eta 
+ 
Fusion of Plumbago. 347 
‘Bhis 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 ina 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 
my hand. I then placed them 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 Jens 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 bour, 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 
solarfocus, we could not reasonably expect to melt them in 
this manner, and they are of a character so highly vitreous, 
that they must neeessarily waste away very slowly, even 
when assailed by oxygen gas. Ina long continued experi- 
ment, it is presumable, that they would be eventually dissi- 
pated, leavin 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. 
_ T have already. stated, that the white fume mentioned 
above, appears when points of charcoal are used. I have 
found that this matter 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, 
