SOME coxruMroR.iRy .mr.ixci-.s i\ I'livsics riii 415 



small l)ut a lii-aiitifui and ooiuincinK scries of experiments, of which 

 the (irst and the most were performed by Sir l-.rnest Rutherford anfl 

 his pupils." These experiments are designed to show that the orbit 

 of a minute ehar^ed particle (usually an alpha-particle), flyinn iIuoukIi 

 a thin film of mel.il, is in certain cases very like the hyperbolic orh't 

 of a comet around the sun. Such an orbit is the [)ath of a particle 

 moving near to an immobile particle, for instance a li^ht particle 

 moving close to a much more massive one, which attracts it or repels 

 it by a force varying inversely as the square of their distance apart. 

 If these e.xperiments show what they are designed to show, then thc\' 

 indicate that the atom includes a particle much more massive than 

 an electron, hearing an electric charge, and sufticiently isolated from 

 the other charges in the atom (such as the electrons) so that its field 

 of force in a measurable space around it is not disturbed by theirs. 

 We cannot, however, trace the entire path of an individual flying 

 charged particle as it swings around through an atom, and are forced 

 to make up for this deficiency by a statistical study of the visible 

 portions of the paths of a great multitude of charge particles. 



Let us consider exactly what these experiments show; for whatever 

 they do prove is the most securely proved of all the beliefs about 

 atoms. In the first place, they show that there is a nucleus; and a 

 vacant space surrounding it, in which an inverse-square force centred 

 upon the nucleus prevails; and they indicate the dimensions of this 

 vacant space. This commences within 10~'- cm. of the nucleus, 

 which is another way of saying that the diameter of the nucleus is 

 less than 10"'- cm.; and it extends beyond a distance given (to take 

 instances) as 14.10"'- cm. for platinum and 10*' cm. for argon, which 

 is another way of saying that nearly all of the negative charge of the 

 atom lies still farther out from the nucleus. If the negative charge 

 is indeed subdi\ ided into electrons, then the atom is formed like a 

 hollow cloud of electrons, with a massive positively-charged nucleus 

 at the centre of the interior hollow. 



The diameter of this cloud of electrons is not furnished li\- tlic 

 experiments on alpha-particle deflections; but considering that the 

 distance between adjacent atoms locked into a crystal lattice is 

 generally a small multiple of 10"' cm., it cannot be much greater than 

 10 ' cm. unless we are prepared to admit interpenetration or violent 

 distortion of atoms; nor does it seem likely that the diameter is very 

 much smaller than this amount. I have already mentioned that 

 some of the properties of gases are adequately explained by assuming 



• For the mathematical theory of these experiments, the second article of this 

 scries may be consulted. 



