SUCCESSION OF CHANGKs IN KADIO.UTIVE BODIES. 207 



The value of T may be taken as a com punitive measure of the stability of the 

 atoms of each product. The atoms of the radium emanation and thorium X have 

 ahout the same stability. The apparent agreement of the rate of change in the two 

 OBBes must !) .'.HIM.!.-!!'.! M ;i "inriilfin-i-. .-mil i !<>t-s n->t. in ;iny uav. imlir.-iti- tli.-il tin- 

 atoms of the two products are the same, for ThX and the radium emanation dift'er 

 both in physical and chemical properties. If the atoms of the two products were 

 identical, it would be expected that the subsequent changes would take place in the 

 same way and at the same rate ; but such is not the case. The stability of the 

 atoms of the products varies over a very wide range. The stability of the atom of 

 ThX, for example, is about 100,000 times greater than that of the actinium 

 emanation. There is every reason to believe that the radio-elements themselves 

 must be regarded as radioactive products of very slow rate of change. In a case like 

 uranium, which is prolmhly half transformed in about 1,000,000,000 years, the atoms 

 must be considered as very stable compared with products like the emanations of 

 thorium or radium. 



27. Itayk'xs Chans/em. The existence of a well-marked change in radium, thorium, 

 and actinium, which is not accompanied by the expulsion of a or ft jKirticles, is of 

 great interest and imj>ortance. 



Since the rayless changes are not accompanied by any appreciable ionization of the 

 gas, their presence cannot be detected by direct means. The rate of change of the 

 substance can, however, lie indirectly determined, as we have seen, by determination 

 of the variation with time of the activity of the succeeding product. The law of 

 change has been found to be the same as for the changes which give rise to a rays. 

 The rayless changes are thus analogous, in some respects, to the monomolecular 

 changes observed in chemistry, with the difference that the changes are in the atoms 

 themselves, and are not due to a molecular combination of the atoms with another 

 su bstance. 



Tt must be supposed that a rayless change is not of so violent a character as one 

 which gives rise to the expulsion of a or ft particles. The change may l>e accounted 

 for either by supposing that there is a re-arrangement of the components of the atom, 

 or that the atom breaks up without the expulsion of its parts with sufficient velocity 

 to produce ionization by collision with the gas. The latter point of view, if correct, 

 at once indicates the possibility that changes of a similar character may be taking 

 place slowly in the non- radioactive elements ; or, in other words, that all matter may 

 be undergoing a slow process of change. The changes taking place in the radio- 

 elements have been detected only in consequence of the expulsion with great velocity 

 of the parts of the disintegrated atom. .If the a particles had been expelled with a 

 velocity less than 10 s centims. per second, it is improbable that any ionizatiou would 

 have been produced, and the changes, in consequence, could not have been followed 

 by the electric method. 



28. Radiations from the Products. The radiations from the successive products of 



