706 REPORT — 1900. 



temperature of 25" C. cannot he attributed to the influence of the pressure whicli 

 lias existed during the natural salt deposition, but must be accounted for by 

 the prevalence of higher temperatures. The presence of these salts in the 

 Stassfurt layer enables us, in fact, to draw conclusions in regard to the tempera- 

 tures which existed during the salt deposition. 



4. Oil the Sensitiveness of Metallic Silver to Light. 

 By Major-General J. Waterhouse, I.S.C. 



The paper is a continuation of that read before the Royal Society on May 31, 

 and contains an account of further experiments on the production of visible 

 photographic images upon plain silver surfaces by the action of solar radiation^. 



The author has found that such visible images are formed when pure silver 

 foils or silvered glass are exposed to sunlight in exhausted glass tubes, and, 

 apparentlv, more readily in the presence of watery vapour. Invisible, but develop- 

 able, images were readily obtained in exhausted tubes in which no signs of the 

 presence of moisture were apparent. By prolonged exposure a visible change 

 also takes place. 



AVhen thin films of silver on glass have been fully exposed in sunlight the 

 action has been found to penetrate the film and produce a distinctly visible image 

 at the back as well as on the face, the exposed parts appearing always lighter than 

 the unexposed. 



Fresh experiments with silver plates used as anode and cathode in a decom- 

 position cell containing distilled water, through which a weak current was allowed 

 to pass, showed that the pale grey deposit on the cathode and the dark olive 

 yellow coating on the anode were both quite sensitive to light, and appeared 

 lighter by exposure, in a manner somewhat analogous to that observed on silvered 

 glass or plain silver foils exposed to light. 



It was noticed that the visible images were not dissolved away either by the 

 usual photographic hxing agents or by dilute nitric acid. 



A very curious action of li^ht upon glass has also been observed. In this 

 case a silvered glass plate was exposed for about a month under a cut-out screen 

 of thin aluminium, the unsilvcred side of the glass being in contact with the 

 aluminium and not protected from the air by a covering glass plate. After 

 exposure the plate was put aside for a few days with the exposed glass side in 

 contact with the silvered surface of another piece of polished silvered glass, which 

 was then found to have received an impressed image from the glass of the design 

 cut out of the aluminium screen. 'I'he image was quite visible, clear and sliarp, 

 and somewhat similar to the images directly impressed by light, though it had not 

 the same appearance of being bleached out, when examined by reHected light. 

 Several days afterwards a second similar image was produced in the same way by 

 contact with the glass upon another freshlj' polished silvered glass plate, and no 

 doubt several more could be produced in the same way. 



These new experiments seem to show that the images formed by the action of 

 light upon plain silver surfaces are due more to molecular or physical changes than 

 to chemical decomposition, though the latter may also probably come into play in 

 the presence of watery vapour, or other conditions favouring oxidation and reduction 

 of the metallic surface. The author is continuing the investigation. 



5. Some Thoughts on Atomic Weights and the Periodic Law. 

 By J. H. Gladstone, D.Sc.^ F.R.S., and George Gladstone, 



The object of this paper w'as to recall attention to a suggestion made during the 

 discussion of a paper by Professor Dumas at the meeting otthe Association at Ipswich 

 in 1851, viz., that the case of ' triads' of analogous elements showed a resemblance 

 to the progression in a series of organic compounds, and might be due to a similar 

 cause. It was shown that the ditlerence in atomic weight between the horizontal 

 lines in Mendeljeff's arrangement is in the lirst instance 16, which afterwards 



