ray made of gutta-percha or porcelain.

28. =The Tray.=--Let us now install ourselves at this table, and let us
cover the margin and the back of the plate with a thick coat of
stopping-out varnish. As soon as the varnish is perfectly dry, we place
the plate into the tray standing horizontally on the table, and pour on
acid enough to cover it to the height of about a centimetre. This depth,
which is sufficient for biting, allows the eye to follow the process in
its various stages.

29. =Strength of the Acid.=--This acid is fresh, and has not yet been
used; bought at forty degrees, I mix it with an equal quantity of
water, which reduces it to twenty degrees. This is the strength
generally adopted for ordinary biting. Its color is clear, and slightly
yellow; but as soon as it takes up the copper it becomes blue, and then
green. As, in its present state, it would act too impetuously, I add to
it a small quantity of acid which has been used before. You may also
throw a few scraps of copper into it the day before using it; the old
etchers used for this purpose a copper coin, larger or smaller,
according to the volume of the bath.[7]

30. =Label your Bottles!=--One day, one of my pupils, having a bad cold,
did not notice the difference between the smell of the acid and that of
the turpentine, and so plunged a plate which he desired to bite, into a
bath of the latter fluid. "It's queer," he said, "this won't bite, and
yet the varnish scales off.... The lines keep enlarging, and run into
one another! What does this curious medley mean, which appears on the
plate?" It was simple enough. The spirits of turpentine had dissolved
the ground, and consequently the plate developed a shining and radiating
surface before the eyes of our wondering student, as if it had just left
the hands of the plate-maker.

Advice to those who are absent-minded, and who are liable to mistake
fluids which look alike for one another,--Label your bottles!

31. =The First Biting.=--Let us make haste now, I say to my pupil, to do
our biting. As the heat of the day abates, the acid becomes less active;
and besides, to judge by the delicate character of the original we are
to render, we shall need at least two or three hours, all told, for this
operation. The task before us consists in the reproduction of a given
work, the merit of which lies in the gradation in the various distances.
It needs time and attention to be able to carry all the necessary
proces