Light by (J asymmetrical Atoms and Molecules. 409 



have a finite electrical moment measured by the product of 

 the charge on the electron and its distance from the positive 

 charge. The atoms would " set " under the action of an 

 external force, and under such a force the collection of 

 atoms would have a finite electrical moment and therefore a 

 specific inductive capacity greater than unity. If it were 

 not for the c >llisions between the atoms their axes would all 

 point in the direction of the electric force and the moment 

 would be independent of the strength of the electric field. 

 These axes are, however, knocked out of line by the collisions 

 between the atoms, so that the resultant moment diminishes 

 as these collisions increase ; in fact, we can show that the 

 resultant moment and therefore the excess of the specific 

 inductive capacity over unity will, on account of this effect, 

 vary inversely as the absolute temperature of the gas. 

 There are gases such as water vapour and the vapours 

 of various alcohols which vary in this way ; but the specific 

 inductive capacity of gases such as hydrogen, helium, 

 nitrogen, or oxygen varies much less rapidly with the 

 temperature, showing that in the normal state the atoms 

 or molecules of these gases have no finite electrical moment. 

 One form of such an atom is that of a double charge 

 at the centre A with an electron on either side of if. If 

 such an electron is acted on by a force X at right angles to 

 the line joining the central charge to the electrons, then, if 

 8j- is the displacement of either electron in the direction of 

 this force and d the distance of an electron from the central 

 charge, we see that 



2 



-7-js o\z — Xtf . d. 



or Zehx = 8^ 3 X. 



Now 'leh.c is the electrical moment of the atom when it is 

 exposed to the electric force, and thus 8d d is proportional to 

 the quantity we have denoted on page 397 by a ; if the axis 

 of y is also at right angles to the line joining the central 

 charge to the electron, b — a. 



We must now consider the effect of a force along the line 

 of electrons : if Sz is the displacement caused by a force Z 

 acting in this direction, 2e 2 ¥ the force exerted by the 

 central charge on an electron, we see that 



dF 

 Ze = 2e 2 f .Sz. 

 dz 



Phil. Mag. S. 6. Vol. 40. No. 238. Oct. 1920. 2 E 



