1908] on Recent Researches in Radio-activity. 38 



uranium, and at the same time for the anomaly observed. This 

 supposes that actinium is a branch descent from some member of the 

 uranium family. It does not appear improbable that at one stage of 

 the disintegration two distinct substances may be produced, one in 

 greater quantity than the other. After the expulsion of an a particle, 

 it may happen that there are two possible arrangements of temporary 

 stability of the residual atom. The great majority of the atoms 

 may fall into one arrangement, and the remainder into the other. 

 Actinium in this case would correspond to the substance in lesser 

 quantity. It would act as a distinct element, and would break up in 

 a different way from the main amount. It is probable that a large 

 amount of accurate work will be required before the position of 

 actinium in the scheme of changes can be fixed with certainty. It is 

 a matter of remark how closely actinium resembles thorium in its 

 series of transformations. It would appear that the atom of actinium 

 has many points in common with thorium, or rather with its product, 

 mesothorium. 



The recent observations on the growth of radium offer a very 

 simple and straightforward method of determining experimentally the 

 period of radium. Suppose that we take a uranium mineral and 

 determine by the emanation method the quantity of radium contained 

 in it. If the immediate parent of radium (i.e. ionium) is next com- 

 pletely separated from the uranium and radium, it will begin to grow 

 radium at a constant rate. Now the rate of growth of radium 

 observed is a measure of the rate of breaking up of the radium parent 

 in the mineral, since before separation the rate of production was 

 equal to the rate of breaking up. Now the growth of radium 

 observed for a short interval, for example, a year, divided by the 

 quantity present in the mineral, gives the fraction of the radium 

 breaking up per year. Proceeding in this way, Boltwood found that 

 the fraction breaking up ])er yeai- is about uoVo» ^^^^^ that the period 

 of radium is about 20(»0 years — a value which Hes between the most 

 ])i'obable values deduced from quite distinct data. 



From an inspection of the radio-active families, it will be seen 

 that out of twenty-six radio-active substances that have been 

 identified, seventeen give out a rays or a and ^ rays, four give out 

 only p rays, and five emit no rays at all. The rayless and /3-ray 

 products are transformed according to the same law as the a-ray pro- 

 ducts, and there is the same sudden change of physical and chemical 

 properties as the result of the transformation. In the case of the 

 substances which tlu'ow off atoms of matter in the form of a particles, 

 there are obvious reasons for anticipating a change in [)]'operties of 

 the substance, but this is not the case for the ray-less or ^-ray 

 products. We must either suppose that the mass of the atom is not 

 appreciably changed by tlie transformation, which consists in an 

 internal rearrangement of the parts of the atom, oi- that the atom 

 expels a particle at too low a velocity to be appreciated by the 



Vol. XIX. (No. Iu2) D 



