January 3, 1919] 



SCIENCE 



attempt on record to compute an atomic 

 weight from hypothetical assumptions. Usually 

 we are wholly at a loss as to the theory under- 

 lying the precise relationships, and must 

 determine our values by careful exijeriment 

 alone. 



The value 206.08 for the atomic weight of 

 lead has further supjwrt in the fact that it 

 is more nearly half way between thallium, 

 204, and bismuth, 208. the two neighboring 

 elements in the periodic system, than is the 

 atomic weight 207.2 possessed by ordinary 

 lead. 



It appears, then, that 206, the value pertain- 

 ing to uranium-lead, is a very reasonable 

 value. 



But, as has been repeatedly pointed out, 

 ordinary lead, constituting the vast bulk of 

 the lead in the world, has without doubt a 

 much higher atomic weight, 207.2, not to be 

 expected from either of the lines of reasoning 

 just given. In order to test the uniformity 

 of this circumstance. Professor Baxter, of 

 Har^-ard, with the help of one of his assist- 

 ants, investigated ordinary lead from non- 

 urauiferous ores from many parts of the world, 

 and discovered that the constancy of its quan- 

 titative behavior is as striking as that of 

 copper or silver. His figures agreed ver;v 

 closely, within the limit of error of experi- 

 mentation, with those obtained as a part of 

 the present comparison of the two kinds of 

 lead, so that there could be no question as 

 to lack of identity of methods or precautions. 



Before leaving the subject of the relative 

 atomic weights of these two types of lead, it 

 is not without interest to note the exact 

 absolute weights of the atoms. If, as we have 

 excellent reason for believing on the basis of 

 the brilliant work of Professor R. A. Millikan, 

 a so-called gram-atom (the atomic weight in 

 grams) contains 606.2 sextillion actual atoms, 

 the weights of the atoms of the two kinds of 

 lead must be respectively 342 and 340 sep- 

 tillionth of a gram. Their extreme smallness, 

 as regards bulk, may perhai>s best be inferred 

 from the consideration that the smallest ob- 

 ject visible as a point in the common micro- 

 scope has a diameter probably about one thou- 



sand times as great as an atom of lead.* 



Evidently, on the basis of the quantitative 

 results just exhibited, we must admit that 

 there is at least one real difference between 

 radioactive lead and the common metal. Are 

 there other differences? 



A question as to the density of each sub- 

 stance, and therefore as to the bulk occupied 

 by the respective atoms, at once arises. Since 

 the atom of uranium-lead weighs less than the 

 other, it must occupy less space, supposing 

 that it has the same density; or else it must 

 have less density, supposing that it should 

 occupy the same space. The identity of the 

 chemical behavior of the two tyjies of lead 

 suggests the probability of the latter alterna- 

 tive, and this was therefore assumed by 

 Soddy; but experimental proof was evidently 

 desirable. Therefore an extended investiga- 

 tion of the density of the various kinds of 

 lead was carried out likewise in the Gibbs 

 Memorial Laboratorj'. As a matter of fact, 

 the densities of the several specimens were 

 found to be very nearly proportional to their 

 atomic weights; that is to say, the bulk of the 

 atom of radioactive lead is almost exactly the 

 same as the bulk of the atom of ordinary 

 lead, although the weights of these atoms are 

 so markedly different. 



DENSITIES AND ATOMIC VOLUMES' 



A distinctive property of elementary sub- 

 stances, which has always been supposed to be 

 concerned more or less definitely with the 

 atomic weight, is the spectrum, depending 

 upon the wave-lengths of light emitted by the 

 vapor. But, surprisingly enough, the spec- 

 trum lines produced by these two sorts of lead, 

 when heated to the high temperature of the 

 electric arc, are so precisel.y alike, both as to 



■• If the smallest object visible in a microscope 

 could be enlarged to the width of this printed page, 

 the atoms in it would appear about the size of the 

 dots on the letters i, or the periods, in the type 

 above. 



