Spectrum wave-length — — - Q-OO O-QO 



Since everv new fact concerning the behaviour o# 

 the elements' gives a new possible means of discover- 

 ing something about their nature, and smce these 

 fact^ arc of an especially significant kmd, the anomajv 

 is of more than passing interest, and mav be said 

 to constitute one of the most interesting. and puzzling 

 situations now presented to the chemist who looks 

 for the deeper meanings of things. 



4 For the sake of better romparison, the value* given are «" »ho»e ^""^ 

 at Harvard, since they all involved nearly the same material. The faults ol 

 experiments els.where, so far as they cover the same ground, are .tsentially 

 identical. 



NO. 2579, VOL. 103] 



18-281 O-OI 0-02 



— o-oi — 



Many new queries arise in one's mind from a study 

 of the data. Among them is a question as to the 

 nature of ordinary lead, which possesses a less reason- 

 able atomic weight than the radio-active variety. Why 

 shpuld this state of things exist? 



Ordinary lead may be either a pure substance or 

 else a mixture of uranium-lead with lead of yet 

 higher atomic weight, perhaps 208. The latter sub- 

 stance might be formed, as Soddy points out, if 

 thorium (over 232) lost six atoms of helium, and he 

 and Honigschmid have found quantitative evidence of 

 its existence in thorium minerals. 



After reviewing all the data. Prof. F. W. Clarke 

 has brought forward an interesting and reasonable 

 hypothesis explaining the 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 evolution 

 from smaller atoms. The hypothesis might be sup- 

 ported by the analogy of the syndiesis and decom- 

 position 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 

 radio-active leads be almost equally capable of inter- 

 pretation in quite a different fashion? Whenever, 

 in the inconceivably distant past, the element lead 

 was evolved, it is scarcely to be supposed that 

 uranium-lead and thorium-lead could have been 

 entirely absent. The conditions must have been . 

 chaotic and favourable to mixture. When the two 

 or more forms were mixed, none of the processes of 

 Nature would separate them. Therefore they must 

 appear aeons 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 above 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 constituting the 

 earth was liquid or gaseous in violent agitation, so 

 that all the kinds of lead were thoroughly commmgled 

 before solidification. This explanation, if it could be 

 confirmed, would furnish important evidence con- 

 cerning 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 observa- 

 tion, carry the inquiring investigator ! 



The true answers to these questions are not to be 

 found by speculation such as that just detailed, how- 

 ever 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 be 

 decided onTv bv discovering whether or not it^ niay be 

 separated 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 separation. Hence the onlv hope 

 of separating isotopes of lead lies in the method ot 

 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 ha.s been ad- 

 vanced by several chemists. Especially if ordinary 

 lead should really be found to be thus complicated, 



