J.BAILEY .-- Humphrey's Journal.
PRACTICAL HINTS ON THE DAGUERREOTYPE.
The following is from Humphrey's Journal, vol.5, and from the pen of Dr.WM.HARRINGTON, one of the most able writers upon the subject of the Daguerreotype in this country: THE CAUSE OF THE DIFFICULTY THAT SOMETIMES OCCURS TO PREVENT THE PRODUCTION OF A CLEAR IMPRESSION UPON A DAGUERREOTYPE PLATE.
Beyond all doubt this is traceable to dampness.

Truly this is not a new thought; but where does this dampness come from?
How does it originate, and where is it located?
Generally it has been referred to a point entirely remote from its real location.
This dampness exists particularly upon the surface of the plate; is obviously derived immediately from the atmosphere; and is owing to a certain relative temperature of the plate with the hygrometric condition of the atmosphere.
Whenever this relation exists between the plate and atmosphere, a precipitation of moisture takes place upon the surface of the plate, which render all efforts at polishing impracticable.

This interference is not confined to the buffing operation alone, but sometimes is discoverable even in the ordinary process of scouring.

Every one at all experienced in this art will remember that it is not always an easy matter for him, by scouring, to bring his plate to the desired lustre.
All his efforts become unavailing; the more he rubs, the duller the surface of his plate appears; and although he renews his cotton repeatedly, still he is obliged to content himself with an unsatisfactory finish.
This relative condition is not confined to any particular season of the year, nor to any certain thermometric temperature; but may occur in summer as well as in winter; the weather being warm or cold, wet or dry, clear or cloudy, raining or shining.

Under any of these circumstances, if the relation of the plate and atmosphere be such as to invite upon the plate a precipitation of humidity from the atmosphere, the prospect of producing a clear impression is quite problematical.
It is reasonable to expect this occurrence from the fact that metal is a good radiator, and radiation reduces the temperature of a metallic body below that of the atmosphere.