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.