ses successfully into practice.

It will be plain to you, from what I have just said, that the operation
you are about to engage in is one of the most delicate in the etcher's
practice. There is the plate in the acid; the liquid has taken hold of
the copper; but your sky must be light, and a prolonged corrosion would
therefore be hurtful to it. Hence we take the plate out of the bath,
pass it through pure water, so that no acid is left in the lines, and
cover it with several sheets of blotting-paper, which, being pressed
against it by the hand, dries the plate. We shall have to go through the
same process after each partial biting, because if the plate were moist,
the stopping-out varnish which we are going to apply to it would not
adhere.

32. =The Use of the Feather.=--You noticed the lively ebullitions on the
plate, which took place twice in succession. After the first, I passed
this feather lightly over the copper, to show you its use. Its vane
removed the bubbles which adhered to the lines. This precaution is
necessary, especially when the ebullitions acquire some intensity and
are prolonged, to facilitate the biting, as the gas by which the bubbles
are formed keeps the acid out of the lines. If these bubbles are not
destroyed, the absence of biting in the lines is shown in the proofs by
a series of little white points. Such points are noticeable in some of
the plates etched by Perelle, who, it seems, ignored this precaution.

33. =Stopping Out.=--The two rapid ebullitions which you saw may serve
you as a standard of measurement; the biting produced by them must be
very light, and sufficient for the tone of the sky. You may, therefore,
cover the entire sky with stopping-out varnish by means of a brush,
taking care to stop short just this side of the outlines of the other
distances. The importance of mixing lamp-black with your stopping-out
varnish to thicken it, comes in just here; because if it remained in its
liquid state, it might be drawn by capillary attraction into the lines
of those parts which you desire to reserve, and thus, by obstructing
them, might stop the biting in places where it ought to continue. Wait
till the varnish has become perfectly dry; you can assure yourself of
this by breathing upon it; if it remains brilliant, it is still soft,
and the acid will eat into it; but as soon as it is dry it will assume a
dull surface under your breath.[8]

34. =Effect of Temperature on Biting.=--Let us now