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LI. The J\Jass of the Long-range Particles from Thorium C. 

 By Sir E. Rutherford, F.E.S., Cavendish Professor of 

 Physics, University of Cambridge *. 



IT is well known that thorium C disintegrates in two ways 

 with the emission of a particles of range §'6 cm. and 

 5-0 cm at 15° C. In 1914, Dr. A. B. Wood and myself 

 showed that a small number of particles — about 1 in 10,000 

 of the total — were expelled with the long range of 113 cm. 

 The range and number of these particles were determined 

 by the scintillation method, and from the brightness of the 

 scintillations it was supposed that the expelled particles 

 were atoms of helium. In a subsequent paper the writer 

 showed that the passage of a particles from radium C through 

 nitrogen and oxygen led to the production of a small number 

 of swift particles, which had a range 1*29 times that of the 

 impinging a particles. Since the ratio of the ranges of the 

 thorium particles, viz. 11'3 cm. and &'6 cm., is of about the 

 same magnitude, viz. 1 32, it was suggested that possibly the 

 long-range particles emitted by thorium C might arise from 

 collision of the a particles of range S'6 cm. with the oxygen 

 of the mica, which was used as an absorbing screen to cut off 

 the particles of range 8'6 cm. This, however, seemed unlikely, 

 .as the number of the long-range particles from thorium C 

 were about ten times greater than would have been expected 

 en this hypothesis. In order to make certain of this point, 

 Dr. A. B. Wood kindly undertook to repeat the experiments, 

 using aluminium instead of mica as an absorbing screen. 

 This experiment, an account of which is given in an accom- 

 panying paper, showed conclusively that the long-range 

 particles could not be ascribed to the oxygen in the mica; 

 and at the same time a more accurate estimate was made of 

 the relative number of long-range particles, which were found 

 to be 1/10,000 of the total number of a. particles from thorium C 

 In the course of recent experiments the writer obtained 

 some evidence that the short-range particles of the a-ray 

 type of the range 9 cm. appearing in oxygen and nitrogen 

 were not atoms of oxygen or nitrogen, but atoms of mass 

 about 3. It was a matter of great interest to examine 

 whether such atoms were liberated in radioactive changes 

 in addition to atoms of helium of mass 4. If this proved to 

 be the case, it would afford a more direct method of deter- 

 mining the mass of the new atoms with accuracy, since they 



* Communicated by the Author. 



