(92) |

Now if we make use of (59), and denote the space-vector which has as the *x-, y-, z*-components by the symbol , then the third component of 92) can be expressed in the form

(93) |

The round bracket denoting the scalar product of the vectors within it.

### § 14. The Ponderomotive Force.

Let us now write out the relation in a more practical form; we have the four equations

(94) |
, |

(95) |
, |

(96) |
, |

(97) |
. |

*It is my opinion that when we calculate the ponderomotive force which acts upon a unit volume at the space-time point* x,y,z,t*, it has got* *x-, y-, z*-