yellow wax. The wax is
melted, then applied very hot, and, when it is solidified and quite cold,
the coating is equalized with a hot iron, whereby the cracks produced by
the contraction of the wax when cooling are filled up.

One of these trays should contain a layer, about three-quarters of an inch
thick, of an almost saturated solution of potassium ferrocyanate (the
developer); the next be filled with water, and the third with water
acidified by sulphuric acid in the proportion of three per cent. in
volumes.

All this being ready, the margin of the proof is turned upwards--so as to
form a disk of which the outside is the impressed surface--in order that
the ferrocyanate solution does not find its way on the back of the proof,
which would produce stains. Now the proof is laid, the lower edge first,
on the developer, and gradually lowered upon it, when, taking immediately
hold of it by the two corners nearest to the body, it is lifted out and
held upright to allow one following the development of the image; and,
presently, if any air-bubbles are seen on the proof, they should at once
be touched up with a brush wetted with the ferrocyanate solution; the
reason explains itself.

The image appears at once. As soon as the fine lines are well defined,
the blue intense, and, especially, when the ground has a tendency to be
tinged blue, the proof is placed in the tray filled with water and in this
turned over two or three times, when it is immersed in the diluted
sulphuric acid. In this bath the print acquires a deep blue coloration,
consisting of Prussian blue, and the ground becomes tinted with a blue
precipitate without adherence, which is easily washed off by throwing the
liquid on the proof with a wooden spatula, or, better, by rubbing with a
rag tied to a stick. When the ground is cleared, and after three or four
minutes immersion to dissolve the iron salts acted on, the proof is rinsed
in water several times renewed to free it from acid, and hung to dry.

There are two causes of failures in this process, viz., over and
under-exposure. In the former case the fine lines are broken or washed out
in clearing the proof (which may also arise from the drawing made with an
ink not opaque enough); in the latter the ground is more or less stained.

The blue stains, the lines for corrections, etc., are erased with the the
potassic oxalate (_blue salving,_ as it is termed) whose formula has been
given.

The additions, co