RADIOACTIVE LEAD RICHARDS. 209 



of quartz in a newly hewn piece of granite seems, and probably is, as 

 compact and perfect as it was just after it was formed, eons ago. We 

 can not imagine that any of its properties have essentially changed 

 during its protracted imprisonment; and, so far as we can guess, 

 the silicon and oxygen of which it was made may have existed for 

 previous eons, first as gas, and then as liquid. The relative weights 

 in Avhich these two elements combine must date at least from the 

 inconceivably distant time when the earth " was without form and 

 void." 



Although, apparently, these numbers were thus determined at the 

 birth of our universe, they are, philosophically speaking, in a differ- 

 ent class from the purely mathematical constants such as the relation 

 of circumference to the diameter of a circle. 3.14159 ... is a geomet- 

 rical magnitude entirely independent of any kind of material, and it 

 therefore belongs in the more general class of numbers, together with 

 simple numerical relations, logarithmic and trigonometric quantities, 

 other mathematical functions. On the other hand, the atomic 

 weights of the primeval elements, although less general than these, 

 are much more general and fundamental than the constants of as- 

 tronomy, such as the so-called constant of gravity, the length of the 

 day and year, the proper motion of the sun, and all the other incom- 

 mensurable magnitudes which have been more or less accidentally 

 ordained in the cosmic system. The physicochemical constants, such 

 as the atomic weights, lie in a group between the mathematical con- 

 stants and the astronomical " constants," and their values have a 

 significance only less important than the former. 



In the lead from uranium we have a comparatively youthful ele- 

 mentary substance, which seems to have been formed since the rocks 

 in wdiich it occurs had crystallized. Is the atomic weight of this 

 youthful lead identical with that of the far more ancient common 

 lead, which seems to be more nearly contemporary as to its origin 

 with the silicon and oxygen of quartz ? 



The idea that different specimens of a given element might have 

 different atomic weights is by no means new — it far antedates the 

 discovery of radioactivity. 



Ever since the discovery of the definite combining proportions of 

 the elements and the ascription of these proportions to the relative 

 weights of the atoms, the complete constancy of the atomic weights 

 has occasionally been questioned. More than once in the past in- 

 vestigators have found apparent differences in the weights of atoms 

 of a single kind, but until very recently all these irregularities have 

 been proved to be due to inaccurate experimentation. Nevertheless, 

 even 30 years ago the question seemed to me not definitively an- 

 swered, and careful experiments were made with copper, silver and 

 sodium,- obtained from widely different sources, in the hope of find- 



