214 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1018. 



gram-atom (the atomic weight in grams) contains 006.2 sextillion 

 actual atoms, the weights of the atoms of the two kinds of lead must 

 be respectively 342 and 340 septillionths of a gram. Their extreme 

 smallness, as regards bulk, may perhaps best be inferred from the 

 consideration that the smallest object visible as a point in the com- 

 mon microscope has a diameter probably about one thousand times 

 as great as an atom of lead. 1 



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 differ- 

 ences ? 



A question as to the density of each substance, 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 types of lead sug- 

 gests the probability of the latter alternative, and this was, there- 

 fore, assumed by Soddy; but experimental proof was evidently 

 desirable. Therefore, an extended investigation of the density of 

 the various kinds of lead was carried out likewise in the Gibbs 

 Memorial Laboratory. 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 substances, which has always 

 been supposed to be concerned more or less definitely with the atomic 

 weight, is the spectrum, depending upon the wavelengths of light 

 emitted by the vapor. But, surprisingly enough, the spectrum lines 

 produced by these two sorts of lead, when heated to the high temper- 

 ature of the electric arc, are so precisely alike, both as to their wave- 

 lengths and their intensities, that no ordinary spectrum analysis 

 shows any difference whatever. This has been proved by careful 



1 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 let:ers i, 

 or the periods in the type used in this footnote. 



