OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
193 
I S3. Papaver orientale . — The chemical habitudes of the sulphurous acid render it 
highly probable that its action, in inducing a dormant state of the colorific principle, 
consists in a partial deoxidizement^ unaccompanied however with disorganization of 
its molecules. And this view is corroborated by the similar action of alcohol already 
spoken of ; similar, that is, in kind, though less complete in degree. Most com- 
monly, vegetable colours, weakened by the action of alcohol, are speedily restored 
on the total evaporation of that ingredient. But one remarkable instance of absolute 
dormancy induced by that agent, has occurred to me in the case of the Papaver 
orientale , a flower of a vivid orange colour, bordering on scarlet, the colouring matter 
of which is not extractable otherwise than by alcohol, and then only in a state so 
completely masked, as to impart no more than a faint yellowish or pinkish hue to 
paper, which it retains when thoroughly dry, and apparently during any length 
of time without perceptible increase of tint. If at any time, however, a drop of 
weak acid be applied to paper prepared with this tincture, a vivid scarlet colour is 
immediately developed, thus demonstrating the continued though latent existence 
of the colouring principle. On observing this, it occurred to me to inquire whether, 
in its dormant state, that principle still retained its susceptibility of being acted on 
by light, since the same powerful and delicate agent which had been shown, in so 
many cases as to constitute a general law, capable of disorganising and destroying 
vegetable colours actually developed, might easily be presumed competent to destroy 
the capacity for assuming colour, in such organic matter as might possess it, under 
the influence of their otherwise appropriate chemical stimuli. A strip of the paper 
was therefore exposed for an hour or two to the spectrum, but without any sensible 
effect, the whole surface being equally reddened by an acid. As this experiment 
sufficiently indicated the action of light, if any, to be very slow, I next placed a strip, 
partly covered, in a south-east window, where it remained from June 19 to August 
19, receiving the few and scanty sunbeams which that interval of the deplorable 
summer of 1841 afforded. When removed, the part exposed could barely be distin- 
guished from the part shaded, as a trifle yellower. But on applying acid, the exposed 
and shaded portions were at once distinguished by the assumption of a vivid red in 
the latter, the former remaining unchanged. 
184. A mezzotinto picture was now pressed on a glazed frame over another por- 
tion of the same paper, and abandoned on the upper shelf of a green-house to what- 
ever sun might occur from August 19 to October 19. The interval proved one of 
almost uninterrupted storm, rain, and darkness. On removal, no appearance what- 
ever of any impressed picture could be discerned, nor was it even possible to tell the 
top of the picture from the bottom. It was then exposed in a glass jar to the fumes 
of muriatic acid, when, after a few minutes, the development of the dormant picture 
commenced, and slowly proceeded, disclosing the details in a soft and pleasing style. 
Being then laid by in a drawer, with free access of air, the picture again faded, by 
very slow degrees, and on January 2, 1842, was found quite obliterated. Being then 
2 c 
MDCCCXLII. 
