the Shape of the Atom. 235 



Some further deductions o£ interest can be made relating 

 to the properties of the atom. If the mass of an atom is 

 denoted by m, its volume, we have seen, is proportional to 



771 



wi 1 / 2 , and its density therefore proportional to — r= or m l/2 . 



Thus the density of atoms increases with increase of atomic 

 weight. The density of a lead atom is thus about 14 times 

 that of a hydrogen atom, and the density of a hydrogen 

 atom about 30 times that of an electron. That the density 

 of an atom should increase with the atomic weight we should 

 expect since there would be a tendency of the atom to contract 

 under the mutual attraction of its parts, and this would 

 increase with the mass of the atom. 



The writer* has shown that the attraction other than 

 gravitational between two molecules a given distance apart 

 is proportional to the product (Xm\ ,2 )(tml /2 ). This attraction 

 gives rise to the surface-tension of liquids, chemical combi- 

 nation, &c. This fact may now be stated in a different form, 

 namely, that the attraction is proportional to the product of 

 the volumes of the molecules. It was also shown in the 

 paper mentioned that the attraction of several atoms close 

 together, as they occur in a molecule, is not exactly 

 additive. We see now how this may be caused. When a 

 number of atoms concentrate to form a molecule each atom 

 must contract slightly owing to the attraction of the different 

 parts of the molecule on one another. The attractive 

 force of each atom at an external point is therefore less 

 than it would be if the other atoms were absent. The con- 

 traction of the atoms would be greatest at their surfaces of 

 contact, which would have the effect of increasing the extent 

 of the total surface of contact. The stability of the structure 

 of the molecule would thereby be increased. 



An electron in the neighbourhood of an atom will be 

 attracted by it due to electrostatic induction. If the atom is 

 considered a perfect conductor of electricity the attraction 

 is given t by 



' e 2 q3(2/ 2 -a 2 ), 



f 3 (f-a 2 ) 2 ' 



where a is the radius of the atom, and f the distance of the 

 electron from the centre of the atom. When / is large in 



* Loc. cit. 



f Maxwell's 'Electricity and Magnetism,' vol. i, 3rd edition, p. 251. 



