352 Experiments upon Diamond, Anthracite and Plumbago> 



space of I an inch at, and around the point, and presenting 

 a beautiful contrast, with the plumbago beneath, like that of 

 a white enamel upon a black ground. 



In subsequent trials, upon pieces from various localities, 

 foreign and domestic, (confined however to very pure spe- 

 cimens,) I obtained still more decided results; the white 

 transparent globules became very numerous and as large as 

 small shot; they scratched window glass — were tasteless — 

 harsh when crushed between the teeth, and they were not 

 magnetic. They very much resembled melted silex, and 

 might be supposed to be derived from impurities in the 

 plumbago, had not their appearance been uniform in the 

 different varieties of that substance, whose analysis has nev- 

 er, I believe, presented any combined silex, and neither 

 good magnifiers, nor friction of the powder between the fin- 

 gers, could discover the slightest trace of any foreign sub- 

 stance in these specimens. Add to this, in different exper- 

 iments, I obtained very numerous perfectly black globules, 

 on the same pieces which afforded the white ones. In one 

 instance they covered an inch in length, all around, many of 

 them were as large as common shot; and they had all 

 the lustre and brilHancy of the most perfect black ena- 

 mel. Among them were observed, here and there, globules 

 of the lighter coloured varieties. In one instance the entire 

 end of the parallelepiped of plumbago was occupied by a 

 single black globule. The dark ones were uniformly at- 

 tracted by the magnet, and I think were rather more sensi- 

 ble to it than the plumbago which had been ignited but not 

 melted. We know how easily, in substances containing 

 iron, the magnetic susceptibility is changed by slight varia- 

 tions of temperature.. I am aware, however, that the dark 

 globules may contain more iron than the plumbago from 

 which they were derived, as the combustion of part of the 

 carbon, may have somewhat diminished the proportion of 

 that substance. I find that the fusion of the plumbago by 

 the compound blow-pipe is by no means difiicult, and the 

 instrument being in good order, good results may be antici- 

 pated with certainty. As the press is waiting while I write, it is 

 not in my power to determine the nature of all of these va- 

 rious coloured j^lobules, and particularly to ascertain wheth- 

 er the abundant white globules are owing to earths combin- 

 ed with the plumbago, or whether they are a different form 



