574 Sir E. Rutherford on Collision of 



The results show that if the collisions of a particles with 

 helium atoms give any long-range scintillations of the order 

 of 28 cm. range, their number is very small compared with 

 that produced in hydrogen under similar conditions. We 

 may consequently conclude that the swift helium atoms 

 produced by collision carry a double charge like the 

 a particle. 



A few experiments have been made to test whether the 

 atoms of lithium, boron, or beryllium have the range to be 

 expected if they carry a single charge. The salts Li 2 C0 3 , 

 B 2 3 , BeO were spread in a thin layer over the active source 

 which was inclined at a small angle with the horizontal, and 

 determinations made of the number of scintillations beyond 

 the range of the a. particle. The air was exhausted and the 

 a particles absorbed in aluminium and silver foils. No 

 certain evidence was obtained of the presence of appreciable 

 numbers of scintillations at the ranges to be expected if the 

 atoms carry a single charge. Experiments of this kind are 

 not easy on account of the difficulty of obtaining thin 

 uniform films of the salts or metal under examination, and 

 of the necessity of getting rid of all traces of hydrogen 

 and water vapour, which give rise to numerous H atoms. 

 It is intended later to make a systematic examination of 

 these elements to determine the range of the atoms produced 

 by close collisions with a particles. 



Experiments in Air and Oxygen. 



Experiments on the range of swift atoms become much 

 easier and more certain when the elements are in the gaseous 

 state, for there is then no uncertainty with regard to the 

 uniformity of the absorbing column and usually no difficulty 

 in ensuring absence of hydrogen and water vapour. Thin 

 films of rolled metals like aluminium, silver, or gold are 

 usually very irregular in thickness. This irregularity comes 

 out very obviously when intense sources of radiation are 

 employed under conditions when one in a million of the 

 incident particles can be detected. It is not unusual in 

 these cases to find that « particles can be detected at a 

 distance 10 per cent, beyond the average range of the 

 a particles as determined by ordinary methods. Mica films 

 are very uniform and show none of these irregularities, but 

 unfortunately mica contains both hydrogen and oxygen and 

 gives rise to numerous H and atoms beyond the range of 

 the bombarding a. particles. 



We have seen that both N and atoms carrying a 

 single charge should be detected beyond the range of the 



