Fusion of Plumbago. 347 



This trial I have this morning made upon the coloured 

 globules obtained in former experiments ; they were easily 

 detached from the plumbago by the slightest touch from the 

 point of a knife, and when collected in a white porcelain 

 dish, they rolled about like shot, when the vessel was turn- 

 ed one way and another. To detach any portions of un- 

 melted plumbago which might adhere to them I carefully 

 rubbed them between my thumb and finger in the palm of 

 ray hand. 1 then placed ihem upon a fragment of wedge- 

 wood ware, floated in a dish of mercury, and slid over them 

 a small jar of very pure oxygen gas, whose entire freedom 

 from carbonic acid, had been fully secured by washing it 

 with solution of caustic soda, and by subsequently testing it 

 with recently prepared lime-water; the globules were now 

 exposed to the solar focus from the lens mentioned volume, 

 5, page 363. It was near noon, and the sky but very slight- 

 ly dimmed by vapour; although they were in the focus for 

 nearly half an hour, they did not melt, disappear, or alter 

 their form; it appeared however, on examining the gas that 

 they had given up part of their substance to the oxygen, for 

 carbonic acid was formed which gave a decided precipitate 

 with lime-water. Indeed when we consider that these globules 

 had been formed in a heat vastly more intense, than that of the 

 solar focus, we could not reasonably expect to melt them in 

 this manner, and they are of a character so highly vitreous, 

 that they must necessarily waste away very slowly, evefl 

 when assailed by oxygen gas. In a long continued experi- 

 ment, it is presumable, that they would be eventually dissi- 

 pated, leaving only a residuum of iron. That they contain 

 iron is manifest, from their being attracted by the magnet, 

 and their colour is evidently owing to this metal. Plumba- 

 go, in its natural state, is not magnetic, but it readily becomes 

 so, by being strongly heated, although without fusion, and 

 even the powder obtained from a black lead crucible after 

 enduring a strong furnace heat, is magnetic. It would be 

 interesting to know, whether the limpid globules are also 

 magnetic, but this trial I have not yet made. 



I have already stated, that the white fume mentioned 

 above, appears when points of charcoal are used. I have 

 found that this malter collects in considerable quantities a 

 little out of the focus of heat around the zinc pole, and occa- 

 sionally exhibits the appearance of a frit of white enamel, 



