solution of ferrous sulphate, FeSO_4. The ferrous sulphate combines
with the nitric acid, forming two new salts--ferric nitrate and ferric
sulphate. The silver is deposited. Any other substance which will remove
oxygen from silver nitrate without combining with the silver would do
the same, and metallic silver would be thrown down. The formula, as
shown on the diagram, explains the interchange.

When the developer is poured over the plate it attacks first the free
silver nitrate, and causes it to deposit extremely fine particles of
metallic silver. The question arises: How is it these particles arrange
themselves to form an image? This is explained by the physical movement
known as molecular attraction or affinity. These particles are attracted
first to the portions of the plate where there is most sub-iodide and
sub-bromide. In the shady parts less silver is deposited. When the image
is once started it follows that particles of silver produced by the iron
developer will cause more to fall down on the face of those already
present, and the image is, of course, built up if the silver nitrate
be all consumed on the plate. The developer then becomes useless or
injurious. The presence of acetic acid checks the reduction of the
silver, and the alcohol facilitates the flow when the bath becomes
charged with ether and spirit.

The molecular attraction just mentioned is made plainer by reference to
the simple lead tree experiment. We have here in this bottle a piece
of zinc rod introduced into a solution of acetate of lead. A chemical
change has taken place. The zinc has abstracted the acetic acid and the
lead is deposited on the zinc, and will continue to be so until the
solution is exhausted. The irregularities of surface and arborescent
appearance are well shown. If the change were rapidly conducted the lead
particles would from their weight sink directly to the bottom instead