January 15, 1915] 



SCIENCE 



75 



Eutherford revived it in 1911 to explain 

 phenomena observed by Geiger and Mars- 

 den,^ and achieved a striking success. The 

 facts observed were these; when a-rays 

 were allowed to pass through thin sheets of 

 metal, a small proportion of them were ob- 

 served to be deflected through very large 

 angles. Eutherford' made a theoretical 

 examination of the results of a single en- 

 counter between an a-particle and an atom 

 of the concentrated-nucleus type, and cal- 

 culated the proportion of the a-partieles 

 which would be deflected through various 

 angles by such encounters. Geiger^ then 

 made a new experimental study of the 

 scattering produced by gold foil and found 

 a very satisfactory verification of Euther- 

 ford 's formula. From the amount of scat- 

 tering at various angles, the value of the 

 nucleus charge was also calculated. For 

 gold it came out about 100 e. The general 

 conclusion was reached that the nucleus 

 charge is about one half the atomic weight 

 times the charge of an electron. But 

 Barkla" had earlier reached the same value 

 for the sum of the electron charges — which 

 in a neutral atom should equal the nucleus 

 charge — by observations on X-rays and the 

 use of a theory developed by J. J. Thom- 

 son. According to these views atoms con- 

 tain only about one sixteenth as many elec- 

 trons as they do on the theory of H. A. 

 Wilson. 



On the assumption that large angles of 

 deflection are sometimes due to single en- 

 counters with an atom, large forces must 

 be postulated to swing the a-partieles so 

 considerably from their paths, forces so 

 large as to require an approach to within an 

 exceedingly small distance from the nu- 

 cleus center. This distance was calculated 

 to be about 1/3,000 of the atom diameter. 

 If this is true, the nucleus can hardly have 

 a diameter exceeding 1/5,000 that of the 

 atom. 



The view that an a-particle may turn 

 through a large angle as the result of a 

 single encounter was strikingly confirmed 

 in 1912 by some remarkable photographs 

 of the paths of a-particles through a gas, 

 taken by C. T. E. Wilson.^" I have here 

 a reproduction of one of these photographs 

 which shows two abrupt bends in the trail 

 of a particle, one of 10.5° and the other of 

 43°. This second bend would certainly 

 seem to be a case of "single scattering." 

 The astonishing conclusion regarding the 

 small size of the nucleus has been confirmed 

 by some recent experiments of Marsden^^ 

 in passing a-radiation through a gas. 



A theory had been worked out by Dar- 

 win that when a-radiation entered hydro- 

 gen, a few H atoms would acquire from 

 close encounters with the a-particles a 

 velocity 1.6 times that of the striking 

 a-particle, corresponding to a range four 

 times that of the radiation. Marsden's 

 experiments were undertaken to test this 

 theory. He passed a-rays into hydrogen and 

 observed the scintillations on a zinc sul- 

 phide screen placed at various distances. 

 The range of the a-particles was found to 

 be 20 cm., but a few scintillations were 

 found when the screen was as much as 90 

 cm. distant, due seemingly to the rapidly 

 moving H atoms in their recoil from colli- 

 sion with the heavier a-particles. This was 

 a striking confirmation of Darwin's theo- 

 retical calculations. Calculation by his 

 method showed that the centers of the 

 nuclei during collision were not over 

 1.7 X 10"^^ cm. apart. This then would 

 be the maximum value of the sum of their 

 radii. This is smaller even than the former 

 result and also smaller than the accepted 

 value of the diameter of an electron. 



Thus the nucleus of the atom appears to 

 be extraordinarily minute, and this suggests 

 an explanation of the somewhat paradoxical 

 result, that practically all of the mass of 



