CONTEMPORARY ADVANCES IN PHYSICS 645 



curves do enter it, and there is a very important feature of their trend: 

 all are horizontal from 30 to 45, then all drop sharply to the axis at the 

 same abscissa. Thus the piling-up of obstruction in the way of the 

 protons does not stop them, so long as the air-equivalent is less than 

 45. But the adding of three further centimeters of air suffices to bar 

 them all. It is as though the nuclei held protons in such a way, that 

 if ejected at all they would automatically be ejected with velocities 

 entailing ranges which lie within this narrow interval of 45 to 48; and 

 alpha-particles acquired the power of setting off the mechanism by 

 which these protons are ejected, when and only when their own range 

 became as great as a critical value somewhere between 2.23 and 2.6. 



Now travel back along the topmost curve into the left-hand half of 

 the figure. The rise to the left of abscissa 30 suggests a second group 

 of protons, having ranges slightly below this amount. But one notices, 

 first, that the rise extends over an interval much wider than that of the 

 steep sharp climbs at the right-hand ends of the curves; beginning at 30, 

 it seems to be still going on at 18. This implies a broad distribution- 

 in-range. One notices next that in the second curve, the correspond- 

 ing rise begins at an abscissa somewhat smaller; in the third, at one 

 w^hich is smaller yet. Moreover, it is easy to draw a smooth curve 

 through the starting-points of these three rises, which on being 

 smoothly prolonged passes near to the points where the two remaining 

 arcs in the lower left-hand corner ascend from the axis of abscissae. 

 All this suggests that in every one of these cases there are protons dis- 

 tributed over a wide interval of speeds, extending upward to a maxi- 

 mum which itself is greater, the higher the energy of the impinging 

 alpha-rays. 



Turn now to Fig. 7. Here we have five curves corresponding to five 

 foils of aluminium, one face of each being exposed to alpha-rays of 

 polonium with their full energy and undiminished range of 3.72 cm. 

 The bottom curve relates to the thinnest foil, equivalent in thickness to 

 0.15 cm. of air. Actually, the distance between its two sides was the 

 equivalent of 1 mm. of air, but many of the alpha-particles traversed it 

 obliquely, so that the atoms of the foil were exposed to the blows of 

 particles varying in range from 3.72 to 3.57 cm.; to this interval of 

 ranges, therefore, the lowest curve refers. Similarly, the second curve 

 from the bottom relates to a foil of air-equivalent 0.62 cm. for the 

 most oblique of the particles, therefore to atoms bombarded by alpha- 

 rays of ranges varying from 3.72 to 3.1 ; the other three, to sheets with 

 the air-equivalents marked beside them. The difference between the 

 second curve and the first is the effect of alpha-rays having ranges be- 

 tween 3.57 and 3.1. Thus, in going from one curve of Fig. 7 to the 



