78 THE CHEMICAL ACTION OF LIGHT. 
could be obtained more quickly and definitely. Many difficulties 
had to be overcome before a completely successful result was 
obtained, but in the end the experiments were absolutely success- 
ful, and proved the complete bactericidal action of light. 
Experiments with spores were also made to determine the 
relative power of the different rays of the spectrum. Screens of 
coloured glass were interposed between the source of light and 
the plate culture, with the result that it was found that the bacteri- 
cidal action of the sun’s rays is due to those in the blue-violet 
half of the spectrum. 
But in addition to the part which light plays in physiological 
chemistry, it also exerts a powerful influence in purely chemical 
actions, as, for instance, in the combination of hydrogen and 
chlorine, which takes place with violence in the presence of light, 
or in the conversion of red into white phosphorus, also in the 
presence of light. 
But of all the actions brought about by the influence of light, 
those concerned in the production of photographs are, perhaps, 
the most widely known and appreciated. The action here is a 
purely chemical one: certain salts of silver are so altered when 
exposed to light that they themselves change colour, or whilst 
themselves undergoing no physical change, can subsequently be 
decomposed by other substances, which have no action upon 
them in the unaltered state. In addition to the ordinary photo- 
graphic processes in which silver salts are concerned, there are 
many others which depend upon the action of light on other 
substances ; thus, for instance, there is the platinotype process, 
depending upon the changes brought about in the salts of 
platinum and iron; the autotype process, which depends upon 
the action of light upon prepared pigmented gelatine, and many 
others. 
An extremely interesting and novel process has lately been 
introduced which depends upon the action of light upon an 
entirely different class of compounds to those just mentioned. 
This is the diazotype process, in which organic compounds are 
alone concerned. The basis of the process is primuline, one of 
