_Blue Prints_.--The best formula for this process, of many that I have
tried, is that furnished by Prof. C.H. Kain, of Camden, N.J., in which
the quantity of ammonio-citrate of iron is exactly double that of the red
prussiate of potash, and the solutions strong. This gives strong prints
of a bright dark blue, and prints very quickly in clear sunlight.

Dissolve six grains of red prussiate of potash in one drm. of distilled
water; in another drm. of distilled water dissolve twelve grains of
ammonio-citrate of iron. Mix the two solutions in a cup or saucer, and at
once brush over the surface of clean strong paper. Cover the surface
thoroughly, but apply no more than the paper will take up at once; it
should become limp and moist, but not wet. The above quantity of
solution, two drms., will suffice to sensitize ten square feet of paper,
or three sheets of the "regular" size of plain paper, 18×22. As fast as
the sheets are washed over with the solution, hang them up to dry by one
corner. The surplus fluid will collect in a drop at the lower corner, and
can be blotted off.

_Black Prints_.--Wash the paper with a saturated solution of bichromate
of potash, made quite acid with acetic acid. After printing, wash the
prints in running water for twenty to thirty minutes, then float them
face down on a weak solution (five to ten per cent.) of protosulphate of
iron for five minutes, and wash as before. If preferred, the iron
solution may be washed over the prints, or they may be immersed in it,
but floating seems preferable. After the second washing, wash the prints
over with a strong solution of pyrogallic acid, when the print will
develop black, and the ground, if the washings were sufficient, will
remain white. A final washing completes the process.

If a solution of yellow prussiate of potash be used in place of the pyro