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.