Bayswater.
_After-dilution of Solutions._--There are in general use two methods of
preparing sensitive paper. In one, as in Mr. Talbot's, the iodide of silver
is formed in a state of purity, before being rendered sensitive: and as,
for this end, a small quantity only of nitrate of silver is necessary, a
very dilute solution will answer the purpose as well, or even better, than
a strong one; but by the other method, the paper being prepared with iodide
of potassium only, or with some other analogous salt, the iodide of silver
has to be formed by the same solution that renders it sensitive. Now as for
every 166.3 parts of iodide of potassium 170.1 parts of nitrate of silver
are required for this purpose, it is evident that a dilute solution could
not be employed unless a very large bulk were taken, and the paper kept in
a considerable time.
The after-washing is to remove from the surface of the paper the great
excess of silver, which is of but little service, and prevents the paper
from keeping.