14 



Proceedings of the Boyal Irish Academy. 



knowledge is not quite definite on the point ; but it may be taken 

 that the multiple at the outside could not exceed 10, and in most 

 cases the chemical formula represents the molecule. The meaning of 

 this result is easily seen. In order that the magnetic field due to the 

 ions should produce the rotation observed, it should be 10,000 times 

 greater than it actually is ; or the rotation which is produced hy the 

 magnetic field of the ions must le at most of the order of one -thousandth 

 of what is actually olser ved. 



The argument can then be summarised as follows : — 



1. Diamagnetism and paramagnetism are explained on the hypo- 

 thesis of molecular currents caused by rotating ions. 



2. The electromagnetic equations, expressing the influence of the 

 external magnetic field, and these ionic circuits on the transmission 

 of light through a substance, show that plane-polarised light would 

 suffer rotation due to the rotating ions. It is then tentatively 

 assumed that all the actual rotation is due to this cause. 



3. The formula, showing how the rotation varies with wave- 

 length, agrees very satisfactorily with observation. 



4. But it is shown in this paper that from this formula can be 



deduced the value of -—-^ where J? is the external field, and is the area, 



and T the periodic time of the circuits in the ultra-violet absorption- 

 band of carbon disulphide and creosote. It is also shown that 



must have a value of the same order in all diamagnetic substances 



in general, from the known value for Yerdet's Constant of magnetic 



rotation, and that this calculated value of leads to absurd results. 



TXt 



5. It is shown that the periodic time of the ionic circuits must be 



/ eir 



taken as r = 2it\ — , as indicated by Langevin, which enables q to 



be evaluated. The resulting value of the ionic orbit is found to be 

 about 100 times the molecular radius. 



6. It is also shown that this value of would require the 



TXl 



magnetic susceptibility to be 10,000 times greater than its observed 

 value. 



7. From the expression (6) given for the magnetic susceptibility, it 

 is easily seen that ions describing orbits of molecular size, or even 

 much smaller than this, would completely account for the observed 



