DOI Seite / Zitierlink:
https://doi.org/10.11588/diglit.19574#0088
74 BUR
fy a nature, that, except it is burnt, it will require a long time
to dry.
The method of burning, or rather drying lamp-black, is as
follows : Put it into an iron ladle or a crucible, and fetitover
a clear fire, letting it remain till it be red-hot, or fo near it,
that there is no manner of fmoke arifes from it.
Umber, which if it be intended for colour for an horfe, or to
be a fhadow for gold, muftbeburnt, which fits itfor thatpurpofe.
In order to burn umber, you muft put it into the naked fire
in large lumps, and not take it out till it it thoroughly red-hot;
if you have a mind to be more curious, you may put it into the
fire in a crucible, till it be red-hot; then take it out, and, when
it is cold, lay it up for ufe.
Ivory alfo muft be burnt to make a black, thus : Fill two cru-
cibles with fhavings of ivory ; then clap their two mouths toge-
ther, and bind them faff with an iron wire, and lute the joints
clofe with clay, fait, and horfe-dung well beaten together; then
fet it in a fire, covering it all over with coals, and let it remain
in the fire, till you are fure the matter inclofed in the crucibles is
thoroughly red-hot; then take it out of the fire, but do not
open the crucibles, till they are perfectly cold ; for, if you fhould
open them while hct, the matter would flame and turn toafhes;
which will alfo be the cafe, if the joints are not luted clofe ; for
it is only the exclufion of the air that prevents any burnt matter
whateverfromturning to a white afh,and prefervesthe blacknefs.
Burning Gbjfes. A machine by which the fun's rays are
collected into a point; and by that means their force and effects
heightened to that extreme degree, as to burn objects placed in
them.
They are of two kinds, either convex or concave ; the convex
tranfmit the rays of light, and in their paffage refract or incline
them towards its axis ; having the property of lens's, and act-
ing according to the laws of refraction; the concave, which
are the more ufual, are very improperly called Burning glaffes,
being ufually made of metal; thefe reflect the rays of light, and,
in that reflection, incline them to a point in their axes; having
the properties of mirrors, and acting according to the laws of
refleclior^
Every concave, mirror, or fpeculum, collects, in rays dif-
pers'd fhrough its whole concavity, after reflection into a point
or focus, and is therefore a Burning mirror.
Hence, as the focus is there, where the rays are moll clofely
contracted, if it be afegment of a large fphere, its breadth mult
not fubtend an arch above eighteen degrees; if it be a fegment
of a fmaller fphere, its breadth may be thirty degrees.
M. Kircher is of opinion, that thofe Burning mirrors, which
do
fy a nature, that, except it is burnt, it will require a long time
to dry.
The method of burning, or rather drying lamp-black, is as
follows : Put it into an iron ladle or a crucible, and fetitover
a clear fire, letting it remain till it be red-hot, or fo near it,
that there is no manner of fmoke arifes from it.
Umber, which if it be intended for colour for an horfe, or to
be a fhadow for gold, muftbeburnt, which fits itfor thatpurpofe.
In order to burn umber, you muft put it into the naked fire
in large lumps, and not take it out till it it thoroughly red-hot;
if you have a mind to be more curious, you may put it into the
fire in a crucible, till it be red-hot; then take it out, and, when
it is cold, lay it up for ufe.
Ivory alfo muft be burnt to make a black, thus : Fill two cru-
cibles with fhavings of ivory ; then clap their two mouths toge-
ther, and bind them faff with an iron wire, and lute the joints
clofe with clay, fait, and horfe-dung well beaten together; then
fet it in a fire, covering it all over with coals, and let it remain
in the fire, till you are fure the matter inclofed in the crucibles is
thoroughly red-hot; then take it out of the fire, but do not
open the crucibles, till they are perfectly cold ; for, if you fhould
open them while hct, the matter would flame and turn toafhes;
which will alfo be the cafe, if the joints are not luted clofe ; for
it is only the exclufion of the air that prevents any burnt matter
whateverfromturning to a white afh,and prefervesthe blacknefs.
Burning Gbjfes. A machine by which the fun's rays are
collected into a point; and by that means their force and effects
heightened to that extreme degree, as to burn objects placed in
them.
They are of two kinds, either convex or concave ; the convex
tranfmit the rays of light, and in their paffage refract or incline
them towards its axis ; having the property of lens's, and act-
ing according to the laws of refraction; the concave, which
are the more ufual, are very improperly called Burning glaffes,
being ufually made of metal; thefe reflect the rays of light, and,
in that reflection, incline them to a point in their axes; having
the properties of mirrors, and acting according to the laws of
refleclior^
Every concave, mirror, or fpeculum, collects, in rays dif-
pers'd fhrough its whole concavity, after reflection into a point
or focus, and is therefore a Burning mirror.
Hence, as the focus is there, where the rays are moll clofely
contracted, if it be afegment of a large fphere, its breadth mult
not fubtend an arch above eighteen degrees; if it be a fegment
of a fmaller fphere, its breadth may be thirty degrees.
M. Kircher is of opinion, that thofe Burning mirrors, which
do