Those furnished by the manufacturers are not
always correct, and it requires some experience to find the proper
degree on the scale.

I would here remark that it is advisable, when placing the spirit lamp
under the bath, to so arrange it that the position of applied heat
should always be on the same point, viz., should the heat be directly
under the bulb containing the thermometer it would raise the mercury in
the tube to the point marked, and the temperature of that in the bath
would be far below what it should be; hence it is (where time is
followed for developing) that many failures occur. This is observed
more readily in the large baths made of thick iron, particularly upon
first heating. In practice I apply the heat as nearly as possible
between the centre of the bottom of the bath and the bulb containing
the mercury tube. It is advisable to keep the lamp lighted under the
bath from the time of commencing in the morning to the close of
business at night. By this means you have a uniformity of action, that
cannot be otherwise obtained.

It is well known to the experienced Daguerreotypist, that different
atmospheres have a decided effect upon the mercury in developing the
Daguerreotype. It will require a greater degree of heat for one
atmosphere than for another. Experience alone determines this little
difference.

In summer, on cloudy and stormy days, mercurial vapors rise more
readily and quickly than in the temperature of autumn or winter. From
60 degrees upwards towards the boiling point (660 deg.), the vapors of
mercury rise in greater abundance and collect in larger globules on
cold surfaces.

For various reasons I prefer a high temperature and short exposure. It
accelerates the process. It renders the lights of the picture more
strong and clear, while the deep shades are more intense. It gives a
finer lustre to the drapery. The solarized portions also are very
seldom blue, especially after gilding. If heated too high, however,
the light parts become of a dead, chalky white, and the shadows are
injured by numerous little globules of mercury deposited over them.
Just the right quantity of mercury leaves the impression of a
transparent, pearly white tone, which improves in the highest degree in
gilding. To mercurialize with exactness is a nice point. If there is
reason to suspect having timed rather short in the camera, reduce the
time over mercury in a corresponding proportion. A dark impres