In a paper, communicated to the Society in February 1897, the author drew attention to increase in electrical conductivity as a characteristic of photo-chemical action.

This increase in electric conductivity may be explained by assuming that light acts either in the direction of increased conductivity, or as a disturbing agent on molecular systems having an inherent tendency towards rearrangements involving increased conductivity.

It may be that both assumptions are correct.

Since making this communication the author has been engaged in experimental investigation, undertaken in order to elucidate this subject further—more particularly in its chemical aspect.

The behaviour of nitric acid with respect to light appeared especially suggestive. Nitric acid on progressive dilution with water becomes less and less subject to decomposition by light, until, as that dilution which corresponds to maximum conductivity is approached, the action of light seems to cease altogether, so that nitric acid at all dilutions greater than that corresponding to its maximum electrolytic conductivity appears to be unaltered by light. It seemed, therefore, desirable to investigate experimentally the purely chemical behaviour of nitric, hydrochloric, and sulphuric acids at different degrees of dilution with water. The investigation is now sufficiently far advanced to justify the communication to the Society, in this preliminary note, of some of the conclusions which have been arrived at.