etric 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. Consequently, if this relative
condition happens, the result will be as I have stated.

Bodies may be colder than the atmosphere and yet derive no moisture
from it; while at the same time the driest atmosphere is not devoid of
moisture, but will part with it under certain conditions.

Assuming for granted that this relative condition between the plate and
atmosphere, disposing the former to receive the humidity of the latter,
constitutes the great obstacle the operator has to contend with in
producing, a clear proof upon the plate, the remedy naturally suggests
itself, and is very simple. It consists in merely heating the plate
above the temperature of the atmosphere, previous to polishing, and
retaining that temperature during the operation. Various measures
might be devised to effect the desired object; one of which consists of
a sheet-iron box, heated from the inside by a spirit-lamp, upon the top
of which are to be kept the plates ready to undergo the process of
being polished; the blocks of the swing or any other vice; or the iron
bed belonging to Lewis's vice.

In cold weather, when it is necessary to keep a fire in the preparation
room, all of the above may be so arranged in the vicinity of the fire
as to receive the requisite degree of heat for the purpose specified.

This part of the subject, however, is left entirely for the ingenuity
of the operator. No matter by hat means he accomplishes the object;
all that is required is to heat the plate above the temperature of the
atmosphere and retain that heat during the process of polishing.

Since the adoption of this method, in connection with my partner, T. J.
Dobyns, even in this humid climate of ours, when everything in the room
is dripping with moisture, it has been attended with invariable success.

CHOICE OF PLATES, ETC.

In the great catalogue of complaints made by operators, none is more
common tha