the period of change of thorium A is 55 minutes or 11 hours. In order to settle the point, it is necessary to find some means of separating the products thorium A and B from each other. In the case of thorium, this is done by electrolyzing a solution of thorium. Pegram obtained an active product which decayed according to an exponential law with the time falling to half value in a little less than one hour. This result shows that the radiating product thorium B has the shorter period. In a similar way, by recourse to electrolysis, it has been found that the change actinium B has a period of 1.5 minutes. In the case of radium, P. Curie and Danne utilized the difference in volatility of radium B and C in order to fix the period of the changes.
It is very remarkable that the third successive product of radium, thorium and actinium should not give out rays. It seems probable that these rayless changes are not of so violent a character as the other changes, and consist either of a rearrangement of the components of the atom or of an expulsion of an α or β particle with so slow a velocity that it fails to ionize the gas. The appearance of such changes in radioactive matter suggests the possibility that ordinary matter may also be undergoing slow 'rayless changes' for such changes can not be detected in the radio-elements unless the succeeding products emit rays.
It is seen that the changes occurring in radium, thorium and actinium are of a very analogous character and indicate that each of these bodies has a very similar atomic constitution.
While there can be no doubt that numerous kinds of radioactive matter with distinct chemical and physical properties are produced in the radio-elements, it is very difficult to obtain direct evidence in some cases that the products are successive and not simultaneous. This is the case for products which have either a very slow or very rapid rate of change compared with the other product. For example, it is difficult to show directly that radium B is the product of radium A and not the direct product of the emanation. In the same way, there is no direct evidence that radium C is the parent of radium D. At the same time, the successive nature of these products is indicated by indirect evidence.
There can be little doubt that each of the radioactive products is a distinct chemical substance and possesses some distinguishing physical or chemical properties. There still remains a large amount of chemical work to be done in comparing and arranging the chemical properties of these products and in determining whether the successive products follow any definite law of variation. The electrolytic method can in many cases be used to find the position of the product in the electrochemical series. The products which change most rapidly are
