I shall touch very lightly on the manipulative part of photography, as
that would be unnecessary; but a brief account of the chemicals in use
is essential to a right appreciation of the theory of developing the
image. In the first place, our object is to get a film of some suitable
material coated with a thin layer of a sensitive salt of silver--say
a bromo-iodide. By mixing certain proportions of ammonium iodide
and cadmium bromide, or an iodide and bromide of cadmium with
collodion--which is pyroxyline, a kind of gun-cotton dissolved in ether
and alcohol--a plate of glass is coated, and before being perfectly dry
is immersed in the nitrate of silver bath. The silver nitrate solution,
adhering and entering to a slight extent the surface of the collodion,
becomes converted by an ordinary chemical action of affinity into silver
iodide and bromide.

The ammonium and cadmium play a secondary part in the process, and
are not absolutely necessary in forming the image. The plate is now
extremely sensitive to light. When we have entered it into the dark
slide and camera, and then exposed to light, the change I mentioned
has taken place. The film is transformed into different quantities of
sub-iodide and sub-bromide of silver, according to brilliancy of light.
In addition, there is on the plate an amount of unchanged silver nitrate
which becomes useful in the second stage, or development. The image is
not seen as yet, being latent, and requiring the well-known developing
solution of sulphate of iron, acetic acid, alcohol, and water.
Practically we all recognize the effect of a nicely-balanced wave of
developer worked round a plate. The high lights are first to appear as a
darker color, till the details of shadow come out; when this is reached
the developer is washed off. The chemical action is briefly thus, and
it can be shown by solutions without a photographic plate, as in a test
tube: Pour into this glass a solution of silver nitrate, AgNO, and add a