THE CHEMICAL ACTION OF LIGHT. 79 
the multifarious derivatives of coal tar. The material on which 
it is desired to produce a photographic image is coated or dyed 
with this substance, and then converted into the diazo-derivative 
by means of a suitable bath. The material so prepared is 
exposed in the usual way under a negative, with the result that in 
the high lights the diazo-compound is decomposed by the action 
of light. The picture is then developed by immersion in a bath 
of a suitable phenol or amine, a wide range of colours from blue 
to red being obtained by varying the developer. In this process 
we have a method of photographic printing which is entirely 
dependent upon organic compounds. 
Time will not allow me to enter into the many applications of 
the action of light in the arts and sciences, but enough has been 
said to indicate the important part which it plays with regard to 
our health and well-being, as well as in relation to the comforts 
and luxuries of life. 
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