IOWA ACADEMY OF SCIENCE 
175 
EVIDENCE FAVORING THE ^RADIOACTIVE DISINTEGRATION OF 
SODIUM AS AN ELEMENT.* 
BY F. C. BROWN. 
By the usual test for radioactivity, i.e., the continued ionization of a gas 
independent of other physical conditions, sodium as an element does not display 
any activity that is definitely greater than that found in all matter. And the 
ionizing activity of ordinary matter is so slight that it can not be stated with 
definiteness whether or not the matter is of itself radioactive. But radioactivity 
implies a more fundamental change than that of emitting matter and energy 
continuously. It implies an atomis disintegration. If a particles are emitted 
the atoms go by leaps and bounds to new atoms of other properties, while if 
j3 and 7 radiations are emitted the wearing away of the atoms must be just as 
certain, though no one has been able to conjecture by what steps the changes 
may take place. 
The fact that a given element does not give out a measurable ionizing 
radiation is not necessarily evidence that it is not radioactive. For example 
we may note the case of Radium C which gives no measurable radiations. Yet 
it disintegrates or decays to half value in 40 years. It is therefore known as 
a radioactive element. 
If sodium is a radioactive element we may at present look for other evidence 
than direct radiations. We shall inquire if in past geologic time sodium has 
accumulated radioactively from other matter or on th^ other hand if sodium 
has disappeared or disintegrated into other forms of matter. 
THE EVIDENCE FROM GEOLOGY. 
The best evidence that we have for considering sodium a radioactive element 
is from geology. If the age of the earth is determined from radioactive data 
and the value accepted, we find that there is not accumulated in the ocean basin 
as much sodium as there should have accumulated during this time. 
Different authorities give the age to range between seventy million and one 
hundred million years. On the other hand, the data of radioactivity gives the 
age to be about ten times as much as the figures noted above. The principles 
of the radioactive method are based on the determination of the amounts of 
helium or lead associated with known quantities of uranium in rocks of 
different epochs. The two principal assumptions that are involved are that 
during the age in question the amount of the uranium and its products 
which give rise to helium shall have remained constant and that the rate of 
production of helium shall have remained unchanged. Naturally these two 
assumptions can not be proved. It can only be said that there is no evidence 
that casts suspicion on these assumptions. 
According to experiments by Rutherford and his colleagues, one gram of 
uranium in equilibrium with its products gives 10.7x10-® cubic centimeters of 
helium per year. Now if we examine the rocks of the different geological 
*This paper was also published in Le Radium, in the October number, 1912. 
