where all occurring are related to the same instant, namely to the instant when
is the local time of .
Since is equal for all points of an ion, then, if we write e for the charge of such a particle, the last integral transforms into
The sum is extending over all ions of the molecule.
Furthermore, if is now the displacement of an ion from its equilibrium position, then
and
This has a simple meaning. We can conveniently call the vector the electric moment of the molecule and denote it by . Then it is
after the things said here, we have to take the value of the derivative for the instant when the local time in is . Obviously we can also write
where means the first component of the electric moment in that very instant. After (by that and by two equations of the same from) we have found , , for the point (x, y, z) and the local time at this place, the study of the propagating oscillations is very simple. The equations (37) give