CONSTITUTION OF THE ATOM—EVE. 187 
photographs showed where a collision had occurred between the 
alpha particle and one of the heavier molecules of air. It immedi- 
ately occurred to Sir Ernest Rutherford that a collision between an 
alpha particle and a lighter atom, such as hydrogen, would result in 
the nucleus of the latter being projected beyond the known range 
of the alpha particle. The poimt was put to the test by Marsden, and 
a complete justification of Rutherford’s nucleus resulted. The 
hydrogen nuclei were found to produce scintillations on a zine sul- 
phide screen at a range about four times as great as that of the alpha 
particles. Some mathematical investigations by G. C. Darwin indi- 
cated that the alpha particle or nucleus of helium, and the hydrogen 
nucleus must have approached so close that their centers were but 
1.710 centimeter apart. This affords further evidence of the 
extreme minuteness of the nucleus compared with the size of an 
atom (10-° centimeter). 
9. It may be well to recall at this point an interesting result of 
Barkla, obtained some years earlier, who showed from the scattering 
of Réntgen rays that the number of electrons in the atom must be 
about 3A, where A is the atomic weight. In the case of an uncharged 
atom, the positive charge on the nucleus must evidently balance the 
negative charges on the electrons revolving in orbits around that 
nucleus. 
Thus we can form a clear mental picture of the general character 
of the atom. It is a miniature solar system. The sun is replaced 
by the positively charged nucleus. The planets, perhaps confined 
to one or more definite orbits or rings, are replaced by negative 
electrons revolving rapidly around the nucleus. The gravitational 
force is replaced by the electrical attraction between the positive 
nucleus and negative electrons. 
10. A brilliant young Dane, Bohr, has gone a step further and 
suggested the structure of an atom capable of explaining the series 
of spectral lines. His work is remarkable as leading to excellent 
numerical verification. He assumes the Rutherfordian nucleus of 
electronic charge about half the atomic weight; he assumes that for 
every revolving electron in every atom the angular momentum is 
some exact multiple of Planck’s constant /2z. 
He further supposes that in a steady stationary orbit even a single 
electron does not radiate away energy. This is entirely contrary to 
classical electrodynamics. Furthermore he imagines that in passing 
from one state of stationary orbit to the next possible, there is homo- 
geneous radiation of amount hn, where n is fhe frequency. This is 
of course Planck’s assumption, and it is certainly unexplained, and 
probably not in accord with Hamilton’s equations as deduced from 
Newton’s laws. Nevertheless, any day we may learn why energy is 
emitted per saltum, and this mystery will vanish. 
