442 



SCIENCE 



[N. S. Vol. XL VII. No. 1218 



as have been the ranges of the turtles. A 

 knowledge of the distribution is of assistance 

 to the herpetologist for the clues to relation- 

 ships which it gives, it aids the student who is 

 not an expert herpetologist to identify his 

 specimens, and it encourages geographical 

 studies and the publication of local lists. In 

 view of its importance in systematic work the 

 subject may well receive careful attention in 

 lists of this kind. 



It should not be concluded that the value of 

 the check list is seriously impaired by the 

 shortcoming just mentioned. The criticism is 

 meant to be constructive, for it must be the 

 hope of aU herpetologists that this very useful 

 book will be the first edition of a permanent 

 check list. That it may be, the reviewer sug- 

 gests that it be officially adopted by the Amer- 

 ican Society of Ichthyologists and Herpetolo- 

 gists, and that the authors be appointed per- 

 manent editors. 



Alexander G. Euthven 



UNivBBSiTr OP Michigan 



SPECIAL ARTICLES 



IDENTITY OF ATOMIC WEIGHT AMONG 

 DIFFERENT ELEMENTS 



AuGUSTE PiccARD has recently suggested^ 

 that the "element" uraniimi may be com- 

 posed of three isotopes, there being in ad- 

 dition to uranium I. and its descendant ura- 

 nium n. a long-lived element of atomic weight 

 " about 240 " which is the parent of the actin- 

 ium series of elements, but has no genetic con- 

 nection with the uranium series. This " actino- 

 mranium " is supposed to undergo an alpha 

 ray change to form uranium T, which through 

 uraniimi Z gives rise to the actinium ele- 

 ments. The hypothesis is attractive for three 

 reasons. It establishes the actinium series 

 as a wholly independent series, as the Geiger- 

 Nuttall relationships between the half-life 

 periods and the alpha ray ranges seem to de- 

 mand. It gives a plausible origin for the 

 puzzling uraniimi Y. Finally, it accounts 

 for the fact that the atomic weight of uranium, 

 instead of being, as would be expected, just 



^Archives des Sciences Physiques et Naturelles, 

 44, 161-64, 1917. 



twelve units higher than that of radium, i. e., 

 238.0, is 238.167 according to Honigschmid's 

 authoritative work; for Piccard assumes that 

 the atomic weight of uranium I. itself, tho 

 chief constituent of the uranium pleiad, is 

 238.0 and that the admixture of the heavier 

 actino-uranium is resjwnsible for the higher 

 value from the analytical determination. 



This hypothesis is so attractive that Wolfke* 

 has already issued a copy of the i)eriodic table 

 of the elements in which the actinitun ele- 

 ments are given atomic weights which follow 

 from the assiunption that the weight of actino- 

 uranium is 240.0. It should be pointed out, 

 however, that this assumption tacitly involves 

 the statement that two elements may occupy 

 the same position in the periodic table, as is 

 commonly accepted for the isotopes, and may 

 in addition have identical atomic weights and 

 yet be different elements. This is a new type 

 of isotopism. In the lead pleiad there are 

 seven elements with atomic weights ranging 

 from 206 to 214, all with identical chemical 

 projverties though differing in stability and in 

 their radiations. According to Wolfke's table 

 not only is the range of atomic weights in 

 this pleiad extended to 216 (for actinium B) 

 so that it covers fully ten units of atomic 

 weight, but there are two elements, actinium 

 D and thorium B, both of which have an 

 atomic weight of 212 and which are there- 

 fore identical in atomic number and atomic 

 weight, and yet the former is apparently 

 stable while the latter has a half-life period 

 of 10.6 hours and emits beta rays. Actinium 

 G'^ and thorium A form another such a pair 

 of elements, actinium X and mesothorium 1 

 stiU another, while the identity between radio- 

 actinium and thoritun itself is perhaps even 

 more striking. With the same atomic num- 

 ber and atomic weight, they are chemically 

 inseparable, they both give alpha rays, yet 

 their periods are 18.9 days and 1.5 X 10"^' 

 years, respectively, and their descendants are 

 quite distinctive. 



According to this hypothesis, then, the 

 atomic weight is almost wholly devoid of in- 



2 ' ' Ueber den inneren Bau der Atome, " Zurich, 

 1917. 



