574 Mass of Long-range Particles from Thorium C. 



of range 8'6 cm. While it is difficult to give a definite 

 proof of this scheme of transformation, the general facts 

 strongly support it. 



The emission of a particles of range 11*3 cm. shows that 

 the modes of disintegration are even more complicated than 

 the above. There is no information to guide us as to the 

 origin of these very swift particles, except that they appear 

 to arise from thorium and decrease in number at the same 

 characteristic rate. The system of transformation may be 

 similar to one of those outlined, or thorium C may break up 

 in two ways with the emission of rays of ranges 8' 6 and 

 11*3 cm. 



Suppose, however, 1/10,000 of the atoms of thorium C 

 break up directly with the emission of these very swift 

 particles. The atomic number of the resulting product, 

 viz. 81, is that of thallium, but the atomic weight is 208 instead 

 of 206 observed for ordinary thallium. It is of interest to 

 note that Merton (Proc. Roy. Soc. A, xcvi. p. 393 (1920)) 

 found that thallium from pitchblende residues gave a longer 

 wave-length than ordinary thallium, indicating the presence 

 of an isotope of higher mass. Soddy * considered the question 

 whether thallium could be an end product of the two 

 main branches of thorium C, but found that the amount 

 of thallium in a particular thorium mineral, Ceylon thorite, 

 was far too small to admit of such a possibility. The amount 

 of lead, mainly thorium lead, in this mineral was 0*4 per cent. 

 If thallium results as the end product of the new 7 branch, the 

 amount of thallium should be about '00004 per cent., or for 

 each 100 grams of thorium-lead, 10 milligrams of the thallium 



© ' © 



isotope — supposed stable — should be obtainable. Soddy states 

 that some thallium was found in the mineral, but in very small 

 amount not determined, but certainly less than '005 per cent. 



It would be of interest to examine whether such a thallium 

 isotope is present in thorium minerals in amount to be ex- 

 pected on the above hypothesis. 



I am much indebted to Dr. Ishida and Mr. Chadwick for 



their assistance in counting scintillations. 



© 



Cavendish Laboratory, 

 Dec. 1920. 



* Soddy, ' Nature/ cii. pp. 356, 444 (1919). 



