616 Dr. H. Geiger and Mr. E. Marsden on tlie Laws of 



diminution in velocity of 'the a particles in passing through 

 the foils, the number of scattered particles increases some- 

 what more rapidly than the thickness. 



Variation with Atomic Weight. 



Assuming that the magnitude of the central charge of the 

 atom is proportional to the atomic weight A, Professor 

 Rutherford has shown that the number of a particles 

 scattered by different foils containing the same number of 

 atoms should be proportional to A 2 . With the thin foils 

 which had to be used experimentally, it was found im- 

 practicable to calculate the number of atoms per unit area by 

 weighing the foils. It proved much more reliable to deduce 

 the required number of atoms from the air equivalent as 

 found by the reduction of the range of « particles by the 

 scintillation method. This method had the advantage that 

 the thickness was determined at the exact part of the foil 

 which served to scatter the a particles, thus eliminating any 

 errors due to variations in the thickness of the foils. Bragg 

 and others have given numbers connecting the thicknesses 

 of foils of various materials and their stopping power, and 

 it has been shown that for different foils of the same air 

 equivalent the numbers of atoms per unit area are inversely 

 proportional to the square roots of the atomic weights. 

 Consequently if the scattering per atom of atomic weight A 

 is proportional to A 2 , the scattering per centimetre air 

 equivalent will be proportional to A 2 x A - *, i. e. to A 3/2 . 



In the experimental investigation the same apparatus was 

 used as in the previous experiments on the variation of 

 scattering with thickness of material. The openings in the 

 disk S were covered with thin foils of different materials, 

 and their thicknesses chosen in such a way that they gave 

 approximately the same effect of scattering. A number of 

 different sets of experiments were made, the foils being- 

 varied in each experiment. The results in a particular 

 experiment are given in Table IV. Columns I. and II. give 

 the foils used and their respective atomic weights. In 

 column III. the air equivalents of the foils are entered. 

 Column IV. gives the number of scintillations observed 

 after correction for the variation in activity of the source 

 and the loss of velocity of the a particles in the foil. 

 Column V. gives the number of scintillations per unit air 

 equivalent of material. In column VI. the values of A 3/ ' 2 are 

 given, and in column VII. the ratios of the numbers of 

 scintillations to A 3/2 are calculated. The figures are constant 

 within the experimental error. 



