ter parts.

40. =Strong Acid and Weak Acid.=--It is, nevertheless, possible to put
such strong acid to good service. A fine gray tint may, for instance,
be imparted to a well-worked sky by passing a broad brush over it,
charged with acid at 40 deg. But the operation must be performed with
lightning speed, and the plate must instantly be plunged into pure
water.

As a corollary of the fourth cause, it is well to know that an acid
overcharged with copper loses much of its force, although it remains at
the same degree. Thus an acid taken at 20 deg., but heavily charged with
copper from having been used, will be found to be materially enfeebled,
and to bite more slowly than fresh acid at 15 deg. to 18 deg. To continue
to use it in this condition would be dangerous, because there is no longer
any affinity between the liquid and the copper, and if, under such
circumstances, you were to trust to the appearance of biting (which
would be interminable, besides), you would find, on removing the
varnish, that the plate had merely lost its polish where the lines ought
to be, without having been bitten. It is best, therefore, always to do
your biting with fresh acid, constantly renewed, as the results will be
more equal, and you will become habituated to certain fixed conditions.

Some engravers, of impetuous spirit and impatient of results, do their
biting with acid of a high degree, while others, more prudent, prefer
slow biting, which eats into the copper uniformly and regularly, and
hence they employ a lower degree. In this way the varnish remains
intact, and there is not that risk of losing the purity of line which
always attends the employment of a stronger acid.

41. =Strength of Acid in relation to certain Kinds of Work.=--Experience
has also shown that, with the same proportion in the time employed, the
values are accentuated more quickly and more completely by a strong than
by a mild acid; this manifests itself at the confluence of the lines,
where the acid would play mischief if the limit of time were
overstepped.

Another effect of biting which follows from the preceding, is noticeable
in lines drawn far apart. Of isolated lines the acid takes hold very
slowly, and they may therefore be executed with a cutting point and
bitten in with tolerably strong acid.

The reverse takes place when the lines are drawn very closely together;
the biting is very lively. Work of this kind, therefore, demands a
needle of moderate s