ong letter to the Editor; but the mistake will, I trust,
be rectified by another communication which I have now sent.

The whole of the formulae, however, as given in "N. & Q.," are quite
correct.

Let me now, however, trespass on your pages by a few more answers to
several other Querists, and which at the same time may be acceptable to
some of your readers.

1. The developing agents which are made with iron are very applicable as
baths to immerse the plate in; and the formic acid, from its powerful
deoxidizing property, renders the iron salt more stable during long use and
exposure to the air.

2. In coating paper with albumen, if the upper edge of the paper be
sufficiently turned back, and the paper be forced down sufficiently on to
the surface of the albumen, no bubbles will form; and {374} the operator
will not be troubled with the streaks so often complained of.

3. No time can possibly be fixed for the exposure of the positive to the
action of the hypo.; and to produce the best effects, the positive must be
continually watched, both while printing and while in the hypo.

4. No hot iron should be applied to the positive after being printed, but
the picture should be allowed to dry spontaneously.

5. The developing agent with the pyrogallic and formic acids will keep good
a very long time, longer, I think, than that in which acetic acid is used,
but cannot be used as a dipping bath.

6. I find the formic acid which I obtain from different chemists rather
variable in its strength. What I use is rather below the average strength,
so that in general about six drachms of the commercial acid will suffice
where I use one ounce; but the excess seems to produce no bad result.

7. A great advantage of the pyrogallic developer which I recommend, is that
of its being able to be diluted to almost any extent, with no other result
than simply making the development slower. Another point is also worthy of
notice, viz. a method by which even a very weak positive on glass may be
converted into a very strong negative.

I take a saturated solution of bichloride of mercury in hydrochloric acid,
and add of this one to six parts of water. This I pour over the collodion
plate, and watch it till the whitening process is quite complete. Having
well washed the surface with water, I pour over it a solution of iodide of
potassium, very weak, not more than two or three grains to the ounce of
water. The effect of this is to turn the wh