ere, 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 a plate 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
that iodine is almost insoluble in water, requiring seven thousand
parts of water to dissolve one of iodine, or one grain to a gallon of
water. Yet its affinities for silver and other substances are so
powerful as to prevent its existing in an insulated state, hence we can
account for the frequent occurrence of a plate presenting parts of an
image over its surface. It is quite evident that those parts of
plate's surface covered with moisture are nothing like as sensitive to
the iodine as those parts perfectly free.

Exposure of the plate in the Camera, and Position.--The time of
exposure necessary to produce an image upon the Daguerreotype plate,
can only be determined by experiment, and requires a liberality of
judgment to be exercised on the part of the operator. The constant
variation of the light renders it impossible to lay down any exact rule
upon this point. Light is not alone to be considered; the amount of
coating exercises a deviating influence, also the subjects to be
represented are n