r a cliche,(5) then immersed in pure water,
whereby the image is developed and at the same time fixed. It is on
account of the great advantages offered by its simplicity that this
process is generally preferred by civil engineers and architects for the
reproduction of their plans.

The sensitizing solution is prepared in mixing by equal volumes the two
solutions following:

A. Iron, ammonio 20 parts
citrate
Water 100 parts
B. Potassium 15 parts
ferricyanate (red
prussiate)
Water 100 parts

Although the mixture keeps pretty well for a certain period in the dark,
it is best to prepare only the quantity wanted for actual use.(6)

The paper is preferably sensitized in operating as follows:

Take hold of the paper by the two opposite corners and fold it into a
loop, lay it on the iron solution, the center of the sheet first placed in
contact with the liquid, and then gradually spread it by lowering the
corners with a little pressure. No solution should run over on the back
of the paper; it would be a cause of stain. This done, and without
allowing the liquid to penetrate _in_ the paper, immediately take hold of
the two corners near the body and withdraw the paper by dragging it over
on a glass rod for this purpose fixed on the edge of the tray. Now pin up
the paper to dry, which should be done rapidly, and sensitize a second
time in proceeding in the same manner. If this second sensitizing be
found objectionable, let float the paper for no more than ten seconds; of
course this method of sensitizing is not applicable to prepare larger
sheets of paper. In this case the paper is pinned by the four corners on
a drawing board or any other support, lined with blotting paper and
quickly brushed over with a sponge sparingly imbued with the sensitizing
mixture, so as to wet the paper with a very small excess of liquid.

The rationale of this manner of sensitizing is to impregnate only the very
surface of the paper with the ferric salts, and thereby to obtain an
intense blue with very good whites, which latter it would be impossible of
obtaining should the sensitizing solution be allowed to reach in the
fibers of the paper, for, in this condition, it is impossible, owing to
the exigencies of the process, to wash out thoroughly the iron salts to
prevent the chemical changes which cause the whites to be tinted blue. It
is for this reason that better