448 TRANSACTIONS OF SECTION B. 
by fractional methods, whether the radio-element was separable from the ordinary 
element. As in most cases the already known element was a common one, the 
non-separability of the two could be shown by using a f-ray electroscope for the 
relative measurement of the radio-elements, whilst the ordinary element was 
estimated by some gravimetric process. 
In one or two cases, as in that of radium-A and polonium and mesothorium-2 
and actinium, where both elements were radio-active and present in unweighable 
quantities, special electroscopic methods of measurement were devised. 
The results of the work show that :— 
1. Uranium-X and radio-actinium are chemically identical with thorium. 
. Mesothorium-2 is chemically identical with actinium. 
. Radium-A is chemically identical with polonium. 
. Radium-(, thorium-C, actinium-C’, and radium-¥ are chemically identical 
with bismuth. 
5. Radium-B, thorium-B, and actinium-B are chemically identical with lead. 
6. Thorium-D and actinium-) are chemically identical with thallium. 
Bm coh 
In the cases in which the inseparable elements are common elements these 
latter have all atomic weights above 200, and occupy one or other of the last 
twelve places of the periodic table. 
(i) Radio-Elements as Indicators in Chemistry and Physics. 
By G. von Hevesy, Ph.D. 
By means of an a-ray electroscope of ordinary sensitiveness it is possible to 
measure accurately as small a quantity as 10-!7grm. of a radioactive substance 
having a half-value period of one hour. The extraordinary simplicity and at the 
same time sensitiveness with which it is possible to measure these extremely 
small quantities of radioactive bodies makes them of the greatest use not only 
in studying substances in great dilution but also as indicators of physical and 
chemical processes. 
Radioactive indicators may be conveniently divided into two principal groups. 
To the first group belong those whose use as indicators depends only on their 
physical properties, and not on their chemical properties. Some examples of the 
use of radioactive indicators of this kind are the following :— 
It is only necessary to know that the radio-elements composing the active 
deposits are metals in order to test the formula of Arrhenius connecting the 
variation of velocity of solution of metals in acids with the temperature. This 
has been lately carried out by Miss Ramstedt. 
It is known from the kinetic theory that the concentration of a solution 
varies with time, and this problem, which could not be attacked by ordinary 
methods, has been made experimentally feasible by the use of radioactive bodies 
as indicators. (Svedberg, Smoluchowskzi.) 
The existence of colloidal solutions of radio-elements has been lately established 
by Paneth and Godlewski, and experiments have been undertaken on the forma- 
tion and precipitation of these colloids using radioactive indicators. 
The emanations, the only gaseous radio-elements, have been employed to 
establish the validity of the gas laws, especially that of Henry’s law for 
extremely small partial pressures. (Bruhat, Boyle.) 
Fick’s Diffusion Law has also been shown to hold accurately for bodies in 
infinitely small concentration by making use of radioactive substances. 
It is often a question of practical interest to the chemist to know how often 
it is necessary to wash out a pipette or a beaker in order to remove the last 
trace of the solution it had contained. This problem can be investigated with 
extreme ease when radioactive indicators are used. 
The fact, however, that most radio-elements are throughout in all chemical 
properties exactly similar to some of the common elements (for instance, radium 
D and thorium B are non-separable from lead, thorium C and radium E from 
bismuth, &c.) allows these bodies to be used chemically as indicators of the 
bodies from which they are known to be non-separable. Radium E can be used 
as an indicator for bismuth, radium D for lead, &c. 
