VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 103 



weigh only 21 grs. ; so that, in this operation, the platina had lost near a 5th 

 part of its weight. 



According therefore to this experiment, the platina does not wholly resist the 

 force of lead in coppellation ; but, by repeated operations of that kind with larger 

 quantities of lead, may probably all be destroyed : and by such repeated coppella.^ 

 tions, gold and silver may very likely be refined from it ; though what was before 

 asserted may hold pretty true, with regard to the common coppellations of the 

 assayers and refiners. 



Mr. Wood said, that, in his experiment, he thought the platina rather gained 

 than lost in weight by coppellation. This might happen from some small mixture 

 of lead, or other metal continuing with it after it remained no longer fused. 



From this single experiment Dr. B. would not be quite positive that lead thus 

 consumes some small quantity of platina, since it was possible the platina used 

 might not be pure. Besides, in order to keep it longer in fusion, he urged on 

 the experiment with an uncommon degree of heat, especially towards the end of 

 the operation ; though he thought no great error could thence arise ; as 4 dr. 

 of silver, which he coppelled at the same time, had lost only 1 grs. in the operation. 



He was told that one Mr. Ord, formerly a factor to the South Sea Company, 

 took in payment from some Spaniards, gold to the value of 5001. sterling, which 

 being mixed with platina, was so brittle that he could not dispose of it, neither 

 could he get it refined in London, so that it was quite useless to him ; though, 

 if no error has been committed in the above-mentioned experiments, it might 

 probably have been rendered pure by a much larger dose of lead than is usually 

 employed for that purpose. 



To his memoir he might have added, that attempting to cleanse a parcel of 

 the native platina from the black sand, with which it was mixed, he found that 

 a great many of its grains were attracted by the magnet he made use of for that 

 purpose. This circumstance he took notice of in a letter to Lord Lonsdale two 

 years before.* 



Of a very Large Human Calculus. By fFilliam Heberden, M. D. -f- F. R. S. 



N° 496, p. 596. 



There is preserved in the library of Trinity College, Cambridge, a stone taken. 



♦ A most ingenious and complete set of experiments was made on platina between 3 and 4 years 

 afterwards by Dr. Lewis. These experiments are inserted in the 48th and 50th vols, of the Phil. Trans. 

 Since then it has occupied the attention of the first chemists in this and other countries ; and lately Dr. 

 Wollaston (Phil. Trans, for 1804 and 1805) has shown that there are 2 or 3 distinct metallic sub- 

 stances contained in the ores of platina. 



+ This eminent physician, as we are informed in his life prefixed to the Commentaries on the 

 History and Cure of Diseases, was born in 17 10, in London, where he received the early part of his 



