494 Prof. E. Butherford. [May 27 ; 



The decay of activity of emanation X of radium depends greatly On 

 whether the a or rays are used as a means of measurements. The 

 curves obtained by the /3 rays are always identical with those obtained 

 by the y rays, showing that the /3 and y rays always occur together 

 and in the same proportion. The complicated decay curves obtained 

 for the different types of rays, and for different times of exposure, can 

 be completely explained on the supposition that there are three rapid 

 successive changes in the matter deposited by the emanation, viz. : — 



(1) A rapid change, giving rise only to a rays, in which half the 

 matter is transformed in about 3 minutes. 



(2) A " rayless " change in which half the matter is transformed in 

 21 minutes. 



(3) A change giving rise to a, /3 and y rays together, in which half 

 the matter is transformed in 28 minutes.* 



A similar rayless change is shown to occur in the " emanating 

 substance " of Giesel. 



The occurrence of a rayless change in the three radio-active bodies 

 is of considerable interest. Since the change is not accompanied by 

 rays, it can only be detected by its effect in the change or changes 

 which follow. The matter of the rayless change is transformed 

 according to the same law as the other changes. The rayless change 

 may be supposed to consist either of a rearrangement of the 

 components of the atom or a disintegration of the atom, in which the 

 products of the disintegration are not set in sufficiently rapid motion 

 to ionise the gas or to affect a photographic plate. The significance of 

 the rayless changes is discussed, and the possibility is pointed out that 

 similar rayless changes may occur in ordinary matter ; for the changes 

 taking place in the radio-active bodies would probably not have been 

 detected if a part of the atom had not been expelled with great 

 velocity. 



The radiations from the different active products have been 

 examined and it is shown that the /3 and y rays appear only in the 

 last rapid change of each of the radio-elements. The other changes 

 are accompanied by the emission of a particles alone. 



Evidence is given that the last rapid change in uranium, radium, and 

 thorium, which gives rise to (3 and y rays, is far more violent and 

 explosive in character than the preceding changes. There is some 

 evidence for supposing that, in addition to the expelled a and /? 

 particles, more than one substance is produced as a result of the 

 disintegration. 



* A statement of the nature of the three changes occurring in emanation X of 

 radium was first given in a paper by Rutherford and Barnes ('Phil. Mag.,' Feb., 

 1904). A brief account of the theory from which the results were deduced his 

 been given in my book "Radioactivity" (Cambridge University Press). Later, 

 Curie and Danne (' Comptes Rendus,' March 14, 1901) arrived, in a similar way, at 

 the same conclusions. 



