490 Major-General J. Waterhouse. The Sensitiveness 



bromine as 63°, whereas we have found the boiling point of specially 

 purified samples to be very close to 58*9°. (See Ramsay and Young,, 

 ' Trans. Chem. Soc.,' vol. 49, p. 454 et seq.) 



" The Sensitiveness of Silver and of some other Metals to Light."" 

 By Major-General J. Waterhouse, I.S.C. (late Assistant Sur- 

 veyor-General of India). Communicated by Sir W. Abney, 

 K.C.B., r.E.S. Eeceived April 25,— Read May 31, 1900. 



During some recent investigations on the Daguerreotype process, the 

 question presented itself as to which of the elements forming the sensi- 

 tive surface of the plate — the silver or the halogens— the sensitiveness 

 was due 1 Now, although the fact that nearly all compounds of silver, 

 especially the haloids, are more or less sensitive to, and decomposed 

 by, the action of light, has long been known, the sensitiveness of 

 metallic silver itself to light, though observed in 1842, by Moser, has 

 never been generally recognised either by chemists or by pho- 

 tographers. 



Mose^-'s Experiment. — Before describing my own experiments, it may 

 ])e as well to give a description of Moser's experiment taken from the 

 original paper in ^ Poggendorff 's Annalen,' vol. 56, 1842, p. 210. 



" A perfectly new silver plate was thoroughly cleaned and polished.. 

 A black tablet with various excised characters was fixed above it, 

 without touching it, and the whole placed in the sun for two hours 

 or more and directed towards it. After the plate, which naturally did 

 not show the least change, was cooled, it was held over mercury, 

 heated as usual to about 60' R. (167° F.). To my great delight a 

 distinct image of the screen was produced in which those parts where 

 the sunlight (which during the course of the experiments was always 

 weak and changeable) had acted, had attracted a quantity of mercury. 

 This interesting experiment was repeated several times with the same 

 result. Sometimes the plates after having been placed in the mercurial 

 vapours were exposed to those of iodine and then placed in the sun,, 

 by which the images usually improved." 



"If we compare this remarkable fact of the action of light upon 

 surfaces of silver with the above-mentioned phenomena produced by 

 contact, we can no longer doubt that light acts on all bodies, modifying 

 them so that they behave differently in condensing the vapours of 

 mercury. A similar experiment was made with copper during un- 

 favourable weather. The copper was not well polished, and, conse- 

 quently, the image produced by the mercurial vapours was faint, 

 although clearly visible. By exposing the plate to the vapour of 

 iodine the image became stronger, and this method was found useful 

 in experiments with copper. A plate of clean mirror-glass was. 



