Incidentally, and mostly just for myself here is : https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338 in which Schrödinger (in 1918!) describes,

Incidentally, and mostly just for myself here is : https://arxiv.org/abs/1211.6338 https://arxiv.org/abs/1211.6338 in which Schrödinger (in 1918!) describes, "matter in the large essentially as a compressible fluid of constant density at rest under a constant spatially isotropic tension which ... must be equal to 1/3 of the rest density of energy". The notation is old-fashioned, but check out Schrödinger's paragraph at the bottom of Harvey's page 5 !