[ 5S7 ]

follows also, that when two metals of known gravities aremelted together, their proportions cannot be found fromthe gravity of the compound, without a previous hydrosta-tic examination of known mixtures of them in differentproportions; that consequently the celebrated proposition,of Archimedes is of more limited use than it has generallybeen supposed; and that the table which Mr. Scheffer hasbeen at the pains to calculate, in the Swedish acts for 1755,for determining the quantities of lead and tin in any givenmixtures of the two, by a statical examination of them,without comparison with standard mixtures, is little to bedepended on.

As the variations of gravity arising from the mixture ofmetals have been ascribed to causes which do not obtainin fluids; it may be proper to observe, that the same thingoften happens in fluids themselves; and here the effect isperhaps still more conspicuous and more strongly marked.One measure of water, and one measure of rectified spiritof wine, mixed together, fall very sensibly short of twomeasures; a proof that their volume is diminished, ortheir weight, under an equal volume, increased by themixture. Mr. Hooke found, that twenty-one measuresof water, and three measures of oil of vitriol, mixed toge-ther, made only twenty-three measures, so that one twenty-fourth part of the bulk was lost.

SECT. VII.

Of the effect of fire and air on mixtures of flat in a with

certain metals.

I. Calcination of fin with platina.

A S gold and tin, rnelted together, and kept in a heatsufficient for calcining the tin, are said byDr.Brandt,in the Swedish transactions, to affect one another in a

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