the Combuflion of the J)tamoncl. 301 



nication through the cemented cork to the mercury, and through this tube a portion of the 

 gas was drawn in order that the mercury might ftand a fmall elevation within the vefTel. 



The diamond which had, by the former treatment, loft a fmall part of its weight, now 

 weighed 199,9 milligrammes, or one ^td part lefs than a jeweller's carat. On the 5th of 

 FruiSlidor, in the 6th year, the experiment was re-commenced by direfting the focus of the 

 great lens of the Inftitute upon this diamond, the value of the oxygen gas having been pre- 

 vioufly determined in cubic centimetres, and reduced to a known temperature and prefTure. 



The diamond at firft exhibited a black fpot at the angle immediately ftruck by the folar 

 rays, after which it became entirely black and of a coaly appearance. An inftant afterwards 

 brilliant and, as it were, boiling fpecks werp diflindly perceived on the black ground. The 

 folar rays being then intercepted for a moment, it appeared of a tranfparent red ; and after- 

 wards, when a cloud obfcured the fun, the ftone was fecn of a much clearer colour than at the 

 commencement of the operation. 



When the fun refumed its force the furface of the diamond afllimed the appearance of 

 metallic brilliancy. It was then perceptibly diminiflied, fcarcely one fourth part remaining; 

 its form oblong, without marked angles or edges, but flill very white and of a beautiful tranf- 

 parency. 



A flight crack appeared at the bottom of the vefTel which fupperted it, but without any 

 reparation of parts. 



At the commencement of the combuflion a purplifh cone was thought to be perceived 

 rifrng from the fupport in the cone of folar rays; but this phenomenon, according to the 

 writer, was merely an optical efFeft, depending on the pofition of the obforver. 



The apparatus was left in its place, without alteration, till the feventh, when the folar focUs 

 being again thrown upon the diamond, the fame appearances were obferved ; namely, black- 

 nefs at the furface, and brilliant boiling points", which difappeared and re-appeared, according 

 to the power of the folar light. An appearance of metallic brilliancy was alfo feen, or rather 

 a leaden colour, according to the expreflion made ufe of by one of the afliftants to charadterizc 

 this phenomenon. 



The diamond was entirely confumed after twenty minutes expofure this fecond time. 

 After a careful examination of the fupport, which was withdrawn, a folution of barytes was 

 introduced above the mercury. It immediately became milky, while the volume of the gas 

 was diminiflied. This barytic water was fhen, for the mod part, withdrawn by means of the 

 inverted bottle (fig. 3.) filled with mercury, which was raifed into the globe by means of 

 an iron ftem, confifting of feveral pieces fcrewed together. A portion of the diftilled water 

 was alfo introduced to wafh out the remaining precipitate ; and this was afterwards added to 

 the barytic water. It was neceflary to add more of the barytes, in order to faturate a portion 

 of carbonic acid, which was more in quantity than the operators expefted. The valuation and 

 reduflion of thefe quantities, and the precautions made ufe of, are given in the Memoir. The 

 refult was, that inftead of 28 parts of combuftible fubftance, with 7^ of the acidyfying principle. 



Vol. III. — October, 1799, Rr as 



