Medicina hydrostatica, or, Hydrostaticks applyed to the materia medica shewing how by the weight that divers bodies, us'd in physick, have in water : one may discover whether they be genuine or adulterate : to which is subjoyn'd a previous hydrostatical way of estimating ores / by the Honourable Robert Boyle ...

SECT. IX.

'TIs known, that, since we be∣gan effectually to cultivate the African Trade, it frequently brings into these Parts, besides things of less value, considerable Quantities of what, from the most usual Size of it,

is by many called Sand-Gold; but which, by reason of the very une∣qual Bulks of the Grains, may per∣haps justly be called Fragments of Gold; since being brought from the Maritime parts, where no Mines of Gold are yet found, they seem to have been broken off and washed away from hidden Veins by the violence of Waters, that, having carried them as far as they were able, left them a Prey to Men. Now, (because that un∣less it be perhaps brought by, or for, some Virtuoso) there is scarce any Gold that comes into Europe in Lumps, under the form of Ore; but a great deal that is brought from Guinea, (and those other parts of Africk, which, for that reason, are comprized under the Name of the Golden Coast) in the Form chiefly of Sand or Gra∣vel, grosser or smaller, and partly also of less minute Pieces; it may conduce to the scope of these Papers to take notice, that, in making E∣stimates of the Genuineness, and the degrees of Purity of these native

Fragments of Gold, our Hydrosta∣cal Way of exploring may be of no small use.

For first, when we have once dis∣covered the Proportion between pure or exquisitely refined Gold, and Wa∣ter equal to it in Bulk; (which Pro∣portion I have lately given exactly enough, for our present purpose,) tis easie, by our Hydrostatical Method, to examine the Fineness of any other Gold proposed; so, at least, as to know, whether it be perfectly Fine; and if it be not, whether it do con∣siderably fall short of perfect Fine∣ness. But since of this I elsewhere treat, I think it more proper to ob∣serve in this place, that when once a Man has found the true Specifick Gravity of a parcel of Sand-Gold, (smaller or courser,) whose Degree of Fineness he knows by Collateral Tryals, or some other Means, (what∣ever they be) He may (as was for∣merly noted when I spoke of Metal∣line Ores,) take this Specifick Gra∣vity for a Standard, with relation

to which, he may make his Estimates of the Fineness of other parcels of the like native Gold, that he is concern∣ed to buy, or to examine. And, by this means, he may oftentimes pre∣vent that chief Fraud of the Negroes, whereof several Traders to the Gol∣den Coast are not a little apprehen∣sive; as being in danger to be much damnified by it. For they complain, that, tho' the Blacks be otherwise, for the most part, but a dull sort of People; yet they have often made a shift to cheat the Traders, by clan∣destinely mixing, with the right Sand-Gold, Filings of Copper, or rather of Brass, whose Colour does so re∣semble that of Gold, that the Fraud is not easily discerned. And in the Account of a late Voyage, made by the French, to the Coast of Africk, to Trade especially for Gold, 'tis ac∣knowledged, that the Officers were egregiously cheated by the Blacks, who, instead of paying them for the Wares they brought, with Powder of true Gold, gave them Powder of

Brass, or gilt Copper, which those that were not accustomed to make Tryal of, are, as the Relater complains, such Wares, in a scarce evitable dan∣ger to be cheated: as these French men confess they were in one day to the worth of a thousand Crowns. But, in regard, that, as Tryal has informed me, Brass is not quite half so heavy as fine Gold of the same Bulk; if there be any considerable Quantity of Filings of Brass with the Gold; This Mix∣ture being put into such an Hydro∣statical Bucket, or wide-mouth'd Glass, as is mentioned in the Essay, will manifestly weigh less in Water, than if it were all Gold. And by comparing its Specifick Gravity, with that formerly found, to the Grain-Gold pitched upon for a Standard; the greater or lesser Decrement of the suspected Gold, will help to make an Estimate of the Quantity of Brass, mingled with the natural Gold.