Purity is always defined as the percentage by weight(mass) of the wanted
substance in the mixture.
9 karat gold implies that 100gms or ounces or pennyweights of the alloy
contains 3/8 gms, ounces or pennyweights of pure gold.
A 10gm piece of 9 carat gold made by alloying 3gm of pure gold with 7gm of
aluminium would not have the same volume as a piece made by alloying 3gm of
gole with 7gm of osmium (if that were possible); its density would be near
21,
The aluminium alloy would be much larger in volume, but would still be
9karat.

When percentages are assumed to be by volume, that is indicated by the
designation (% v/v). The coresponding (% w/w) is used occasionally, when it
is required to make absolutely clear that the weight percentage is implied.

Hope that helps to clarify the situation. - G.H.Ireland.

--
igor
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Acorn RISC OS4
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