It will be observed that we keep the bromide in excess while mixing. I
must not forget to mention that to those experienced in mixing, by
far the best method is that described by Captain Abney in his Cantor
lectures, of keeping the silver in excess.

The emulsion, being properly mixed, has now to be placed in the water
bath, and kept at the boiling point for forty-five minutes. As,
obviously, I cannot keep you waiting while this is done, I propose to
divide our emulsion into two portions, allowing one portion to stew, and
to proceed with the next operation with the remainder.

Supposing, then, this emulsion has been boiled, it is placed in cold
water to cool. While it is cooling, let us consider for a moment what
takes place during the boiling. It is found that during this time the
emulsion undergoes two remarkable changes:

1. The molecules of silver bromide gradually aggregate together, forming
larger and larger particles.

2. The emulsion increases rapidly in sensitiveness. Now what is the
cause, in the first place, of this aggregation of molecules: and, in the
second place, of the increase of sensitiveness? We know that the two
invariably go together, so that we are right in concluding that the same
cause produces both.

It might be thought that heat is the cause, but the same changes take
place more slowly in the cold, so we can only say that heat accelerates
the action, and hence must conclude that the prime cause is one of the
materials in the emulsion itself.