200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
In the case of lead, six isotopes are already known which differ from 
one another either in atomic or radioactive properties. On the nu- 
cleus theory of the atom this indicates that the charges on the nuclei 
are the same, but that the masses differ. The proof of the presence of 
isotopes promises to open up a new and very fundamental field oi 
chemical inquiry which must inevitably exercise a great influence on 
atomic weight determinations and also on our ideas of atomic con- 
stitution. In a recent letter to this journal Merton has indicated that 
the minute change in the wave length of spectrum lines of isotopes 
may give us a simple method of attack on this problem. 
While the subject of radioactivity belongs in essence to the border 
line of physics and chemistry, with affiliations to both sciences, it has 
had numerous connections with other fields of work. The examina- 
tion of the earth's crust has shown that radioactive matter is very 
widely distributed, and has disclosed, notably through the work of 
Strutt and Joly, that the heating effect due to this matter vitiates to 
a large extent the old arguments of the duration of the earth's heat. 
While showing that the old views are not tenable, radioacti^dty has 
at the same time supplied new methods of estimating the age of 
minerals and the dura^tion of geological epochs. The minimum age 
of minerals can be deduced from the helium accumulated from the 
transformation of radioactive matter, and the maximum age from 
the accumulated lead which is the product of both uranium and 
thorium. Now that the atomic weights of the lead isotopes are well 
established, the atomic weight of the lead in a uranium mineral should 
serve as a definite guide to the fraction of lead present which is due 
to the transformation of uranium and thus give a trustworthy esti- 
mate of the age of the mineral. Joly has demonstrated in a striking 
way that the pleochroic halos observed in mica are of radioactive 
origin, and he has also estimated their age. The presence of radio- 
active matter in the atmosphere has been shown to account for its 
electrical conductivity. Just before the war e\ddence was obtained 
indicating the presence of a very penetrating type of y radiation in 
the upper atmosphere. It is to be hoped that soon a further study 
will be made to determine the nature and origin of this interesting 
radiation. Finally, numerous investigations have been carried out 
to determine the effects of the radioactive rays on living tissue and 
on the growth of plants and organisms. With the increased use of 
radium for therapeutic purposes, it is likely that our knowledge of 
this important field of inquiry will grow rapidly. 
It is a matter of remark that while the study of radioactivity hag 
disclosed in a striking way the transformation of heavy atoms 
through a long series of stages, it has at the same time provided us 
with indubitable proof of the correctness of the old atomic theory of 
