Oxford.
_Sir W. Newton on Use of Common Soda and Alum._--In reply to W. ADRIAN
DELFERIER, who is desirous of knowing the "_rationale_ of the _action_ of
the _common soda_ and powdered alum, &c.," my motive for using _common
soda_ to cleanse the _negatives_ is, that it not only removes the
hyposulphite of soda more readily, but any impurities which may be in the
paper, as well as the _whole_ of the _size_, such being absolutely
necessary for the after _waxing_ process; which, when done, the negative
should appear nearly as transparent as glass.
The reason why I prefer _alum_ for the positives is, that while it has the
effect of removing the hyposulphite of soda and other impurities in the
paper, it does not act upon the _size_, which in this instance it is
desirous to retain.
I have been induced to make a series of experiments, with a view to prevent
the _fading_ of the _positives_, or, indeed, that any portion should be, as
it were, _eaten away in parts_; and since I have adopted the foregoing, in
no one instance has any change taken place whatever.