sulphuric acid, repeating the
quantity as often as necessary, providing always that acid be not used
in manufacturing the quick.

4. Have the plates lost their sensitiveness by being many times
exposed to mercury? Clean and burn them; but if French plates, burn
light, or you spoil them.

5. Are the buff s dry and clean? Examine the plate critically after
buffing to detect any appearance of scum or film on the surface. If
so, the longer you buff the more it shows. Scrape and dry the buffs
thoroughly.

6. Is the mercury free from scum and dirt? If not, filter. Is it
also far enough from the coating boxes? Should be at least three feet,
and kept covered.

7. Is the mercury sufficiently heated? This is important. Long
exposure, however, will answer the same purpose.

8. Are your lenses clean, and in proper place?

9. Are the tablets in focus with the ground-glass? If you can
attribute the failure to none of these, mix a new box of some other
kind of quick, say the dry, for instance. If you fail in the same
manner here, take time, wash your buffs, overhaul all the chemicals,
and start anew. Do not be discouraged.

There is no day so dark but that the sun will shine again. We will
close with this brief summary of advice:

Clean your plates. Keep everything dry. Keep the mercury hot. Follow
these instructions carefully, and you must succeed.

HINTS AND CAUTIONS.

First of all, cleanliness should be observed. When there is dust or
dirt about your room, particularly about the work-bench, failures will
be frequent; for the smallest particles of rotten-stone, when allowed
to come in contact with the buffs, will produce scratches on the
surface of the plate, which very much injures the operation, and often
causes failures.

Dust flying about the room is injurious, if allowed to fall on the
plate, either before or after it has been coated, as it causes black
spots which cannot be removed.

The polished plate should not be allowed to come in contact with a
strong current of air, for it tends to oxidize the surface. Breathing
on the surface should also be avoided, for the same reason.

The plate should, in all cases, be buffed immediately before using, and
not allowed to stand any length of time. It should be held with the
polished face downward.

It is always best that the plate should be of the same temperature of
the atmosphere in the room.

Keep the camera and mercury-bath perfectly f