I will notice one of the principal causes having a tendency
to prevent the perfect uniformity of chemical action,
between the iodine and silver; hydrogen, or the moisture
in the atmosphere, makes a very perceptible barrier.
This moisture may arise as the result of the cold, from a want
of friction in the buffing of the plate, which, coming in contact
with the warmer air, as a writer on this subject says:

"It is well known that as often as bodies, when cold, are exposed
to a warmer air, the humidity contained in them is condensed.
It is to this effect that we must attribute the difficulty
experienced in operating in most cases." This is corroborated
by the results experienced by our operators. So it is seen that
the plate should be of a temperature above that of the atmosphere.
Mr. Gurney submits his plates to a gentle heat from a spirit
lamp just before exposing them to the vapor of iodine.
Experience has convinced me that a plate heated to about 80 deg.
before being exposed to iodine will present a far better
defined image than aplate at a temperature of 50 deg.
I account for this by noticing that, at a higher temperature,
the plate throws off any larger crystals that might otherwise
be deposited, receiving only the finer, thus producing
a more perfect chemical combination of iodide of silver.
I would call the attention of the operator to this point,
as presenting something of interest, and which may direct
in a way of accelerating the future operations.

That the presence of a film of moisture over the plate is a preventive
of uniform chemical action, may be readily understood from the fact