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gravity of this mixture, we were to compute that of the tinemployed in it, we should make it above a fourth partgreater than it really is.

Mr. Hooke made an experiment of the fame kind, be-fore the Royal Society, on a mixture of tin and silver. Thegravity of the tin was about 7, and that of the silver 10,666:of equal parts of the two metals melted together, the gra-vity was 10,812. By applying Mr. Scheffers principle tothis mixture, if silver was a metal of unknown gravity, weshould conclude, that its gravity must be upwards of 23.Several other experiments of the gravities of metallicmixtures are given in Dr. Birch’s history of the RoyalSociety; but the reader must observe, that the computedgravities are no where to be relied on, Mr. Hooke havingfallen into the fame mistake, in regard to the calculations,as I had done in the tables published in the PhilosophicalTransactions.

Dr. Brandt, in the Swedish acts for 1744, where we like-wise find an inadvertence of the same kind in the methodof calculation, gives three experiments on mixtures of leadand tin; in two of which there is such an increase of gra-vity, as would make the specific weight of lead above 13,and in the third a more remarkable one : 531 grains offine tin lost in water 757, so that 100 parts lost 14,218 :531 grains of a mixture of 87 parts fine tin and 3 partslead, lost in water 72s, so that 100 parts of this mixturelost 1 3,633 - the quantity of tin in it ought to have lost more,or to have occupied a greater space in the water, than thewhole mixt did; so that the lead and tin, by their mixture,were contracted into less volume than that of the tin by itself.

It appears therefore that the gravity of a metal can neverbe with any certainty deduced from that of its mixturewith another metal, as a dilatation or contraction of thevolume may result from their action on one another. It

follows