470 ScicHiiJic LUeUigente. [Dec. 



not magnetic. They very much resembled melted slleago, had not' 

 be supposed to be derived from impurities in the plumbx, and might 

 their appearance been uniform in the different varieties of that sub- 

 stance, whose analysis has never, I believe, presented any combined 

 silex ; and neither good magnifiers, nor friction of the powder between 

 the fingers, could discover the slightest trace of any foreign substance 

 in these specimens. Add to this, in different experiments, 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 brilliancy of the most perfect black enamel. 

 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 attracted by the magnet, and 1 think were rather more 

 sensible to it than the plumbago, which had been ignited, but not melt- 

 ed. We know how easily, in substances containing iron, the magnetic 

 susceptibility is changed by slight variations 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 difficult: and the instrument being in good 

 order, good results may be anticipated with certainty. As the press 

 is waiting while I write, it is not in my power to determine the nature 

 of all of these various coloured globules, and particularly to ascertain 

 whether the abundant white globules are owing to earths combined 

 with the plumbago, or whether they are a different form of carbon. 

 If the former be true, it proves that no existing analysis of plumbago 

 can be correct, and would still leave the remarkable white fume, so 

 abundantly exhaled between the poles of the deflagrator, and so ra- 

 pidly transferred from the copper to the zinc pole, entirely unaccount- 

 ed for. I would add, that^r the merejusioti of plumbago, the blow- 

 pipe is much preferable to the deflagrator ; but a variety of interest- 

 ing phenomena in relation to both plumbago and charcoal are exhi- 

 bited by the latter, and not by the former. 



A postscript to this communication, dated April 18, gives the fol- 

 lowing statement : — 



The anthracite of Rhode-Island is thought to be very pure. Dr. 

 William Meade (see Bruce's Journal, p. 36), estimates its proportion 

 of carbon at ninety-four per cent. This anthracite I have just suc- 

 ceeded in melting by the compound blow-pipe. It gives large bril- 

 liant black globules, not attractable by the magnet, but in other re- 

 spects not to be distinguished from the dark globules of melted plum- 

 bago. The experiment was entirely successful in every trial ; and the 

 great number of the globules, and their evident flow from, and con- 

 nexion with, the entire mass, permitted no doubt as to their being 

 really the melted anthracite. 



The Kilkenny coal gave only white and transparent globules ; but 

 it seems rather difficult to impute this to impurities, since this anthra- 

 cite is stated to contain ninety-seven per cent, of carbon. 



I have exposed a diamond this afternoon to the solar focus in a jar 

 of pure oxygen gas, but observed no signs effusion, nor indeed did 



