912 
/ 
tioned in our last volume, of only a par- 
tial adherence to the Linnean nomen- 
clature. Thus the large genus dnas is 
converted merely into a reference, the 
species being described under their spe- 
cific names. We turned therefore to dn- 
ser for the anas anser, where we found a 
meagre description, with a further refe- 
rence to Goose; to Antarctica for anas 
antarctica, which is wholly omitted ; to 
Arborea for anas arborea, under which 
mame the compleat description of this 
species is placed; so that by this absurd 
confusion several species will be omitted, 
and others referred on from article to 
article, to the great trouble and per- 
plexity of the reader. Animalcule is 
superficial, from the writer having re- 
course tonone but English authors. In 
the zoological articles use has been pro- 
perly made of Dr. Shaw’s work ; and the 
botany is largely extracted from Martyn’s 
Miller. 
There is no mineralogical article of any 
importance; but the chemical part of the 
volume before us is very rich. Ammonia, 
Animal matter, Antimony, Arama, and 
Arsenic, are the most elaborate, and ma- 
nifest in the writers extensive enquiry, 
and an intimate acquaintance with labo- 
ratory details. 
In‘the department of drts and AZfa- 
nufactures there are only a few articles, 
but these are excellent, and contain much 
new and accurate information : as a spe- 
cimen, we shall quote the whole article 
Aquatinta. 
«© AquatrnTa, in the history of the arts, 
a method of producing engravings very much 
xeseinbling drawings in Indian ink. 
«« The principle of this process consists in 
corroding the copper with aqua fortis, in 
such a n:*nner, that an impression from it 
has the appearance of a tint laid on the paper. 
This is effected by covering the copper with 
a powder or some substance which takes a 
granulated form, so as to prevent the aqua 
fortis from acting where the particles adhere, 
and bythis meanscause it to corrode the copper 
partially and in the interstices only. When 
these particles are extremely minute and near 
to each other, the impression from the plate 
appears to the naked eye exactly like a wash 
f Indian ink; but when they are larger, the 
granulation is more distinct: and as thts may 
be varied at pleasure, it is capable of being 
adapted with great success to a variety of 
purposes and subjects. i arn 
« This powder or granulation is called 
the aquatinta grain, and there are two gene- 
ral moczs of produc’ ng it. 
ss We eH first describe what is called the 
powder grain, because it was the first that 
GENERAL SCIENCE. 
was used. Having etched the outline on a 
copper plate prepared in the usual way by the 
coppersmith (for which see the article Bex 
ing) some substance must be finely powdered 
and sifted which will melt with heat, and 
when cold adhere to the plate, and resist the 
action of aqua fortis. ‘The substances which 
have been used for this purpose, either sepa- 
rately or mixed, are, asphaltnm, Burgundy, 
pitch, rosin, gum copal, and gum mastic ; 
and in a degree all the resins and gum resins 
will answer the purpose. Common resin has 
been most generally used, and answers toler- 
ably well; though gum copal makes 4 grain 
that resists the aqua fortis Conia The sub- 
stance intended to be used for the grain must 
now be distributed over the plate, as equally 
as possible; and different methods of per= 
forming this essential part of the operation 
have been used by different engravers, and at: 
different times. The most usual way is to 
tie up some powder in a piece of muslin, and 
to strike it against a piece of stick held ata 
considerable height above the plate. By this, 
the powder that issues falls gently, and settles 
equally over the plate. Every one must have, 
observed how uniformly hair powder settles 
upon the furniture afier the operations of the, 
hair dresser: this may afford a hint towards 
the best mode of performing this part of the 
process. The powder must fall upon it from 
a considerable height, and there must be a, 
sufficiently large cloud of dust formed. The 
plate being covered equally over with the dust 
or powder, the operator is next: to proceed to 
fix it upon whe plate, by heating it gently, so 
as to melt thg particles. This may. be eflected 
by holding under the plate lighted pieces of 
brown paper rolled up, and moying them 
about tll every part of the powder is melted. 
This will be known by its change of colour, 
which will turn brownish. It must now be 
suffered to'cool, when it may be examined 
with a magnifier; and if the grains or par- 
ticles appear to be uniformly distributed, it is 
ready for the next part of the process. he 
design or drawing to be engraved must now 
be examined, and suéh parts of it as are per- 
fectly white, areto be remarked. "Those cor- 
responding parts of the plate must be covered, 
or stopped out, as it is called, with turpen- 
tine, or what is better, mastic varnish, dilated: 
with turpentine to a proper eonsis ence, to 
work freely with, the pencil, and nixed with 
lamp-black to give it colour; for, if transpa- 
rent, the touches of the pencil would not be 
so distinctly seen. The margin of the plate, 
must also be covered with varnish. When 
the stopping out is sufliciently dry, a border 
of wax must be raised round the plate in the 
same manner as in etching, and the aqua, 
fortis properly diluted with water, poured on. 
This is called biting in; and it is that part 
of the process which is most uncertain, and 
which requires the greatest degree of experi- 
ence. When the aqua fortis has been on so 
long thatthe plate, when printed, would pro- 
duce the lightest tint in. the drawing, it is 
