﻿426 Sir E. Rutherford and Dr. J. Chadwick on the 



It should be pointed out that the ranges of the forward 

 particles from boron, fluorine, sodium, and phosphorus may 

 be subject to considerable error, owing to the use of a film of 

 powder as the bombarded material. The particles of 

 maximum range are produced on the surface of the grains of 

 powder, and therefore to find the true range the size and air 

 equivalent of the grains of powder must be known. For the 

 ranges given above it has been assumed that the grains were 

 uniform in size and an average value of the air equivalent of 

 the film of powder has been calculated from its weight per 

 sq. cm. The ranges so determined are obviously somewhat 

 less than the true ranges. The ranges of the backward 

 particles are, of course, not subject to this source of error. 



It was observed that the number of particles liberated 

 from the different elements appeared all to be of the same 

 order of magnitude when allowance is made for the differ- 

 ence in range. In our original experiments we found that 

 the number ojj particles from boron was somewhat smaller 

 than the numbers from the other elements, but this was 

 due to the use of an irregular film. Using a film of more 

 finely powdered boron it was found that the number of 

 particles from boron was about the same as from the other 

 elements. 



Examination of other Elements. 



In our former experiments we examined all the light 

 elements, with the exception of the rare gases, as far as 

 calcium. Of these only the six elements of the above table 

 were found to emit H particles in detectable amount under 

 the bombardment of & rays. As was pointed out in that 

 paper, the atomic masses of these elements can be represented 

 by 4n + a where n is a whole number, a result which 

 receives a simple explanation on the assumption that the 

 nuclei of these elements are composed of helium nuclei of 

 mass 4 and hydrogen nuclei. On the other hand, some 

 of the light elements which gave no detectable number of H 

 particles also had atomic masses given by 4w + a. It was 

 thus a point of great importance to repeat the examination of 

 these elements with the improved microscope, and to search, 

 if possible, for the emission of particles of shorter range than 

 free H nuclei. In some cases it was only possible, on account 

 of hydrogen contamination of the materials, to observe at 

 absorptions greater than 30 cm. of air, while in others the 

 observations were carried well within this range. 



Lithium was examined as oxide and as metal, a thin sheet 

 of the latter being obtained by pressing molten lithium 



