20 Action of the Solar Spectrum on Vegetable Colours. 
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 other- 
wise 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 ap- 
parently during any length of time without perceptible in- 
crease 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 con- 
tinued 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 consti- 
tute a general law, capable of disorganising and destroying 
vegetable colours actually developed, might easily be pre- 
sumed 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 suf- 
ficiently 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 distinguished 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 portion of the same paper, and abandoned 
on the upper shelf of a green-house to whatever sun might 
occur from August 19 to October 19. ‘The interval proved 
one of almost uninterrupted storm, rain, and darkness. On 
removal, no appearance whatever 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, 
