CONTEMPORARY ADVANCES IN PHYSICS 647 



are released, if they are released at all, with a speed corresponding to a 

 range in the neighborhood of 57.5 centimeters. The upper curves 

 coalesce with this second-lowest over the descending arc at the right- 

 hand end; the addition of slower alpha-rays to the bombarding stream 

 does not increase the rate at which the members of this group are liber- 

 ated; the power of dislodging them is confined to particles of a narrow 

 interval of speeds. 



The second-lowest and the middle curve are indistinguishable over 

 the right-hand half of Fig. 7. In going from them to the second-highest 

 curve, however, we meet another significant contrast. Another 

 "group" of protons makes its appearance: its distinctive range is close 

 to 47, it is evidently the very one ^ which was detected by scrutiny of 

 Fig. 6. As alpha-particles slow down, they reach a critical speed at 

 which they acquire the power of releasing this group. The corres- 

 ponding critical range must be lower than (3.72 — 1.26) or 2.46 cm., 

 for otherwise the rise near 47 would appear on the middle curve. It 

 must be higher than (3.72 — 1.88) or 1.84 cm., or the rise would not 

 appear on the second-highest curve. 



Evidently there are both an upper and a lower critical range, R2 and 

 Ri, such that alpha-particles can cause the ejection of the protons of 

 this group if and only if their ranges lie between Ri and R^- The curves 

 of Fig. 7 "bracket" the upper limit of this interval, fixing R2 between 

 1.84 and 2.46. Likewise the curves of Fig. 6 bracket the lower limit, 

 locating it between 2.23 and 2.6. The interval must therefore lie 

 between 2.23 and 2.46. By a more minute analysis of the curves, 

 Pose locates it in the neighborhood of 2.42. 



From the left-hand parts of the curves of Fig. 7, one makes the same 

 deductions as from those of Fig. 6. Whatever their speed (within the 

 scope of these experiments) alpha-particles possess the power of liberat- 

 ing protons with a wide distribution in range. The breadth of this 

 distribution, i.e. the difference between the longest and the shortest 

 ranges comprised within it, decreases with decreasing speed of the 

 particles ; so also does the longest range. 



It appears, therefore, that there are two mechanisms of disruption. 



One seems to be controlled by the internal structure of the nucleus ; the 



alpha-particle serves only to touch it off; it can be touched off, or 



actuated (to use a more dignified word) only by alpha-particles of a 



narrowly-delimited range of speeds; once it is actuated, it ejects a 



proton with a velocity strictly defined. The other accords more closely 



^ The group of range 57 cm. does not appear on tlie uppermost curve of Fig. 6, but 

 Dr. Pose writes me tfiat it was actuafly apparent in tlie data, and ttiat fie deducted 

 it in order to maiie obvious tfie resemblance wliicfi exists between tlie rigfit-liand 

 ends of tfiis and the next two curves when that group is disregarded. 



