The author, having prosecuted the inquiry, the first steps of which he communicated in a paper read to the Royal Society in February 1840, relating to the effects of the solar spectrum on the colouring matter of the
Viola tricolor
, and on the resin of guaiacum, relates, in the present paper, the results of an extensive series of similar experiments, both on those substances, and also on a great number of vegetable colours, derived from the petals of flowers, and the leaves of various plants. In the case of the destruction of colour of the preparations of guaiacum, which takes place by the action of heat, as well as by the more refrangible rays of light, he ascertained that although the non-luminous thermic rays produce an effect, in as far as they communicate heat, they are yet incapable of effecting that peculiar chemical change which other rays, much less copiously endowed with heating power, produce in the same experiment. He also found that the discoloration produced by the less refrangible rays is much accelerated by the application of artificial terrestrial heat, whether communicated by conduction or by radiation; while, on the other hand, it is in no degree promoted by the purely thermic rays beyond the spectrum, acting under precisely similar circumstances, and in an equal degree of condensation. The author proceeds to describe, in great detail, the photographic effects produced on papers coloured by various vegetable juices, and afterwards washed with solutions of particular salts; and gives a minute account of the manipulations he employed for the purpose of imparting to paper the greatest degree of sensitiveness to the action of solar light. This action he found to be exceedingly various, both as regards its total intensity and the distribution of the active rays over the spectrum. He observed, however, that the following peculiarities obtain almost universally in the species of action exerted. First, the action is
positive
; that is to say, light destroys colour, either totally, or leaving a residual tint, on which it has no further, or a very much slower action; thus effecting a sort of chromatic analysis, in which two distinct elements of colour are separated, by destroying the one and leaving the other outstanding. The older the paper, or the tincture with which it is stained, the greater is the amount of this residual tint.
XXII. Bakerian lecture.—On the laws connexion between the conditions of a chemical change and its amount.—III. Further researches on the reaction of hydrogen dioxide and hydrogen iodide
