the accident; later on you shall hear something of the remedy. We
will now continue our biting. Plunge your plate into the bath again, if
you please.

38. =Means of ascertaining the Depth of the Lines.=--"My dear sir, I see
that my drawing turns black; it disappears almost entirely, and is lost
in the color of the ground.[9] I am quite perplexed. My mind endeavors
to penetrate beneath this varnish, so as to be able to witness the
mysterious birth of my _oeuvre_. See these violent ebullitions! What
do you think of them?"

Let them go on a moment longer, and then withdraw your plate. We have
now arrived at a point where the eye cannot judge of the work of the
acid as easily as before; henceforth we must, therefore, examine the
depth of our bitings by uncovering a single line, as, for instance, this
one here in the ground. Or we may even lay bare, by the aid of spirits
of turpentine, a part of the foreground, provided, however, that we must
not forget to cover it again with the brush. This will give us an idea
of the total effect so far produced by the biting, and we can then
regulate the partial bitings which are still to follow, either by a
comparison of the time employed on those that have gone before, or by
the intensity of the ebullitions, the action of which on the copper we
have already studied. You perceive that, while it is difficult to fix a
standard of time for the bitings at the beginning of the operation, it
is yet possible to calculate those to come by what we have so far done.

39. =The Rules which govern the Biting are subordinated to various
Causes.=--In reality, it is impossible to establish fixed rules for the
biting, for the following reasons:--

1. Owing to the varying intensity of the stroke of the needle. The
etcher who confines himself to gently baring his copper must bite longer
than he who attacks his plate more vigorously, and therefore exposes it
more to the action of the acid.

2. Owing to the different quality of the plates.

3. Owing to the difference in temperature of the surrounding air:--of
this we have before spoken.

4. Owing to difference of strength in the acid, as it is impossible
always to have it of absolutely the same number of degrees. At 15 deg.
to 18 deg. the biting is gentle and slow; at 20 deg. it is moderate;
at 22 deg. to 24 deg. it becomes more rapid. It would be dangerous to
employ a still higher degree for the complete biting-in of a plate,
especially in the ligh