It was invented by Venetian mirror makers and continued to be used till the year 1835, when a german cgemist - Justus von Liebig, discovered silvering mirrors with real silver. In the amalgam the proportion was 1 atom of Hg per 6-10 atoms of tin. A mercury atom can replace any tin atom, yet it one must be surrounded only by tin atoms. Otherwise mercury becomes liquid. Mercury atoms are scattered in a quite regular way. You can easily check that here the proportion is 1:8, as on the right a piece of amalgam was chopped into prymitive cells. To obtain still more even distribution every second atom should be moved to the lower layer..