Although we intended to only describe the printing processes without the
use of silver salts, we thought it would be well to complete this work by
giving the most practical and interesting processes ever published to
obtain permanent photographs; as they may give rise in the hand of
experimenters to useful applications.
From time to time processes are published under “queer” names, which are
based on the well known actions of reagents on the ferric salts reduced by
light. They are derived from those described in the following pages.
We call specially the attention of the reader to the process of Poitevin,
by which one can experiment with every ferric salts, citrate, lactate,
oxalate, tartrate, benzoate, etc., by simply exciting with the
corresponding acid. Observe that to obtain good results the paper should
be strongly sized; it is a sine qua non, although not recommended by
Poitevin.