ABSTRACT
In photographic processes, where silver and other metals are used, the effect of light is so rapid that the state of the weather, as to gloom or sunshine, is of little moment. If papers tinged with vegetable colours are intended to be preserved, they must be kept perfectly dry and in darkness. A close tin vessel, the air of which is dried by quicklime (carefully enclosed in double paper bags, well pasted at the edges to prevent the dust escaping), is useful for this purpose. Moisture destroys them for the most part rapidly, though some (as the colour of the Senecio splendens) resist obstinately. Paper stained with the tincture of the flower is changed to a vivid scarlet by acids, and to green by alkalies; if ammonia be used, the red colour is restored as the ammonia evaporates, proving the absence of any acid quality in the colouring matter sufficiently energetic to coerce the elastic force of the alkaline gas.
