g-wax, which will perfectly adhere to the glass.
If the commoner sorts of wax are used, some marine glue must be added to it
to temper it. The side slips should be fixed a quarter of an inch apart, so
as to form a cavity, which must be entirely filled up with wax. The wax may
be used as in sealing a letter in the first instance; but, in order to give
the whole bath solidity, and expel every particle of air from between the
glass, I use a heated pointed iron, as a plumber does in the act of
soldering. This, passed over the external parts of the wax, also gives it a
hardness and smooth finish.

These details may appeal trifling, and others may have more ingenious modes
of accomplishing the object; but having used baths so constructed upwards
of twelve months without leakage, I believe they will be found to be most
economical, and far more to be relied on than gutta percha. A good bath so
made should require about six ounces of solution of nitrate of silver to
take a picture eight inches square. Your observations in a former Number,
respecting the uncertainty of gutta percha, I have found to be perfectly
true. Samples of gutta percha constantly vary; and one may contain
impurities acted upon by the chemicals, which another does not. A small rim
formed by sealing-wax dissolved in spirits of wine, and applied twice or
thrice along the upper edge of the bath, is sufficient to protect the
prepared glass from adhering to the front of the bath when in use.

H. W. D.

_Securing Calotype Negatives._--Will any of your correspondents be good
enough to say what they consider the best method of securing a calotype
paper negative for a few days or a week, in cases where it may be
difficult, from lack of conveniences during that time, to use hyposulph.,
with its consequent washings, &c.? Some, I believe, recommend bromide of
potassium; some, the iodide; others, common salt: but I should like to know
which is considered the _best_; what strength, and how applied. Also,
whether any subsequent treatment is necessary previous to the final
application of the hypo.

W. T.

* * * * *

Replies to Minor Queries.

_Wood of the Cross_ (Vol. vii., pp. 177. 334.).--I find, in your 179th
Number, p. 334., a communication on "The Wood of the Cross." Mention is
made of the several kinds of wood of which the cross is said to have been
made--elder, olive, &c. It is a somewhat curious coincidence, that
yesterday I w