448 New Facts concerning the Diamond. [October, 



symmetrically shaped pits can easily be seen by heating a 

 thin plate of boart in a blowpipe flame and examining 

 it under the microscope. By prolonged heating several 

 small triangular pits will often merge into one large one. 

 A crystal of diamond, even when so reduced in size by 

 oxidation as to be only visible with difficulty, continues to 

 exhibit sharp edges and angles. A dodecahedron with very 

 rounded faces but smooth and brilliant surface also exhibited 

 the triangular pits often very distinctly ; moreover, it had a 

 brown colour, which was not destroyed by heat, and must, 

 therefore, be of a totally different nature from that of the 

 topaz or smoky quartz. 



Several experimentalists, M. Jacquelain amongst others, 

 affirm that at an extremely high temperature, such as is 

 attainable at the focus of a large burning-glass, or between 

 the charcoal points of a powerful galvanic battery, such as 

 100 elements of Bunsen, the diamond softens, that it 

 passes into an allotropic state, is changed into true coke, 

 capable of employment as an excellent conductor of elec- 

 tricity, and diminishes greatly in density, as much as from 

 3*336 to 2*6778. It has also been stated, that upon watch- 

 ing through smoked glass the combustion of a diamond 

 under the focus of a burning glass, it was seen to melt, and 

 even to undergo a kind of ebullition. 



M. Schrotter informs us that the R. T. cabinet of mine- 

 ralogy, at Vienna, contains a diamond which was placed 

 under the focus of a burning glass in 1751, by Francis I., 

 the husband of Maria Theresa, and allowed to burn for 

 some time, and that after this partial combustion the 

 diamond, a very limpid well-cut stone, became black both 

 externally and internally. 



Clarke, having burnt a diamond in the flame of oxy- 

 hydrogen gas, relates that it first become opaque, like ivory, 

 then the angles of the octahedron were rounded, the surface 

 was covered with bubbles, and there remained a globe of 

 metallic brilliancy, which finally disappeared entirely. Silli- 

 man, upon burning a diamond upon magnesia, found it turn 

 black and burst, and Murray and Macquer also speak of the 

 diamond turning black under combustion. 



Messrs. Rose and Siemens heated the diamond between 

 the two charcoal points of a large magneto-electric machine, 

 the poles being enclosed in a glass cylinder from which air 

 was excluded. During two separate experiments, upon the 

 charcoal becoming incandescent, the diamond exploded into 

 numerous fragments, all of which were black; examination 

 showing, however, that the colouring was wholly superficial, 



