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.
