Why is microcosmic salt called so?
Devoting sufficient time on Google didn't help at all.

Hmmm... I found this PDF in just a few seconds, where we read the following.

The origin of the name microcosmic salt.
Beads of sodium metaphosphate
saturated with some oxides cool to a clear glass. Small crystals grow
in the glass on repeated heating and cooling. A microscope with a 80
to 100 x magnification is needed to inspect these crystals with
various morphologies. Such beads resemble a microcosmos. Some authors
tried to use the effect for analytical purposes in the 19th century,
but these efforts failed. Borax beads sometimes show the same effect.

Microcosmos here is understood (as usual) as a &quot;small world&quot;, a &quot;world in miniature&quot;.