468 



SCIENCE 



[N. S. Vol. L. No. 1299 



be scattered into a comparatively broad beam. 

 Geiger and Marsden sbowed not only that 

 mudh small scattering occurred, but also that 

 in passing tbrough tbe atoms of a heavy ele- 

 ment some of the a-particles vrere actually 

 turned back in their path. Considering the 

 great energy of motion of the a-partiele, this 

 is an arresting fact, showing that the a-par- 

 ticle must encounter very intense forces in 

 penetrating the structure of the atom. In 

 order to explain such results, the idea of the 

 nucleus atom was developed in which the main 

 mass of the atom is concentrated in a posi- 

 tively charged nucleus of very small dimen- 

 sions compared with the space occupied by the 

 electrons which surround it. The scattering 

 of a-particles through large angles was shown 

 to be the result of a single collision where the 

 a-particle passed close to this charged nucleus. 

 From a study of the distribution of the par- 

 ticles scattered at different angles, results of 

 first importance emerged. It was found that 

 the results could be explained only if the 

 electric forces between the a-particle and 

 charged nucleus followed the law of inverse 

 squares for distances apart of the order of 

 10-^1 cm. Darwin pointed out that the varia- 

 tion of scattering with velocity was explicable 

 only on the same law. This is an important 

 step, for it affords an experimental proof that, 

 at any rate to a first approximation, the ordi- 

 nary law of force holds for electrified bodies 

 at such exceedingly minute distances. It was 

 also found that a resultant charge on the nu- 

 cleus measured in fundamental units was 

 about equal to the atomic number of the ele- 

 ment. In the case of gold this number is be- 

 lieved from the work of Moseley to be 79. 



Knowing the mass of the impinging a-par- 

 ticle and of the atom with which it collides, 

 we can determine from direct mechanical prin- 

 ciples the distribution of velocities after the 

 collision, assuming that there is no loss of 

 energy due to radiation or other causes. It 

 is important to notice that in such a calcula- 

 tion we need make no assumption as to the 

 nature of the atoms or of the forces involved 

 in the approach and separation of the atoms. 

 For example, if an a-particle collides with 



another helium atom, we should expect the 

 a-particle to give its energy to the heliimi atom, 

 which could thus travel on with the speed 

 of the a-particle. If an a^article collides 

 directly with a heavy atom — e. g., of gold of 

 atomic weight 197 — the a-particle should re- 

 trace its path with only slightly diminished 

 velocity, while the gold atom moves onward in 

 the original direction of the a-particle, but 

 with about one fiftieth of its velocity. Next, 

 consider the important case where the a-par- 

 ticle of mass 4 makes a direct collision with a 

 hydrogen atom of mass 1. From the laws of 

 impact, the hydrogen atom is shot forward 

 with a velocity 1.6 times that of the direc- 

 tion, but with only 0.6 of its initial speed. 

 Marsden showed that swift hydrogen atoms set 

 in motion by impact with a-particles can be de- 

 tected like a-particles by the scintillations 

 produced in a zinc sulphide crystal. Recently 

 I have been able to measure the speed of such 

 H atoms and found it to be in good accord 

 with the calculated value, so that we may con- 

 clude that the ordinary laws of impact may be 

 applied with confidence in such cases. The 

 relative velocities of the a-particles and recoil 

 atom after collision can thus be simply illus- 

 trated by impact of two perfectly elastic balls 

 of masses proportional to the masses of the 

 atoms. 



While the velocities of the recoil atoms can 

 be easily calculated, the distance which they 

 travel before being brought to rest depends on 

 both the mass and the charge carried by the 

 recoil atom. Experiment shows that the range 

 of H atoms, like the range of a-particles, varies 

 nearly as the cube of their initial velocity. If 

 the H atom carries a single charge, Darwin 

 showed that its range should be about four 

 times the range of the a-particle. This has 

 been confirmed by experiment. Generally, it 

 can be shown that the range of a charged atom 

 carrying a single charge is mu^TH, where m is 

 the atomic weight, and u the ratio of the veloc- 

 ity of the recoil atom to that of the a-particle, 

 and E the range of the a-particle before col- 

 lision. In comparison of theory with experi- 

 ment, the results agree better if the index is 

 taken as 2.9 instead of 3. If, however, the 



