(75) | ,
, u.s.f. , , u.s.f. , |

(76) | , |

These quantities are all real. In the theory for bodies at rest, the combinations ( are known as Maxwell's Stresses", are known as the Poynting's Vector, as the electromagnetic energy-density, and *L* as the Langrangian function.

On the other hand, by multiplying the alternating matrices of *f* and *F*, we obtain

(77) |

and hence, we can put

(78) | , |

where by *L*, we mean *L*-times the unit matrix, *i.e*. the matrix with elements

Since here , we deduce that,

and find, since , we arrive at the interesting conclusion

(79) | , |

*i.e.* *the product of the matrix* S *into itself can be expressed as the multiple of a unit matrix* — a matrix in which all the elements except those in the principal diagonal are zero, the elements in the principal diagonal are all equal