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4. Marggraf used an aqua regia composed of one partos sal ammoniac and sixteen parts of aquafortis; and foundthat twenty-four ounces of this menstruum were necessaryfor one ounce of platina. It may be suspected that thequantity of sal ammoniac was not here sufficient for enablingall the aquafortis to act on the platina, so that the metalwas dissolved only by a part of the menstruum, the restbeing superfluous aquafortis. The author observes, thatthe solution deposited in the cold small reddish crystals :yet he distilled off one half of it in a retort, and does nottake notice of any crystallisation happening in the concen-trated residuum; from whence it seems to follow, thatone half of the liquor was inessential to the dissolution.

5. Macquer made an experiment of the fame kind,whichagrees nearer with mine : of his aqua regia, composed ofequal parts of the nitrous and marine acids, sixteen ouncesdissolved, by digestion, one ounce of platina ; and in mytrials, about fourteen ounces were found to suffice. Acidspirits differ so much in their strength, and the dissolutionis so much influenced by the vapours being more or lessconfined during the process, as appears from the expe-riments above related, that an exact agreement in thispoint is not to be expected.

6. In all the dissolutions of platina, a portion of black-ish matter remained at the bottom, whether the platinahad or had not been separated from its black dust. Insome experiments, where the purer grains of the metalwere used, the quantity of this indissoluble matter amount-ed to about six grains on the ounce, or an eightieth part:where the mineral was taken entire, without any separa-tion of its heterogeneous mixtures, the residuum was inone trial above a fortieth, and in another about a thir-tieth part. The proportion could not be determined withmuch exactness, the indissoluble substance defending

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