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li. It is well known to the chemists, that the metalscalled imperfect, or those which calcine in the fire, gainweight in their calcination ; a phenomenon not a littleastonishing, and of which they have not been able to assignany probable cause, unless it be the absorption of air. Asplatina appears plainly, from many of its properties, notto be One of the imperfect metals, Mr. Macquer veryjustly suspects, that the increase of weight in the aboveexperiments was owing to the calcination of some hetero-geneous substances mixed with the platina. The ferrugi-ileous lining which it left in the crucible, and the obscura-tion of the colour, seemed to confirm this conjecture, andhe further took notice, that after the second calcinationthere were some grains of a friable matter like scales ofiron, and that the magnetic sand was no longer black andbrilliant, but of the fame state grey colour with the pla-tina. It may here be observed, that if there was no mistakein Mr. Macquers weights, the quantity of this heteroge-neous calcinable matter must be very considerable. Of allthe experiments I can recollect of the calcination of bo-dies, there is no one in which the increase was so great asthat which Mr. 8chester allows to iron, viz. one third ofits weight, as we shall fee hereafter in the sixth section ofthis history : admitting even this augmentation to the cal-cinable matter in platina, the quantity of this matter, toproduce an augmentation of sixteen grains on the ounce,must be forty-eight grains, or one eleventh part of theplatina.

12. The observations in the foregoing paragraph accountfor the difference between my experiments No. 7, andthose of M. Marggraf and Macquer in No, 8, 9, and lo;mine having been made with the purer grains, and theirswith the entire mineral containing its common mixture ofcalcinable parts. For further satisfaction in this point, I

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