The photosynthesis of naturally occurring compounds.—II. The photosynthesis of carbohydrates from carbonic acid by means of visible light

Abstract

In the preceding communication evidence was brought forward in favour of the photosynthesis of carbohydrates when pure carbonic acid, adsorbed on a surface suspended in water, is exposed to ultra-violet light. Although possibly the results described may seem to mark a step forward towards the explanation of the phenomenon of natural photosynthesis of carbohydrates from carbonic dioxide and water, there still remains unexplained the greatest difficulty of all, namely, the utilisation by the living plant of visible light only. Furthermore, as we were the first to recognise at this stage, the evidence of the photosynthesis of carbohydrates given by the previous experiments is somewhat slender. In all the experiments described in the preceding paper carbonic acid adsorbed on a white surface was exposed to ultra-violet light, it being assumed that in the case of aluminium powder there is formed a layer of hydroxide which is effective. The possibility occurred to one of us (W. E. S.) that if a visibly coloured powder, capable of adsorbing carbonic acid, be employed, the photosynthesis may be brought about by visible light. Two powders which satisfy the conditions are the basic carbonates of nickel and cobalt in that they are visibly coloured and undoubtedly adsorb carbonic acid on their surface when suspended in water through which carbon dioxide is passed. Preliminary experiments were carried out in which suspensions of each of these powders in water were placed in vessels of ordinary German soda glass with walls from 1·5 to 2 mm. in thickness. Carbon dioxide was passed continuously through each vessel, and the results were illuminated by the light from an ordinary tungsten filament lamp. After filtration and evaporation the exposed solutions in each yielded a gummy residue which was undoubtedly organic in nature, since it readily charred with strong sulphuric acid. This experiment was repeated several times with the same results, and these convinced us that it is possible by the use of coloured powders to photosynthesise organic compounds from carbonic acid with the help of visible light.