Page:The Meaning of Relativity - Albert Einstein (1922).djvu/68

momentum of the whole system, multiplied by $i$, together with the negative energy of the system, so that (49) expresses the conservation principles in their integral form. That this gives the right conception of energy and

the conservation principles will be seen from the following considerations.

Phenomenological Representation of the Energy Tensor of Matter.

Hydrodynamical Equations. We know that matter is built up of electrically charged particles, but we do not