XII. On the action of the rays of the solar spectrum on vegetable colours, and on some new photographic processes

Abstract

149. In my paper on the "Chemical Action of the Solar Spectrum on preparations of Silver and other substances,” read to the Royal Society in February 1840, and of which the present communication is intended as a continuation or supplement, some experiments on the effect of the spectrum on the colouring matter of the Viola tricolor , and on the resin of guaiacum are described, which the extreme deficiency of sunshine during the summer and autumn of the year 1839 prevented me from prosecuting efficiently up to the date of that communication. The ensuing year 1840 was quite as remarkable for an excess of sunshine as its predecessor for the reverse. Unfortunately the derangements consequent on a change of residence prevented my availing myself of that most favourable conjuncture, and it was not till the autumn of that year that the inquiry could be resumed. From that time to the present date it has been prosecuted at intervals as the weather would allow, though owing to the almost unprecedented continuance of bad weather during the whole of the past summer and autumn (1841), it has of late been almost wholly suspended. 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. It is otherwise in the class of photographic actions now to be considered, in which exposure to the concentrated spectrum for many hours, to clear sunshine for several days, or to dispersed light for whole months, is requisite to bring on many of the effects described, and those some of the most curious. Moreover, in such experiments, when unduly prolonged by bad weather, the effects due to the action of light become mixed and confounded with those of spontaneous changes in the organic substances employed, arising from the influence of air, and especially of moisture, &c., and so give rise to contradictory conclusions, or at all events preclude definite results, and obscure the perception of characters which might serve as guides in an intricate in quiry, and afford hints for the conduct of future experiment. It is owing to these causes that I am unable to present the results at which I have arrived, in any sort of regular or systematic connection; nor should I have ventured to present them at all to the Royal Society, but in the hope that, desultory as they are, there may yet be found in them matter of sufficient interest to render their longer suppression unadvisable, and to induce others more favourably situated as to climate, to prosecute the subject.