It can be ascertained better in London than here. Mexican Dollars pass current in China at the rate of 1000 dollars to 717 Taels alike weight, at which rate a dollar weighing 415.74 Grains troy is accepted as of sufficient weight.
But Mexican dollars are nearly always heavier than this, and it will be of importance that the British Dollar shall be fully equal in weight to an average Mexican dollar, which will probably be found to weigh 416 or 416.5 Grains troy.
As regards the purity of the new Coin, I am of the opinion that it should be of 9/10th fineness - that is composed of Metal containing 9/10th of pure silver and 1/10th of alloy - as described out here, where the notation of fineness is centesimal "of Silver of 90 touch". Mexican Dollars issued by different Mints in that Country are not precisely uniform in fineness, but the average fineness of Mexican Dollars of all Mints in parcels is, I believe, found to be about 897 thousandths. If this calculation be correct, the new British Dollar of 9/10 fineness, and of equal weight to the Mexican Dollar, will be intrinsically more valuable than the two to the extent of 3 mils, which would be a decided advantage.