CONTEMPORARY ADVANCES IN PHYSICS 137 



the symbol o«^ in equation (5) standing for a neutron. When there 

 are three fragments, conservation of momentum no longer demands 

 that the available energy be equally divided among the three, but 

 admits of an infinity of distributions. It is not difficult to find the 

 highest fraction of Ti which either of the two alpha-particles in case 

 (5), or any of the three in case (6), may receive; this amounts to very 

 nearly one-half in the former, to two-thirds in the latter case. 



In equation (5) the rest-masses of all the charged nuclei are known; 

 that of the neutron is still subject to some controversy, but if we 



Fig. 15 — Various tracks produced during bombardment of lithium by deutons. 

 (Dee & Walton; Proceedings) 



tentatively put Chadwick's value 1.0065 for it we get for (Ti — To) 

 the value 16 millions of electron-volts. 2"o again is negligible, so that 

 we are to compare half of this figure with the energy corresponding 

 to the range 7.8 cm. — the right-hand end of the sloping part of the 

 curve of Fig. 9 — which is 8.3 millions. The agreement is entirely 

 satisfactory. With boron the result is not so pleasing, for Ti by 

 equation (6) should be more than eleven millions, and two-thirds of 

 this differs rather seriously from the energy-value corresponding to the 

 end of the curve of Fig. 11, which is 6 millions. Kirchner got a 

 photograph in which three coplanar tracks of the same appearance 



