Prof. J. J. Thomson on Cathode Rays. 313 



by the two oppositely electrified atoms which form the mole- 

 cule of the gas. The measurements of the specific inductive 

 capacity show, however, that this is very approximately an 

 additive quantity : that is, that we can assign a certain value 

 to each element, and find the specific inductive capacity of 

 HC1 by adding the value for hydrogen to the value for 

 chlorine ; the value of H 2 by adding twice the value for 

 hydrogen to the value for oxygen, and so on. Now the elec- 

 trical moment of the doublet formed by a positive charge on 

 one atom of the molecule and a negative charge on the other 

 atom would not be an additive property ; if, however, each 

 atom had a definite electrical moment, and this were large 

 compared with the electrical "moment of the two atoms in the 

 molecule, then the electrical moment of any compound, and 

 hence its specific inductive capacity, would be an additive 

 property. For the electrical moment of the atom, however, 

 to be large compared with that of the molecule, the charge 

 on the corpuscles would have to be very large compared with 

 those on the ion. 



If we regard the chemical atom as an aggregation of a 

 number of primordial atoms, the problem of finding the 

 configurations of stable equilibrium for a number of equal 

 particles acting on each other according to some law of force 

 — whether that of Boscovich, where the force between them is 

 a repulsion when they are separated by less than a certain 

 critical distance, and an attraction when they are separated 

 by a greater distance, or even the simpler case of a number 

 of mutually repellent particles held together by a central force 

 — is of great interest in connexion with the relation between 

 the properties of an element and its atomic weight. Unfor- 

 tunately the equations which determine the stability of such 

 a collection of particles increase so rapidly in complexity with 

 the number of particles that a general mathematical investi- 

 gation is scarcely possible. We can, however, obtain a good 

 deal of insight into the general laws which govern such con- 

 figurations by the use of models, the simplest of which is the 

 floating magnets of Professor Mayer. In this model the 

 magnets arrange themselves in equilibrium under their mutual 

 repulsions and a central attraction caused by the pole of a 

 large magnet placed above the floating magnets. 



A study of the forms taken by these magnets seems to me to 

 be suggestive in relation to the periodic law. Mayer showed 

 that when the number of floating magnets did not exceed 5 

 they arranged themselves at the corners of a regular polygon — 

 5 at the corners of a pentagon, 4 at the corners of a square, 

 and so on. When the number exceeds 5, however, this law 



