The action of resin and allied bodies on a photographic plate in the dark

In former papers it has been shown that certain metals, woods, juices of plants, etc., have the property of acting on a photographic plate in the dark; that a similar action is exerted by coal resins and allied bodies is proved by the following experiments. Ordinary resin or colophony is the solid, remaining on the distillation of crude turpentine, and the substance known in commerce as “amber resin” is ordinary resin slightly purified, and is of a lighter colour.