6. =The Work of the Acid.=--After the subject has been drawn on the ground, the acid steps in to give variety to the forms which were laid out for it by the needle, to impart vibration to this work of uniform aspect, and to inform it with the all-pervading warmth of life. In principle, a single biting ought to be sufficient; but if the artist desires to secure greater variety in the result by a succession of partial bitings, the different distances may be made to detach themselves from one another by covering up with varnish the parts sufficiently bitten each time the plate is withdrawn from the bath. The different parts which the mordant is to play must be regulated by the feeling: discreet and prudent, it will impart delicacy to the tender values; controlled in its subtle functions, it will carefully mark the relative tones of the various distances; less restrained and used more incisively, it will dig into the accentuated parts and will give them force.
7. =The Use of the Dry Point.=--If harmony has not been sufficiently attained, the _dry point_ is used on the bare metal, to modify the values incompletely rendered, or expressed too harshly. Its office is to cover such insufficient passages with a delicate tint, and to serve, as Charles Blanc has very well expressed it, as a _glaze_ in engraving.
8. =Spirit in which the Etcher must work.=--Follow your feeling, combine your modes of expression, establish points of comparison, and adopt from among the practical means at command (which depend on the effect, and on which the effect depends) those which will best render the effect desired: this is the course to be followed by the etcher. There is plenty of the instinctive which practice will develop in him, and in this he will find a growing charm and an irresistible attraction. What happy effects, what surprises, what unforeseen discoveries, when the varnish is removed from the plate! A bit of good luck and of inspiration often does more than a methodical rule, whether we are engaged on subjects of our own invention,--_capricci_, as the Italians call them,--or whether we are drawing from nature directly on the copper. The great aim is to arrive at the first onset at the realization of our ideas as they are present in our mind. An etching must be virginal, like an improvisation.