CHEMISTRY: F. W. CLARKE 187 
helium, however, is now relatively rare; but there is evidence to make us be- 
lieve that it was largely consumed in building other elements. Its present 
observed emission by radium is evidence in favor of this supposition. 
In this evolutionary hypothesis with its subsidiary speculations there is, 
I think, nothing incompatible with present knowledge. In matters of detail 
it is unavoidably incomplete; but notwithstanding its imperfections it bears 
very^directly upon a consideration of the later phenomena of radioactive decay. 
Here the process of evolution is reversed and rapid changes take the place 
of slow ones. Furthermore, the normal elements are supposed to be veri- 
table store-houses of potential energy; which, in radioactive changes becomes 
partly kinetic. Radium, for example, gives forth heat continuously; and 
its rate of decay can be observed in the laboratory. 
Through the investigation of radioactive transformations more than thirty 
new substances, elements or pseudo-elements, have been discovered. Some 
of these are extremely evanescent, lasting only for seconds or even fractions 
of a second; others are relatively long lived. All of them, however, are 
more or less unstable, and change, slowly or swiftly, into other things. Some 
of them are metallic, like radium, polonium, and actinium; others appear 
as emanations which belong to the group of the chemically inert gases. One 
of these, helium, is continuously being generated from radium. Some, 
again, are iso topic with bismuth or thallium; and four of them are said to 
be isotopes of lead. These are Radium B, Thorium B, Actinium B, and 
Radium D. The first three are short lived, and endure only for a few min- 
utes or hours, but Radium D, also known as radio-lead, is assigned a probable 
life period of 24 years, and given theoretically an atomic weight not far from 
210. In its chemical relations it cannot be distinguished from lead. 
All four of these isotopes may have been present in uranium lead at the 
time of its formation, but it does not seem possible that even a trace of them 
could persist in the lead which is now extracted from uraninite or thorianite. 
They are therefore negligible in our consideration of the evidence which 
is now supplied by the study of the atomic weights, except in so far as they 
show the probable derivation of uranium lead and thorium lead from the two 
higher elements. The essential point is that all these varieties of lead are 
products of degradation, and in that respect differ fundamentally from the 
normal product of evolution. The thirty or more new substances which 
have been revealed to us by the study of radioactivity are all matter in a 
a state of transition from instability towards some stable form, which may 
be lead, or bismuth, or thallium, or some other element which has not yet 
been recognized as an end product of these mysterious changes. As these 
products are approached we have them in an incomplete condition, nearly 
but not quite identical with the permanent elements: This may be the char- 
acter of isotopic lead. The fact that uranium lead is radioactive shows 
that it is still undergoing change; and that its atoms have not acquired the 
