’ I 
MIC 
very different from all other kinds of en- 
graving and etching, since, instead of form- 
ing the figures with lines and scratches 
made with the point of a graver, or by 
means of aqua fortis, they are wholly form- 
ed by scraping and burnishing. Mezzotin- 
tos are made in the following manner; take 
a well-polished copper-plate, and, begin- 
ning at the corner, rake or furrow the sur- 
face all over with a knife or instrument 
made for the purpose, first one way, and 
flien the other, till the whole is of a regular 
roughness, without the le^st smooth part to 
be seen ; in which state, if a paper was to 
be worked off from it at the copper plate 
press it would be all over black. When 
this is done, the plate is rubbed over with 
charcoal, black chalk, or black lead, and 
then the design is drawn with white chalk, 
after which the out-lines are traced out, 
and the plate finished by scraping off the 
roughness, so as to leave the figure on the 
plate. The out-lines and deepest shades 
are not scraped at all, the next shades are 
.scraped but little, the next more, and so 
on, till the shades gradually falling off, 
leave the paper white, in which places the 
plate is neatly burnished. 
By an artful disposition of the shades, 
and different parts of a figure on different 
plates, mezzo tintos have been printed in co- 
lours, so as nearly to represent very beauti- 
ful paintings. 
MICA, in mineralogy, a species of the 
Clay genus, is commonly of a grey colour, 
passing into brown and black. It occurs 
disseminated in thin tables and layers in 
other stones, also crystallized. It feels 
smooth, but not greasy. Specific gravity 
about 2.8 or 2.9. It may be converted by 
the blow pipe into a white enamel, and it 
consists, according to Kirwan, of 
Silica 38 
Alumina 28 
Oxide of iron 14 
Magnesia ...20 
_t()(^ 
By an analysis of Vauquelin, the differ- 
ence is very considerable, as will be seen. 
Silica 50 
Alumina 35 
Oxide of iron 7 
Magnesia 1.35 
Lime 
94.68 
Loss ••• 5 . 32 
’ 100' 
MIC 
Mica is one of the constituent parts of 
granite, gneiss, and mica slate; it is also 
found in syenite, porphyry, and wacce, in 
almost every part of the world. It was 
formerly used for windows and lanterns, in- 
stead of glass, and in the Russian navy it is 
still used for the same purpose, being, on 
account of its elasticity, less liable to break 
than glass, on the discharge of cannon. Mi- 
ca, used by the Russians, is dug up in Si- 
beria. 
MICHAUXIA, in botany, so named in 
memory of Andrew Michanx, botanist ; a 
genus of the Octandria Monogynia class 
and order. Natural order of Carapanacca;. 
Campanulaceas, Jussieu. Essential charac- 
ter: calyx sixteen-parted ; corolla wheel- 
shaped, eight-parted ; nectary eight-valved, 
staminiferous ; capsule eight celled, many- 
seeded. There is but one species, viz. M. 
campanuloides, rough-leaved michaiixia : 
this is a handsome biennial plant, having 
the habit of a campanula; it has a simple 
stem, panicled when in flower; upright, 
herbaceous, rough-haired, green, two feet 
high ; root-leaves petioled, cordate ; stem- 
leaves half embracing, lanceolate; flowers 
in a panicle, peduncled, bracted, hanging 
down. It is a native of Aleppo. 
MICHELIA, in botany, so named in ho- 
nour of Pietro Antonio Micheli of Flo- 
rence ; a genus of the Polyandria Polygy- 
nia class and order. Natural order of Co- 
adunatse. Magnolim, Jussieu. Essential 
character: calyx three-leaved; petals fif- 
teen ; berries many, four-seeded. There are 
two species, natives of the East Indies. 
MICROMETER, an instrument usually 
fitted to a telescope, in the focus of the 
object-gla«s, for measuring small angles or 
distances, as the apparent diameter of the 
planets. The general principle of this in- 
strument is, that it moves a fine wire, paral- 
lel to itself, in the plane of the picture of 
an object, formed in the focus of a teles- 
cope, and thus measures its perpendicular 
distance from a fixed wire in the same 
plane. 
This instrument was invented about the 
year 1666; and it has, of course, undergone 
many improvements since that time. Or. 
Gascoigne divided the image of an object, 
in the focus of the object-glass, by the ap- 
proach of two pieces of metal, ground to a 
very fine edge ; instead of which. Dr. Hook 
would substitute two fine hairs, stretched 
parallel to each other : and two other me- 
thods of Dr. Hook, different from this, are 
described in his postliumous works. An 
