4 2 4 SCIENCE PROGRESS. 



Faraday looked for this and Zeeman found it. The direct 

 observation of an alteration in the vibrations of molecules 

 by magnetic force is one of the most interesting and im- 

 portant observations made of recent years. It is most 

 interesting and important for several reasons. It enables 

 us to study the internal vibrations of molecules by modi- 

 fying in a simple way the conditions under which these 

 vibrations are executed. The laws of action of magnetic 

 force are so well known that we can be fairly sure how it 

 acts on the molecular motions and from a study of the effects 

 produced we can already make inferences as to the nature 

 of those motions in the molecules to which the spectral lines 

 are due. For example, the hypothesis has received con- 

 firmation that the lines are due to the motions of electric 

 charges, and there seems reason to suppose that these 

 electric charges are accompanied in their motions by only a 

 small part of the mass of the molecule. Further, we are 

 now able for the first time to produce polarised radiations 

 from the molecules of a gas. Hertz showed how to produce 

 polarised radiations from electromagnetic oscillations on con- 

 ductors, but hitherto attempts to produce polarised radiations 

 from gaseous molecules have failed. This is in itself a great 

 advance in our means of studying these vibrations : it simpli- 

 fies the problem to be studied by analysing it into compo- 

 nents. We know that different lines in each spectrum have 

 different physical characteristics such as belonging to 

 different so-called series of lines, being reversed with more 

 or less ease, becoming more or less expanded, etc., under 

 changes of temperature and pressure and we are now pro- 

 vided with a further characteristic difference between lines 

 in that they are differently acted on by magnetic force. 



The effect of magnetic force on the vibrations of mole- 

 cules is a complicated one. The apparently simplest effect 

 upon a vibration is to produce the result already described 

 as required in order to explain the Faraday effect and which 

 Lorentz has taught us we should expect to be produced on 

 a vibrating electron. Every simple vibration of a point may 

 be analysed into two circular rotations, one right- and the 

 other left-handed, in a plane at right angles to the magnetic 



