82 THE WOMBAT. 



energy, — a red light does not represent a sufficient amount of 

 energy to act on the chemicals in the plate, but white light 

 does. 



Now let us try to follow the changes that light actually 

 produces in the silver salt in the plate. If we take a lump of 

 bromide of silver and break it into pieces, each piece is still 

 a piece of bromide of silver ; if we continue the process we 

 shall at last get pieces so small that they cannot be broken or 

 split again without destroying their properties as bromide of 

 silver. These pieces are called molecules — that is, a molecule 

 is the smallest particle of a substance that can exist as that 

 substance. If a molecule of bromide of silver be further split 

 up we get an atom of bromine and an atom of silver. Atoms 

 are the particles of which molecules are composed, and which, 

 in chemical reactions, migrate from molecule to molecule 

 without any change in themselves, although they affect the 

 properties of the molecule in which they "reside." (They 

 •cannot be rendered smaller in size by any means whatever). 



The atoms in a molecule are held together by a certain 

 amount of energy, and we have seen that red light does not 

 represent a sufficient amount of energy to separate them, but 

 white light does. It is for this reason that a plate is not 

 sensitive to a red light, while it is sensitive to a white light. 



In making a gelatine plate, bromide of potash is dissolved 

 in a solution of gelatine, and some solution of nitrate of silver 

 is then poured in and mixed. A molecule of potassium 

 bromide is composed of one atom of potassium (K) and one 

 atom of bromine (Br.) ; a molecule of nitrate of silver contains 

 one atom of silver (Ag.), one atom of nitrogen (N.) and three 

 atoms of oxygen ( ;! ). Thus we get the following reaction 

 when making a gelatine plate : — K Br + Ag NOo = Ag Br + 

 K NO.', ; that is, one molecule of bromide of potash plus one 

 molecule of nitrate of silver = one molecule of bromide of silver 

 plus one molecule of nitrate of potash. The nitrate of potash is 

 washed out by means of water, and the mixture of gelatine 

 and silver bromide is poured over the glass plate and dried. 

 When white light is allowed to fall on this sensitive plate, it 

 reduces the bromide of silver (Ag Br) to sub-bromide — 

 Ago Br. Both of these substances are white, and therefore 

 the image is invisible and is said to be latent. The latent 

 image is rendered visible by development. Although different 

 developers produce negatives differing slightly in appearance, 

 the chemical action is the same in every case, namely, a re- 

 duction of the white silver salt to black metallic silver, so we 

 may take as a typical case the action of ferrous oxalate. You 

 will remember that the plate which has been acted on by 

 light contains bromide and sub-bromide of silver, and both of 

 these salts are reduced to black metallic silver by the action 



