A Manual Of Photography

67 THE COLOURING MATTER OE FLOWERS. injure the tint. The presence of alcohol prevents the solution of the gummy principle, which, when present, gives a smeary surface ; but the evenness of tint given by this process results chiefly from that singular intestine movement which always takes place when alcohol is in the act of separation from water by evaporation ; a movement which disperses knots and blots in the film of liquid with great energy, and spreads them over the surrounding surface. 44 Corchorus Japonica.—The flowers of this common and hardy but highly ornamental plant are of a fine yellow, somewhat inclining to orange, and tins is also the colour the expressed juice imparts to paper. As the flower begins to fade the petals whiten —an indication of their photographic sensibility which is amply verified on exposure of the stained paper to sunshine. I have hitherto met with no vegetable colour so sensitive. If the flowers be gathered in the height of their season, paper so coloured (which is of a very even and beautiful yellow) begins to discolour in ten or twelve minutes in clear sunshine, and in half an hour is completely whitened. The colour seems to resist the first impression of the light, as if by some remains of vitality, which being overcome, the tint gives way at once, and the dis- colouration, when commenced, goes on rapidly. It does not even cease in the dark when once begun. Hence it happens that photographic impressions taken on such paper, which, when fresh, are very sharp and beautiful, fade by keeping, visibly from day to day, however carefully preserved from light, they require from half an hour to an hour to complete, according to the sun- shine. Hydriodate of potash cautiously applied retards consider- ably, but does not ultimately prevent, this spontaneous discharge. “Common Ten Weeks’ Stocks: Mathiola annua. -I aper stainet with the tincture of this 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. Sul- phurous acid whitens it, as do the alkaline sulphites ; but tins effect is transient, and the red colour is slowly restored by free exposure to air, especially with the aid of light, whose influence in this case is the more remarkable, being exactly the reverse of its ordinary action on this colouring principle, which it destroys irrecoverably, as above stated. The following experiments were made to trace and illustrate this curious change: “ Two photographic copies of engravings taken on paper tinted with this colour were placed in a jar of sulphurous acid gas, by which they were completely whitened, and all traces of the