at its conclusion, to say a few words as to our principal object. As the
subject for this evening's lecture I have chosen, "The Preparation of
Gelatine Plates," as it is probably one of very general interest to
photographers.

Before preparing our emulsion, we must first decide upon the particular
materials we are going to use, and of these the first requisite is
nitrate of silver. Nitrate of silver is supplied by chemists in three
principal conditions:

1. The ordinary crystallized salt, prepared by dissolving silver in
nitric acid, and evaporating the solution until the salt crystallizes
out. This sample usually presents the appearance of imperfect crystals,
having a faint yellowish tinge, and a strong odor of nitrous fumes, and
contains, as might be expected, a considerable amount of free acid.

2. Fused nitrate, or "lunar caustic," prepared by fusing the
crystallized salt and casting it into sticks. Lunar caustic is usually
alkaline to test paper.

3. Recrystallized silver nitrate, prepared by redissolving the ordinary
salt in distilled water, and again evaporating to the crystallizing
point. By this means the impurities and free acid are removed.

I have a specimen of this on the table, and it consists, as you observe,
of fine crystals which are perfectly colorless and transparent; it is
also perfectly neutral to test paper. No doubt either of these samples
can be used with success in preparing emulsions, but to those who are
inexperienced, I recommend that the recrystallized salt be employed. We
make, then, a solution of recrystallized silver nitrate in distilled