whole
sheet of paper curls up into a roll, and is quite unmanageable. I want to
know, therefore, whether there is any objection to allowing the paper to
remain on the iodizing solution until it lies flat on it, so that on
removal it will not curl, and may be easily and conveniently laid on the
dry side to pass the glass rod over it. As soon as the paper is floated on
the solution (I speak of Turner's) it has a great tendency to curl, and
takes some time before the expansion of both surfaces becoming equal allows
it to lie quite flat on the liquid. May this operation be performed by the
glass rod, without floating at all?

Photographers, like myself, at a distance from practical instruction, are
so much obliged for plain and simple directions such as those given by DR.
DIAMOND, which are the result of experience, that I am sure he will not
mind being troubled with a few inquiries relative to them.

C. E. F.

_Hockin's Short Sketch._--Mr. Hockin is so well known as a thoroughly
practical chemist, that it may suffice to call attention to the fact of his
having published a little brochure entitled _How to obtain Positive and
Negative Pictures on Collodionized Glass, and copy the latter upon Paper. A
Short Sketch adapted for the Tyro in Photography._ As the question of the
_alkalinity_ of the nitrate bath is one which has lately been discussed, we
will give, as a specimen of Mr. Hockin's book, a quotation, showing his
opinion upon that question:

"_The sensitizing agent_, nitrate of silver in crystals, not the
ordinary fused in sticks, is nearly always confessedly adulterated; it
is thus employed:

"_The silver or nitrate bath._--Nitrate of silver five drachms,
distilled water ten ounces; dissolve and add iodized collodion two
drachms.

"Shake these well together, allow them to macerate twelve hours, and
filter through paper. Before adding the nitric acid, test the liquid
with a piece of blue litmus paper; if it remain blue after being
immersed one minute, add one drop of dilute nitric acid[3], and test
again for a minute; and so on, until a claret red is indicated on the
paper. It is necessary to test the bath in a similar manner, frequently
adding half a drop to a drop of dilute acid when required. This
precaution will prevent the fogging due to alkalinity of the bath, so
formidable an obstacle to young hands."

[Footnote 3: "Dilute nitric acid.--Water fi